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package java.util;

import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Function;
import java.io.Serializable;


An object that maps keys to values.  A map cannot contain duplicate keys;
each key can map to at most one value.

This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface. 
The Map interface provides three collection views, which
allow a map's contents to be viewed as a set of keys, collection of values,
or set of key-value mappings.  The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not. 
Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map. 
All general-purpose map implementation classes should provide two "standard" constructors: a void (no arguments) constructor which creates an empty map, and a constructor with a single argument of type Map, which creates a new map with the same key-value mappings as its argument. In effect, the latter constructor allows the user to copy any map, producing an equivalent map of the desired class. There is no way to enforce this recommendation (as interfaces cannot contain constructors) but all of the general-purpose map implementations in the JDK comply. 
The "destructive" methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw UnsupportedOperationException if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an UnsupportedOperationException if the invocation would have no effect on the map. For example, invoking the putAll(Map) method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be "superimposed" is empty. 
Some map implementations have restrictions on the keys and values they may contain. For example, some implementations prohibit null keys and values, and some have restrictions on the types of their keys. Attempting to insert an ineligible key or value throws an unchecked exception, typically NullPointerException or ClassCastException. Attempting to query the presence of an ineligible key or value may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible key or value whose completion would not result in the insertion of an ineligible element into the map may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface. 
Many methods in Collections Framework interfaces are defined in terms of the equals method. For example, the specification for the 
containsKey(Object key) method says: "returns true if and only if this map contains a mapping for a key k such that (key==null ? k==null : key.equals(k))." This specification should not be construed to imply that invoking Map.containsKey with a non-null argument key will cause key.equals(k) to be invoked for any key k. Implementations are free to implement optimizations whereby the equals invocation is avoided, for example, by first comparing the hash codes of the two keys. (The Object.hashCode() specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying Object methods wherever the implementor deems it appropriate. 
Some map operations which perform recursive traversal of the map may fail with an exception for self-referential instances where the map directly or indirectly contains itself. This includes the clone(), equals(), hashCode() and toString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so. 
Unmodifiable Maps


The Map.of, Map.ofEntries, and Map.copyOf static factory methods provide a convenient way to create unmodifiable maps. The Map instances created by these methods have the following characteristics: 
    

  • They are unmodifiable. Keys and values cannot be added, removed, or updated. Calling any mutator method on the Map will always cause UnsupportedOperationException to be thrown. However, if the contained keys or values are themselves mutable, this may cause the Map to behave inconsistently or its contents to appear to change. 
  • They disallow null keys and values. Attempts to create them with null keys or values result in NullPointerException. 
  • They are serializable if all keys and values are serializable.

  • They reject duplicate keys at creation time. Duplicate keys passed to a static factory method result in IllegalArgumentException. 
  • The iteration order of mappings is unspecified and is subject to change.

  • They are value-based. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones. Therefore, identity-sensitive operations on these instances (reference equality (==), identity hash code, and synchronization) are unreliable and should be avoided. 
  • They are serialized as specified on the
Serialized Form
page.



This interface is a member of the

Java Collections Framework.

Author: Josh Bloch
Type parameters:
  • <K> – the type of keys maintained by this map
  • <V> – the type of mapped values
See Also:
Since:1.2
/**
 * An object that maps keys to values.  A map cannot contain duplicate keys;
 * each key can map to at most one value.
 *
 * <p>This interface takes the place of the {@code Dictionary} class, which
 * was a totally abstract class rather than an interface.
 *
 * <p>The {@code Map} interface provides three <i>collection views</i>, which
 * allow a map's contents to be viewed as a set of keys, collection of values,
 * or set of key-value mappings.  The <i>order</i> of a map is defined as
 * the order in which the iterators on the map's collection views return their
 * elements.  Some map implementations, like the {@code TreeMap} class, make
 * specific guarantees as to their order; others, like the {@code HashMap}
 * class, do not.
 *
 * <p>Note: great care must be exercised if mutable objects are used as map
 * keys.  The behavior of a map is not specified if the value of an object is
 * changed in a manner that affects {@code equals} comparisons while the
 * object is a key in the map.  A special case of this prohibition is that it
 * is not permissible for a map to contain itself as a key.  While it is
 * permissible for a map to contain itself as a value, extreme caution is
 * advised: the {@code equals} and {@code hashCode} methods are no longer
 * well defined on such a map.
 *
 * <p>All general-purpose map implementation classes should provide two
 * "standard" constructors: a void (no arguments) constructor which creates an
 * empty map, and a constructor with a single argument of type {@code Map},
 * which creates a new map with the same key-value mappings as its argument.
 * In effect, the latter constructor allows the user to copy any map,
 * producing an equivalent map of the desired class.  There is no way to
 * enforce this recommendation (as interfaces cannot contain constructors) but
 * all of the general-purpose map implementations in the JDK comply.
 *
 * <p>The "destructive" methods contained in this interface, that is, the
 * methods that modify the map on which they operate, are specified to throw
 * {@code UnsupportedOperationException} if this map does not support the
 * operation.  If this is the case, these methods may, but are not required
 * to, throw an {@code UnsupportedOperationException} if the invocation would
 * have no effect on the map.  For example, invoking the {@link #putAll(Map)}
 * method on an unmodifiable map may, but is not required to, throw the
 * exception if the map whose mappings are to be "superimposed" is empty.
 *
 * <p>Some map implementations have restrictions on the keys and values they
 * may contain.  For example, some implementations prohibit null keys and
 * values, and some have restrictions on the types of their keys.  Attempting
 * to insert an ineligible key or value throws an unchecked exception,
 * typically {@code NullPointerException} or {@code ClassCastException}.
 * Attempting to query the presence of an ineligible key or value may throw an
 * exception, or it may simply return false; some implementations will exhibit
 * the former behavior and some will exhibit the latter.  More generally,
 * attempting an operation on an ineligible key or value whose completion
 * would not result in the insertion of an ineligible element into the map may
 * throw an exception or it may succeed, at the option of the implementation.
 * Such exceptions are marked as "optional" in the specification for this
 * interface.
 *
 * <p>Many methods in Collections Framework interfaces are defined
 * in terms of the {@link Object#equals(Object) equals} method.  For
 * example, the specification for the {@link #containsKey(Object)
 * containsKey(Object key)} method says: "returns {@code true} if and
 * only if this map contains a mapping for a key {@code k} such that
 * {@code (key==null ? k==null : key.equals(k))}." This specification should
 * <i>not</i> be construed to imply that invoking {@code Map.containsKey}
 * with a non-null argument {@code key} will cause {@code key.equals(k)} to
 * be invoked for any key {@code k}.  Implementations are free to
 * implement optimizations whereby the {@code equals} invocation is avoided,
 * for example, by first comparing the hash codes of the two keys.  (The
 * {@link Object#hashCode()} specification guarantees that two objects with
 * unequal hash codes cannot be equal.)  More generally, implementations of
 * the various Collections Framework interfaces are free to take advantage of
 * the specified behavior of underlying {@link Object} methods wherever the
 * implementor deems it appropriate.
 *
 * <p>Some map operations which perform recursive traversal of the map may fail
 * with an exception for self-referential instances where the map directly or
 * indirectly contains itself. This includes the {@code clone()},
 * {@code equals()}, {@code hashCode()} and {@code toString()} methods.
 * Implementations may optionally handle the self-referential scenario, however
 * most current implementations do not do so.
 *
 * <h2><a id="unmodifiable">Unmodifiable Maps</a></h2>
 * <p>The {@link Map#of() Map.of},
 * {@link Map#ofEntries(Map.Entry...) Map.ofEntries}, and
 * {@link Map#copyOf Map.copyOf}
 * static factory methods provide a convenient way to create unmodifiable maps.
 * The {@code Map}
 * instances created by these methods have the following characteristics:
 *
 * <ul>
 * <li>They are <a href="Collection.html#unmodifiable"><i>unmodifiable</i></a>. Keys and values
 * cannot be added, removed, or updated. Calling any mutator method on the Map
 * will always cause {@code UnsupportedOperationException} to be thrown.
 * However, if the contained keys or values are themselves mutable, this may cause the
 * Map to behave inconsistently or its contents to appear to change.
 * <li>They disallow {@code null} keys and values. Attempts to create them with
 * {@code null} keys or values result in {@code NullPointerException}.
 * <li>They are serializable if all keys and values are serializable.
 * <li>They reject duplicate keys at creation time. Duplicate keys
 * passed to a static factory method result in {@code IllegalArgumentException}.
 * <li>The iteration order of mappings is unspecified and is subject to change.
 * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
 * Callers should make no assumptions about the identity of the returned instances.
 * Factories are free to create new instances or reuse existing ones. Therefore,
 * identity-sensitive operations on these instances (reference equality ({@code ==}),
 * identity hash code, and synchronization) are unreliable and should be avoided.
 * <li>They are serialized as specified on the
 * <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
 * page.
 * </ul>
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 *
 * @author  Josh Bloch
 * @see HashMap
 * @see TreeMap
 * @see Hashtable
 * @see SortedMap
 * @see Collection
 * @see Set
 * @since 1.2
 */
public interface Map<K, V> {
    // Query Operations

    
Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE. 
Returns:the number of key-value mappings in this map
/**
     * Returns the number of key-value mappings in this map.  If the
     * map contains more than {@code Integer.MAX_VALUE} elements, returns
     * {@code Integer.MAX_VALUE}.
     *
     * @return the number of key-value mappings in this map
     */
    int size();

    
Returns true if this map contains no key-value mappings. 
Returns:true if this map contains no key-value mappings
/**
     * Returns {@code true} if this map contains no key-value mappings.
     *
     * @return {@code true} if this map contains no key-value mappings
     */
    boolean isEmpty();

    
Returns true if this map contains a mapping for the specified key. More formally, returns true if and only if this map contains a mapping for a key k such that Objects.equals(key, k). (There can be at most one such mapping.) 
Params:
  • key – key whose presence in this map is to be tested
Throws:
Returns:true if this map contains a mapping for the specified key
/**
     * Returns {@code true} if this map contains a mapping for the specified
     * key.  More formally, returns {@code true} if and only if
     * this map contains a mapping for a key {@code k} such that
     * {@code Objects.equals(key, k)}.  (There can be
     * at most one such mapping.)
     *
     * @param key key whose presence in this map is to be tested
     * @return {@code true} if this map contains a mapping for the specified
     *         key
     * @throws ClassCastException if the key is of an inappropriate type for
     *         this map
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key is null and this map
     *         does not permit null keys
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    boolean containsKey(Object key);

    
Returns true if this map maps one or more keys to the specified value. More formally, returns true if and only if this map contains at least one mapping to a value v such that Objects.equals(value, v). This operation will probably require time linear in the map size for most implementations of the Map interface. 
Params:
  • value – value whose presence in this map is to be tested
Throws:
Returns:true if this map maps one or more keys to the specified value
/**
     * Returns {@code true} if this map maps one or more keys to the
     * specified value.  More formally, returns {@code true} if and only if
     * this map contains at least one mapping to a value {@code v} such that
     * {@code Objects.equals(value, v)}.  This operation
     * will probably require time linear in the map size for most
     * implementations of the {@code Map} interface.
     *
     * @param value value whose presence in this map is to be tested
     * @return {@code true} if this map maps one or more keys to the
     *         specified value
     * @throws ClassCastException if the value is of an inappropriate type for
     *         this map
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified value is null and this
     *         map does not permit null values
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    boolean containsValue(Object value);

    
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. 
More formally, if this map contains a mapping from a key k to a value v such that Objects.equals(key, k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.) 
If this map permits null values, then a return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The 
containsKey operation may be used to distinguish these two cases. 
Params:
  • key – the key whose associated value is to be returned
Throws:
Returns:the value to which the specified key is mapped, or null if this map contains no mapping for the key
/**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that
     * {@code Objects.equals(key, k)},
     * then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>If this map permits null values, then a return value of
     * {@code null} does not <i>necessarily</i> indicate that the map
     * contains no mapping for the key; it's also possible that the map
     * explicitly maps the key to {@code null}.  The {@link #containsKey
     * containsKey} operation may be used to distinguish these two cases.
     *
     * @param key the key whose associated value is to be returned
     * @return the value to which the specified key is mapped, or
     *         {@code null} if this map contains no mapping for the key
     * @throws ClassCastException if the key is of an inappropriate type for
     *         this map
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key is null and this map
     *         does not permit null keys
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    V get(Object key);

    // Modification Operations

    
Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for the key, the old value is replaced by the specified value. (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.) 
Params:
  • key – key with which the specified value is to be associated
  • value – value to be associated with the specified key
Throws:
Returns:the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key, if the implementation supports null values.)
/**
     * Associates the specified value with the specified key in this map
     * (optional operation).  If the map previously contained a mapping for
     * the key, the old value is replaced by the specified value.  (A map
     * {@code m} is said to contain a mapping for a key {@code k} if and only
     * if {@link #containsKey(Object) m.containsKey(k)} would return
     * {@code true}.)
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with {@code key}, or
     *         {@code null} if there was no mapping for {@code key}.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with {@code key},
     *         if the implementation supports {@code null} values.)
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     * @throws NullPointerException if the specified key or value is null
     *         and this map does not permit null keys or values
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     */
    V put(K key, V value);

    
Removes the mapping for a key from this map if it is present (optional operation). More formally, if this map contains a mapping from key k to value v such that Objects.equals(key, k), that mapping is removed. (The map can contain at most one such mapping.) 
Returns the value to which this map previously associated the key, or null if the map contained no mapping for the key. 
If this map permits null values, then a return value of null does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to null. 
The map will not contain a mapping for the specified key once the
call returns.

Params:
  • key – key whose mapping is to be removed from the map
Throws:
Returns:the previous value associated with key, or null if there was no mapping for key.
/**
     * Removes the mapping for a key from this map if it is present
     * (optional operation).   More formally, if this map contains a mapping
     * from key {@code k} to value {@code v} such that
     * {@code Objects.equals(key, k)}, that mapping
     * is removed.  (The map can contain at most one such mapping.)
     *
     * <p>Returns the value to which this map previously associated the key,
     * or {@code null} if the map contained no mapping for the key.
     *
     * <p>If this map permits null values, then a return value of
     * {@code null} does not <i>necessarily</i> indicate that the map
     * contained no mapping for the key; it's also possible that the map
     * explicitly mapped the key to {@code null}.
     *
     * <p>The map will not contain a mapping for the specified key once the
     * call returns.
     *
     * @param key key whose mapping is to be removed from the map
     * @return the previous value associated with {@code key}, or
     *         {@code null} if there was no mapping for {@code key}.
     * @throws UnsupportedOperationException if the {@code remove} operation
     *         is not supported by this map
     * @throws ClassCastException if the key is of an inappropriate type for
     *         this map
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key is null and this
     *         map does not permit null keys
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    V remove(Object key);


    // Bulk Operations

    
Copies all of the mappings from the specified map to this map (optional operation). The effect of this call is equivalent to that of calling put(k, v) on this map once for each mapping from key k to value v in the specified map. The behavior of this operation is undefined if the specified map is modified while the operation is in progress. 
Params:
  • m – mappings to be stored in this map
Throws:
  • UnsupportedOperationException – if the putAll operation is not supported by this map
  • ClassCastException – if the class of a key or value in the
        specified map prevents it from being stored in this map
  • NullPointerException – if the specified map is null, or if
        this map does not permit null keys or values, and the
        specified map contains null keys or values
  • IllegalArgumentException – if some property of a key or value in
        the specified map prevents it from being stored in this map
/**
     * Copies all of the mappings from the specified map to this map
     * (optional operation).  The effect of this call is equivalent to that
     * of calling {@link #put(Object,Object) put(k, v)} on this map once
     * for each mapping from key {@code k} to value {@code v} in the
     * specified map.  The behavior of this operation is undefined if the
     * specified map is modified while the operation is in progress.
     *
     * @param m mappings to be stored in this map
     * @throws UnsupportedOperationException if the {@code putAll} operation
     *         is not supported by this map
     * @throws ClassCastException if the class of a key or value in the
     *         specified map prevents it from being stored in this map
     * @throws NullPointerException if the specified map is null, or if
     *         this map does not permit null keys or values, and the
     *         specified map contains null keys or values
     * @throws IllegalArgumentException if some property of a key or value in
     *         the specified map prevents it from being stored in this map
     */
    void putAll(Map<? extends K, ? extends V> m);

    
Removes all of the mappings from this map (optional operation).
The map will be empty after this call returns.

Throws:
  • UnsupportedOperationException – if the clear operation is not supported by this map
/**
     * Removes all of the mappings from this map (optional operation).
     * The map will be empty after this call returns.
     *
     * @throws UnsupportedOperationException if the {@code clear} operation
     *         is not supported by this map
     */
    void clear();


    // Views

    
Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations. 
Returns:a set view of the keys contained in this map
/**
     * Returns a {@link Set} view of the keys contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  If the map is modified
     * while an iteration over the set is in progress (except through
     * the iterator's own {@code remove} operation), the results of
     * the iteration are undefined.  The set supports element removal,
     * which removes the corresponding mapping from the map, via the
     * {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.  It does not support the {@code add} or {@code addAll}
     * operations.
     *
     * @return a set view of the keys contained in this map
     */
    Set<K> keySet();

    
Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. 
Returns:a collection view of the values contained in this map
/**
     * Returns a {@link Collection} view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are
     * reflected in the collection, and vice-versa.  If the map is
     * modified while an iteration over the collection is in progress
     * (except through the iterator's own {@code remove} operation),
     * the results of the iteration are undefined.  The collection
     * supports element removal, which removes the corresponding
     * mapping from the map, via the {@code Iterator.remove},
     * {@code Collection.remove}, {@code removeAll},
     * {@code retainAll} and {@code clear} operations.  It does not
     * support the {@code add} or {@code addAll} operations.
     *
     * @return a collection view of the values contained in this map
     */
    Collection<V> values();

    
Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations. 
Returns:a set view of the mappings contained in this map
/**
     * Returns a {@link Set} view of the mappings contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  If the map is modified
     * while an iteration over the set is in progress (except through
     * the iterator's own {@code remove} operation, or through the
     * {@code setValue} operation on a map entry returned by the
     * iterator) the results of the iteration are undefined.  The set
     * supports element removal, which removes the corresponding
     * mapping from the map, via the {@code Iterator.remove},
     * {@code Set.remove}, {@code removeAll}, {@code retainAll} and
     * {@code clear} operations.  It does not support the
     * {@code add} or {@code addAll} operations.
     *
     * @return a set view of the mappings contained in this map
     */
    Set<Map.Entry<K, V>> entrySet();

    
A map entry (key-value pair). The Map.entrySet method returns a collection-view of the map, whose elements are of this class. The only way to obtain a reference to a map entry is from the iterator of this collection-view. These Map.Entry objects are valid only for the duration of the iteration; more formally, the behavior of a map entry is undefined if the backing map has been modified after the entry was returned by the iterator, except through the setValue operation on the map entry. 
See Also:
Since:1.2
/**
     * A map entry (key-value pair).  The {@code Map.entrySet} method returns
     * a collection-view of the map, whose elements are of this class.  The
     * <i>only</i> way to obtain a reference to a map entry is from the
     * iterator of this collection-view.  These {@code Map.Entry} objects are
     * valid <i>only</i> for the duration of the iteration; more formally,
     * the behavior of a map entry is undefined if the backing map has been
     * modified after the entry was returned by the iterator, except through
     * the {@code setValue} operation on the map entry.
     *
     * @see Map#entrySet()
     * @since 1.2
     */
    interface Entry<K, V> {
        
Returns the key corresponding to this entry.

Throws:
  • IllegalStateException – implementations may, but are not
        required to, throw this exception if the entry has been
        removed from the backing map.
Returns:the key corresponding to this entry
/**
         * Returns the key corresponding to this entry.
         *
         * @return the key corresponding to this entry
         * @throws IllegalStateException implementations may, but are not
         *         required to, throw this exception if the entry has been
         *         removed from the backing map.
         */
        K getKey();

        
Returns the value corresponding to this entry. If the mapping has been removed from the backing map (by the iterator's remove operation), the results of this call are undefined. 
Throws:
  • IllegalStateException – implementations may, but are not
        required to, throw this exception if the entry has been
        removed from the backing map.
Returns:the value corresponding to this entry
/**
         * Returns the value corresponding to this entry.  If the mapping
         * has been removed from the backing map (by the iterator's
         * {@code remove} operation), the results of this call are undefined.
         *
         * @return the value corresponding to this entry
         * @throws IllegalStateException implementations may, but are not
         *         required to, throw this exception if the entry has been
         *         removed from the backing map.
         */
        V getValue();

        
Replaces the value corresponding to this entry with the specified value (optional operation). (Writes through to the map.) The behavior of this call is undefined if the mapping has already been removed from the map (by the iterator's remove operation). 
Params:
  • value – new value to be stored in this entry
Throws:
Returns:old value corresponding to the entry
/**
         * Replaces the value corresponding to this entry with the specified
         * value (optional operation).  (Writes through to the map.)  The
         * behavior of this call is undefined if the mapping has already been
         * removed from the map (by the iterator's {@code remove} operation).
         *
         * @param value new value to be stored in this entry
         * @return old value corresponding to the entry
         * @throws UnsupportedOperationException if the {@code put} operation
         *         is not supported by the backing map
         * @throws ClassCastException if the class of the specified value
         *         prevents it from being stored in the backing map
         * @throws NullPointerException if the backing map does not permit
         *         null values, and the specified value is null
         * @throws IllegalArgumentException if some property of this value
         *         prevents it from being stored in the backing map
         * @throws IllegalStateException implementations may, but are not
         *         required to, throw this exception if the entry has been
         *         removed from the backing map.
         */
        V setValue(V value);

        
Compares the specified object with this entry for equality. Returns true if the given object is also a map entry and the two entries represent the same mapping. More formally, two entries e1 and e2 represent the same mapping if
    (e1.getKey()==null ?
     e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &&
    (e1.getValue()==null ?
     e2.getValue()==null : e1.getValue().equals(e2.getValue()))

 This ensures that the equals method works properly across different implementations of the Map.Entry interface. 
Params:
  • o – object to be compared for equality with this map entry
Returns:true if the specified object is equal to this map entry
/**
         * Compares the specified object with this entry for equality.
         * Returns {@code true} if the given object is also a map entry and
         * the two entries represent the same mapping.  More formally, two
         * entries {@code e1} and {@code e2} represent the same mapping
         * if<pre>
         *     (e1.getKey()==null ?
         *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &amp;&amp;
         *     (e1.getValue()==null ?
         *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
         * </pre>
         * This ensures that the {@code equals} method works properly across
         * different implementations of the {@code Map.Entry} interface.
         *
         * @param o object to be compared for equality with this map entry
         * @return {@code true} if the specified object is equal to this map
         *         entry
         */
        boolean equals(Object o);

        
Returns the hash code value for this map entry. The hash code of a map entry e is defined to be: 
    (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
    (e.getValue()==null ? 0 : e.getValue().hashCode())

 This ensures that e1.equals(e2) implies that e1.hashCode()==e2.hashCode() for any two Entries e1 and e2, as required by the general contract of Object.hashCode. 
See Also:
Returns:the hash code value for this map entry
/**
         * Returns the hash code value for this map entry.  The hash code
         * of a map entry {@code e} is defined to be: <pre>
         *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
         *     (e.getValue()==null ? 0 : e.getValue().hashCode())
         * </pre>
         * This ensures that {@code e1.equals(e2)} implies that
         * {@code e1.hashCode()==e2.hashCode()} for any two Entries
         * {@code e1} and {@code e2}, as required by the general
         * contract of {@code Object.hashCode}.
         *
         * @return the hash code value for this map entry
         * @see Object#hashCode()
         * @see Object#equals(Object)
         * @see #equals(Object)
         */
        int hashCode();

        
Returns a comparator that compares Entry in natural order on key. 
The returned comparator is serializable and throws NullPointerException when comparing an entry with a null key. 
Type parameters:
  • <K> – the Comparable type of then map keys
  • <V> – the type of the map values
See Also:
Returns:a comparator that compares Entry in natural order on key.
Since:1.8
/**
         * Returns a comparator that compares {@link Map.Entry} in natural order on key.
         *
         * <p>The returned comparator is serializable and throws {@link
         * NullPointerException} when comparing an entry with a null key.
         *
         * @param  <K> the {@link Comparable} type of then map keys
         * @param  <V> the type of the map values
         * @return a comparator that compares {@link Map.Entry} in natural order on key.
         * @see Comparable
         * @since 1.8
         */
        public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K, V>> comparingByKey() {
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> c1.getKey().compareTo(c2.getKey());
        }

        
Returns a comparator that compares Entry in natural order on value. 
The returned comparator is serializable and throws NullPointerException when comparing an entry with null values. 
Type parameters:
  • <K> – the type of the map keys
  • <V> – the Comparable type of the map values
See Also:
Returns:a comparator that compares Entry in natural order on value.
Since:1.8
/**
         * Returns a comparator that compares {@link Map.Entry} in natural order on value.
         *
         * <p>The returned comparator is serializable and throws {@link
         * NullPointerException} when comparing an entry with null values.
         *
         * @param <K> the type of the map keys
         * @param <V> the {@link Comparable} type of the map values
         * @return a comparator that compares {@link Map.Entry} in natural order on value.
         * @see Comparable
         * @since 1.8
         */
        public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K, V>> comparingByValue() {
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> c1.getValue().compareTo(c2.getValue());
        }

        
Returns a comparator that compares Entry by key using the given Comparator. 
The returned comparator is serializable if the specified comparator
is also serializable.

Params:
Type parameters:
  • <K> – the type of the map keys
  • <V> – the type of the map values
Returns:a comparator that compares Entry by the key.
Since:1.8
/**
         * Returns a comparator that compares {@link Map.Entry} by key using the given
         * {@link Comparator}.
         *
         * <p>The returned comparator is serializable if the specified comparator
         * is also serializable.
         *
         * @param  <K> the type of the map keys
         * @param  <V> the type of the map values
         * @param  cmp the key {@link Comparator}
         * @return a comparator that compares {@link Map.Entry} by the key.
         * @since 1.8
         */
        public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
            Objects.requireNonNull(cmp);
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
        }

        
Returns a comparator that compares Entry by value using the given Comparator. 
The returned comparator is serializable if the specified comparator
is also serializable.

Params:
Type parameters:
  • <K> – the type of the map keys
  • <V> – the type of the map values
Returns:a comparator that compares Entry by the value.
Since:1.8
/**
         * Returns a comparator that compares {@link Map.Entry} by value using the given
         * {@link Comparator}.
         *
         * <p>The returned comparator is serializable if the specified comparator
         * is also serializable.
         *
         * @param  <K> the type of the map keys
         * @param  <V> the type of the map values
         * @param  cmp the value {@link Comparator}
         * @return a comparator that compares {@link Map.Entry} by the value.
         * @since 1.8
         */
        public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
            Objects.requireNonNull(cmp);
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
        }
    }

    // Comparison and hashing

    
Compares the specified object with this map for equality. Returns true if the given object is also a map and the two maps represent the same mappings. More formally, two maps m1 and m2 represent the same mappings if m1.entrySet().equals(m2.entrySet()). This ensures that the equals method works properly across different implementations of the Map interface. 
Params:
  • o – object to be compared for equality with this map
Returns:true if the specified object is equal to this map
/**
     * Compares the specified object with this map for equality.  Returns
     * {@code true} if the given object is also a map and the two maps
     * represent the same mappings.  More formally, two maps {@code m1} and
     * {@code m2} represent the same mappings if
     * {@code m1.entrySet().equals(m2.entrySet())}.  This ensures that the
     * {@code equals} method works properly across different implementations
     * of the {@code Map} interface.
     *
     * @param o object to be compared for equality with this map
     * @return {@code true} if the specified object is equal to this map
     */
    boolean equals(Object o);

    
Returns the hash code value for this map. The hash code of a map is defined to be the sum of the hash codes of each entry in the map's entrySet() view. This ensures that m1.equals(m2) implies that m1.hashCode()==m2.hashCode() for any two maps m1 and m2, as required by the general contract of Object.hashCode. 
See Also:
Returns:the hash code value for this map
/**
     * Returns the hash code value for this map.  The hash code of a map is
     * defined to be the sum of the hash codes of each entry in the map's
     * {@code entrySet()} view.  This ensures that {@code m1.equals(m2)}
     * implies that {@code m1.hashCode()==m2.hashCode()} for any two maps
     * {@code m1} and {@code m2}, as required by the general contract of
     * {@link Object#hashCode}.
     *
     * @return the hash code value for this map
     * @see Map.Entry#hashCode()
     * @see Object#equals(Object)
     * @see #equals(Object)
     */
    int hashCode();

    // Defaultable methods

    
Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key. 
Params:
  • key – the key whose associated value is to be returned
  • defaultValue – the default mapping of the key
Throws:
Implementation Requirements:
The default implementation makes no guarantees about synchronization
or atomicity properties of this method. Any implementation providing
atomicity guarantees must override this method and document its
concurrency properties.
Returns:the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key
Since:1.8
/**
     * Returns the value to which the specified key is mapped, or
     * {@code defaultValue} if this map contains no mapping for the key.
     *
     * @implSpec
     * The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param key the key whose associated value is to be returned
     * @param defaultValue the default mapping of the key
     * @return the value to which the specified key is mapped, or
     * {@code defaultValue} if this map contains no mapping for the key
     * @throws ClassCastException if the key is of an inappropriate type for
     * this map
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key is null and this map
     * does not permit null keys
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V getOrDefault(Object key, V defaultValue) {
        V v;
        return (((v = get(key)) != null) || containsKey(key))
            ? v
            : defaultValue;
    }

    
Performs the given action for each entry in this map until all entries
have been processed or the action throws an exception.   Unless
otherwise specified by the implementing class, actions are performed in
the order of entry set iteration (if an iteration order is specified.)
Exceptions thrown by the action are relayed to the caller.

Params:
  • action – The action to be performed for each entry
Throws:
Implementation Requirements: The default implementation is equivalent to, for this map: 
 
for (Map.Entry<K, V> entry : map.entrySet())
    action.accept(entry.getKey(), entry.getValue());


The default implementation makes no guarantees about synchronization
or atomicity properties of this method. Any implementation providing
atomicity guarantees must override this method and document its
concurrency properties.
Since:1.8
/**
     * Performs the given action for each entry in this map until all entries
     * have been processed or the action throws an exception.   Unless
     * otherwise specified by the implementing class, actions are performed in
     * the order of entry set iteration (if an iteration order is specified.)
     * Exceptions thrown by the action are relayed to the caller.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code map}:
     * <pre> {@code
     * for (Map.Entry<K, V> entry : map.entrySet())
     *     action.accept(entry.getKey(), entry.getValue());
     * }</pre>
     *
     * The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param action The action to be performed for each entry
     * @throws NullPointerException if the specified action is null
     * @throws ConcurrentModificationException if an entry is found to be
     * removed during iteration
     * @since 1.8
     */
    default void forEach(BiConsumer<? super K, ? super V> action) {
        Objects.requireNonNull(action);
        for (Map.Entry<K, V> entry : entrySet()) {
            K k;
            V v;
            try {
                k = entry.getKey();
                v = entry.getValue();
            } catch (IllegalStateException ise) {
                // this usually means the entry is no longer in the map.
                throw new ConcurrentModificationException(ise);
            }
            action.accept(k, v);
        }
    }

    
Replaces each entry's value with the result of invoking the given
function on that entry until all entries have been processed or the
function throws an exception.  Exceptions thrown by the function are
relayed to the caller.

Params:
  • function – the function to apply to each entry
Throws:
Implementation Requirements:

The default implementation is equivalent to, for this map: 
 
for (Map.Entry<K, V> entry : map.entrySet())
    entry.setValue(function.apply(entry.getKey(), entry.getValue()));



The default implementation makes no guarantees about synchronization
or atomicity properties of this method. Any implementation providing
atomicity guarantees must override this method and document its
concurrency properties.
Since:1.8
/**
     * Replaces each entry's value with the result of invoking the given
     * function on that entry until all entries have been processed or the
     * function throws an exception.  Exceptions thrown by the function are
     * relayed to the caller.
     *
     * @implSpec
     * <p>The default implementation is equivalent to, for this {@code map}:
     * <pre> {@code
     * for (Map.Entry<K, V> entry : map.entrySet())
     *     entry.setValue(function.apply(entry.getKey(), entry.getValue()));
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param function the function to apply to each entry
     * @throws UnsupportedOperationException if the {@code set} operation
     * is not supported by this map's entry set iterator.
     * @throws ClassCastException if the class of a replacement value
     * prevents it from being stored in this map
     * @throws NullPointerException if the specified function is null, or the
     * specified replacement value is null, and this map does not permit null
     * values
     * @throws ClassCastException if a replacement value is of an inappropriate
     *         type for this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if function or a replacement value is null,
     *         and this map does not permit null keys or values
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of a replacement value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ConcurrentModificationException if an entry is found to be
     * removed during iteration
     * @since 1.8
     */
    default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        Objects.requireNonNull(function);
        for (Map.Entry<K, V> entry : entrySet()) {
            K k;
            V v;
            try {
                k = entry.getKey();
                v = entry.getValue();
            } catch (IllegalStateException ise) {
                // this usually means the entry is no longer in the map.
                throw new ConcurrentModificationException(ise);
            }

            // ise thrown from function is not a cme.
            v = function.apply(k, v);

            try {
                entry.setValue(v);
            } catch (IllegalStateException ise) {
                // this usually means the entry is no longer in the map.
                throw new ConcurrentModificationException(ise);
            }
        }
    }

    
If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value. 
Params:
  • key – key with which the specified value is to be associated
  • value – value to be associated with the specified key
Throws:
Implementation Requirements: The default implementation is equivalent to, for this 
map: 
 
V v = map.get(key);
if (v == null)
    v = map.put(key, value);
return v;



The default implementation makes no guarantees about synchronization
or atomicity properties of this method. Any implementation providing
atomicity guarantees must override this method and document its
concurrency properties.
Returns:the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
Since:1.8
/**
     * If the specified key is not already associated with a value (or is mapped
     * to {@code null}) associates it with the given value and returns
     * {@code null}, else returns the current value.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code
     * map}:
     *
     * <pre> {@code
     * V v = map.get(key);
     * if (v == null)
     *     v = map.put(key, value);
     *
     * return v;
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with the specified key, or
     *         {@code null} if there was no mapping for the key.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with the key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the key or value is of an inappropriate
     *         type for this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V putIfAbsent(K key, V value) {
        V v = get(key);
        if (v == null) {
            v = put(key, value);
        }

        return v;
    }

    
Removes the entry for the specified key only if it is currently
mapped to the specified value.

Params:
  • key – key with which the specified value is associated
  • value – value expected to be associated with the specified key
Throws:
Implementation Requirements: The default implementation is equivalent to, for this map: 
 
if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
    map.remove(key);
    return true;
 } else
    return false;



The default implementation makes no guarantees about synchronization
or atomicity properties of this method. Any implementation providing
atomicity guarantees must override this method and document its
concurrency properties.
Returns:true if the value was removed
Since:1.8
/**
     * Removes the entry for the specified key only if it is currently
     * mapped to the specified value.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code map}:
     *
     * <pre> {@code
     * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     *     map.remove(key);
     *     return true;
     * } else
     *     return false;
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param key key with which the specified value is associated
     * @param value value expected to be associated with the specified key
     * @return {@code true} if the value was removed
     * @throws UnsupportedOperationException if the {@code remove} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the key or value is of an inappropriate
     *         type for this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default boolean remove(Object key, Object value) {
        Object curValue = get(key);
        if (!Objects.equals(curValue, value) ||
            (curValue == null && !containsKey(key))) {
            return false;
        }
        remove(key);
        return true;
    }

    
Replaces the entry for the specified key only if currently
mapped to the specified value.

Params:
  • key – key with which the specified value is associated
  • oldValue – value expected to be associated with the specified key
  • newValue – value to be associated with the specified key
Throws:
Implementation Requirements: The default implementation is equivalent to, for this map: 
 
if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
    map.put(key, newValue);
    return true;
 } else
    return false;


The default implementation does not throw NullPointerException
for maps that do not support null values if oldValue is null unless
newValue is also null.

The default implementation makes no guarantees about synchronization
or atomicity properties of this method. Any implementation providing
atomicity guarantees must override this method and document its
concurrency properties.
Returns:true if the value was replaced
Since:1.8
/**
     * Replaces the entry for the specified key only if currently
     * mapped to the specified value.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code map}:
     *
     * <pre> {@code
     * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     *     map.put(key, newValue);
     *     return true;
     * } else
     *     return false;
     * }</pre>
     *
     * The default implementation does not throw NullPointerException
     * for maps that do not support null values if oldValue is null unless
     * newValue is also null.
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param key key with which the specified value is associated
     * @param oldValue value expected to be associated with the specified key
     * @param newValue value to be associated with the specified key
     * @return {@code true} if the value was replaced
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of a specified key or value
     *         prevents it from being stored in this map
     * @throws NullPointerException if a specified key or newValue is null,
     *         and this map does not permit null keys or values
     * @throws NullPointerException if oldValue is null and this map does not
     *         permit null values
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of a specified key
     *         or value prevents it from being stored in this map
     * @since 1.8
     */
    default boolean replace(K key, V oldValue, V newValue) {
        Object curValue = get(key);
        if (!Objects.equals(curValue, oldValue) ||
            (curValue == null && !containsKey(key))) {
            return false;
        }
        put(key, newValue);
        return true;
    }

    
Replaces the entry for the specified key only if it is
currently mapped to some value.

Params:
  • key – key with which the specified value is associated
  • value – value to be associated with the specified key
Throws:
Implementation Requirements: The default implementation is equivalent to, for this map: 
 
if (map.containsKey(key)) {
    return map.put(key, value);
 } else
    return null;



The default implementation makes no guarantees about synchronization
or atomicity properties of this method. Any implementation providing
atomicity guarantees must override this method and document its
concurrency properties.
Returns:the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
Since:1.8
/**
     * Replaces the entry for the specified key only if it is
     * currently mapped to some value.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code map}:
     *
     * <pre> {@code
     * if (map.containsKey(key)) {
     *     return map.put(key, value);
     * } else
     *     return null;
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param key key with which the specified value is associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with the specified key, or
     *         {@code null} if there was no mapping for the key.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with the key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     * @since 1.8
     */
    default V replace(K key, V value) {
        V curValue;
        if (((curValue = get(key)) != null) || containsKey(key)) {
            curValue = put(key, value);
        }
        return curValue;
    }

    
If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null. 
If the mapping function returns null, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in: 
 
map.computeIfAbsent(key, k -> new Value(f(k)));



Or to implement a multi-value map, Map<K,Collection<V>>, supporting multiple values per key: 
 
map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);



The mapping function should not modify this map during computation.

Params:
  • key – key with which the specified value is to be associated
  • mappingFunction – the mapping function to compute a value
Throws:
Implementation Requirements: The default implementation is equivalent to the following steps for this map, then returning the current value or null if now absent: 
 
if (map.get(key) == null) {
    V newValue = mappingFunction.apply(key);
    if (newValue != null)
        map.put(key, newValue);
 }



The default implementation makes no guarantees about detecting if the mapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the mapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the mapping function modifies this map during computation and as a result computation would never complete. 
The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the mapping function is applied once atomically only if the value is not present.
Returns:the current (existing or computed) value associated with
        the specified key, or null if the computed value is null
Since:1.8
/**
     * If the specified key is not already associated with a value (or is mapped
     * to {@code null}), attempts to compute its value using the given mapping
     * function and enters it into this map unless {@code null}.
     *
     * <p>If the mapping function returns {@code null}, no mapping is recorded.
     * If the mapping function itself throws an (unchecked) exception, the
     * exception is rethrown, and no mapping is recorded.  The most
     * common usage is to construct a new object serving as an initial
     * mapped value or memoized result, as in:
     *
     * <pre> {@code
     * map.computeIfAbsent(key, k -> new Value(f(k)));
     * }</pre>
     *
     * <p>Or to implement a multi-value map, {@code Map<K,Collection<V>>},
     * supporting multiple values per key:
     *
     * <pre> {@code
     * map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
     * }</pre>
     *
     * <p>The mapping function should not modify this map during computation.
     *
     * @implSpec
     * The default implementation is equivalent to the following steps for this
     * {@code map}, then returning the current value or {@code null} if now
     * absent:
     *
     * <pre> {@code
     * if (map.get(key) == null) {
     *     V newValue = mappingFunction.apply(key);
     *     if (newValue != null)
     *         map.put(key, newValue);
     * }
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about detecting if the
     * mapping function modifies this map during computation and, if
     * appropriate, reporting an error. Non-concurrent implementations should
     * override this method and, on a best-effort basis, throw a
     * {@code ConcurrentModificationException} if it is detected that the
     * mapping function modifies this map during computation. Concurrent
     * implementations should override this method and, on a best-effort basis,
     * throw an {@code IllegalStateException} if it is detected that the
     * mapping function modifies this map during computation and as a result
     * computation would never complete.
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the mapping function is applied once atomically only if the value
     * is not present.
     *
     * @param key key with which the specified value is to be associated
     * @param mappingFunction the mapping function to compute a value
     * @return the current (existing or computed) value associated with
     *         the specified key, or null if the computed value is null
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the mappingFunction
     *         is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V computeIfAbsent(K key,
            Function<? super K, ? extends V> mappingFunction) {
        Objects.requireNonNull(mappingFunction);
        V v;
        if ((v = get(key)) == null) {
            V newValue;
            if ((newValue = mappingFunction.apply(key)) != null) {
                put(key, newValue);
                return newValue;
            }
        }

        return v;
    }

    
If the value for the specified key is present and non-null, attempts to
compute a new mapping given the key and its current mapped value.

If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged. 
The remapping function should not modify this map during computation.

Params:
  • key – key with which the specified value is to be associated
  • remappingFunction – the remapping function to compute a value
Throws:
Implementation Requirements: The default implementation is equivalent to performing the following steps for this map, then returning the current value or null if now absent: 
 
if (map.get(key) != null) {
    V oldValue = map.get(key);
    V newValue = remappingFunction.apply(key, oldValue);
    if (newValue != null)
        map.put(key, newValue);
    else
        map.remove(key);
 }



The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the remapping function modifies this map during computation and as a result computation would never complete. 
The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the remapping function is applied once atomically only if the value is not present.
Returns:the new value associated with the specified key, or null if none
Since:1.8
/**
     * If the value for the specified key is present and non-null, attempts to
     * compute a new mapping given the key and its current mapped value.
     *
     * <p>If the remapping function returns {@code null}, the mapping is removed.
     * If the remapping function itself throws an (unchecked) exception, the
     * exception is rethrown, and the current mapping is left unchanged.
     *
     * <p>The remapping function should not modify this map during computation.
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if now absent:
     *
     * <pre> {@code
     * if (map.get(key) != null) {
     *     V oldValue = map.get(key);
     *     V newValue = remappingFunction.apply(key, oldValue);
     *     if (newValue != null)
     *         map.put(key, newValue);
     *     else
     *         map.remove(key);
     * }
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about detecting if the
     * remapping function modifies this map during computation and, if
     * appropriate, reporting an error. Non-concurrent implementations should
     * override this method and, on a best-effort basis, throw a
     * {@code ConcurrentModificationException} if it is detected that the
     * remapping function modifies this map during computation. Concurrent
     * implementations should override this method and, on a best-effort basis,
     * throw an {@code IllegalStateException} if it is detected that the
     * remapping function modifies this map during computation and as a result
     * computation would never complete.
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the remapping function is applied once atomically only if the
     * value is not present.
     *
     * @param key key with which the specified value is to be associated
     * @param remappingFunction the remapping function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the
     *         remappingFunction is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V computeIfPresent(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue;
        if ((oldValue = get(key)) != null) {
            V newValue = remappingFunction.apply(key, oldValue);
            if (newValue != null) {
                put(key, newValue);
                return newValue;
            } else {
                remove(key);
                return null;
            }
        } else {
            return null;
        }
    }

    
Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). For example, to either create or append a String msg to a value mapping: 
 
map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
 (Method merge() is often simpler to use for such purposes.) 
If the remapping function returns null, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged. 
The remapping function should not modify this map during computation.

Params:
  • key – key with which the specified value is to be associated
  • remappingFunction – the remapping function to compute a value
Throws:
Implementation Requirements: The default implementation is equivalent to performing the following steps for this map, then returning the current value or null if absent: 
 
V oldValue = map.get(key);
V newValue = remappingFunction.apply(key, oldValue);
if (oldValue != null) {
   if (newValue != null)
      map.put(key, newValue);
   else
      map.remove(key);
 } else {
   if (newValue != null)
      map.put(key, newValue);
   else
      return null;
 }



The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the remapping function modifies this map during computation and as a result computation would never complete. 
The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the remapping function is applied once atomically only if the value is not present.
Returns:the new value associated with the specified key, or null if none
Since:1.8
/**
     * Attempts to compute a mapping for the specified key and its current
     * mapped value (or {@code null} if there is no current mapping). For
     * example, to either create or append a {@code String} msg to a value
     * mapping:
     *
     * <pre> {@code
     * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
     * (Method {@link #merge merge()} is often simpler to use for such purposes.)
     *
     * <p>If the remapping function returns {@code null}, the mapping is removed
     * (or remains absent if initially absent).  If the remapping function
     * itself throws an (unchecked) exception, the exception is rethrown, and
     * the current mapping is left unchanged.
     *
     * <p>The remapping function should not modify this map during computation.
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * <pre> {@code
     * V oldValue = map.get(key);
     * V newValue = remappingFunction.apply(key, oldValue);
     * if (oldValue != null) {
     *    if (newValue != null)
     *       map.put(key, newValue);
     *    else
     *       map.remove(key);
     * } else {
     *    if (newValue != null)
     *       map.put(key, newValue);
     *    else
     *       return null;
     * }
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about detecting if the
     * remapping function modifies this map during computation and, if
     * appropriate, reporting an error. Non-concurrent implementations should
     * override this method and, on a best-effort basis, throw a
     * {@code ConcurrentModificationException} if it is detected that the
     * remapping function modifies this map during computation. Concurrent
     * implementations should override this method and, on a best-effort basis,
     * throw an {@code IllegalStateException} if it is detected that the
     * remapping function modifies this map during computation and as a result
     * computation would never complete.
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the remapping function is applied once atomically only if the
     * value is not present.
     *
     * @param key key with which the specified value is to be associated
     * @param remappingFunction the remapping function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the
     *         remappingFunction is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V compute(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue = get(key);

        V newValue = remappingFunction.apply(key, oldValue);
        if (newValue == null) {
            // delete mapping
            if (oldValue != null || containsKey(key)) {
                // something to remove
                remove(key);
                return null;
            } else {
                // nothing to do. Leave things as they were.
                return null;
            }
        } else {
            // add or replace old mapping
            put(key, newValue);
            return newValue;
        }
    }

    
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is null. This method may be of use when combining multiple mapped values for a key. For example, to either create or append a String msg to a value mapping: 
 
map.merge(key, msg, String::concat)



If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged. 
The remapping function should not modify this map during computation.

Params:
  • key – key with which the resulting value is to be associated
  • value – the non-null value to be merged with the existing value
       associated with the key or, if no existing value or a null value
       is associated with the key, to be associated with the key
  • remappingFunction – the remapping function to recompute a value if
       present
Throws:
Implementation Requirements: The default implementation is equivalent to performing the following steps for this map, then returning the current value or null if absent: 
 
V oldValue = map.get(key);
V newValue = (oldValue == null) ? value :
             remappingFunction.apply(oldValue, value);
if (newValue == null)
    map.remove(key);
else
    map.put(key, newValue);



The default implementation makes no guarantees about detecting if the remapping function modifies this map during computation and, if appropriate, reporting an error. Non-concurrent implementations should override this method and, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the remapping function modifies this map during computation and as a result computation would never complete. 
The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface ConcurrentMap must document whether the remapping function is applied once atomically only if the value is not present.
Returns:the new value associated with the specified key, or null if no
        value is associated with the key
Since:1.8
/**
     * If the specified key is not already associated with a value or is
     * associated with null, associates it with the given non-null value.
     * Otherwise, replaces the associated value with the results of the given
     * remapping function, or removes if the result is {@code null}. This
     * method may be of use when combining multiple mapped values for a key.
     * For example, to either create or append a {@code String msg} to a
     * value mapping:
     *
     * <pre> {@code
     * map.merge(key, msg, String::concat)
     * }</pre>
     *
     * <p>If the remapping function returns {@code null}, the mapping is removed.
     * If the remapping function itself throws an (unchecked) exception, the
     * exception is rethrown, and the current mapping is left unchanged.
     *
     * <p>The remapping function should not modify this map during computation.
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * <pre> {@code
     * V oldValue = map.get(key);
     * V newValue = (oldValue == null) ? value :
     *              remappingFunction.apply(oldValue, value);
     * if (newValue == null)
     *     map.remove(key);
     * else
     *     map.put(key, newValue);
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about detecting if the
     * remapping function modifies this map during computation and, if
     * appropriate, reporting an error. Non-concurrent implementations should
     * override this method and, on a best-effort basis, throw a
     * {@code ConcurrentModificationException} if it is detected that the
     * remapping function modifies this map during computation. Concurrent
     * implementations should override this method and, on a best-effort basis,
     * throw an {@code IllegalStateException} if it is detected that the
     * remapping function modifies this map during computation and as a result
     * computation would never complete.
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the remapping function is applied once atomically only if the
     * value is not present.
     *
     * @param key key with which the resulting value is to be associated
     * @param value the non-null value to be merged with the existing value
     *        associated with the key or, if no existing value or a null value
     *        is associated with the key, to be associated with the key
     * @param remappingFunction the remapping function to recompute a value if
     *        present
     * @return the new value associated with the specified key, or null if no
     *         value is associated with the key
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key is null and this map
     *         does not support null keys or the value or remappingFunction is
     *         null
     * @since 1.8
     */
    default V merge(K key, V value,
            BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        Objects.requireNonNull(value);
        V oldValue = get(key);
        V newValue = (oldValue == null) ? value :
                   remappingFunction.apply(oldValue, value);
        if (newValue == null) {
            remove(key);
        } else {
            put(key, newValue);
        }
        return newValue;
    }

    
Returns an unmodifiable map containing zero mappings.
See Unmodifiable Maps for details.

Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Returns:an empty Map
Since:9
/**
     * Returns an unmodifiable map containing zero mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @return an empty {@code Map}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of() {
        return ImmutableCollections.emptyMap();
    }

    
Returns an unmodifiable map containing a single mapping.
See Unmodifiable Maps for details.

Params:
  • k1 – the mapping's key
  • v1 – the mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mapping
Since:9
/**
     * Returns an unmodifiable map containing a single mapping.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the mapping's key
     * @param v1 the mapping's value
     * @return a {@code Map} containing the specified mapping
     * @throws NullPointerException if the key or the value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1) {
        return new ImmutableCollections.Map1<>(k1, v1);
    }

    
Returns an unmodifiable map containing two mappings.
See Unmodifiable Maps for details.

Params:
  • k1 – the first mapping's key
  • v1 – the first mapping's value
  • k2 – the second mapping's key
  • v2 – the second mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing two mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the first mapping's key
     * @param v1 the first mapping's value
     * @param k2 the second mapping's key
     * @param v2 the second mapping's value
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if the keys are duplicates
     * @throws NullPointerException if any key or value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2) {
        return new ImmutableCollections.MapN<>(k1, v1, k2, v2);
    }

    
Returns an unmodifiable map containing three mappings.
See Unmodifiable Maps for details.

Params:
  • k1 – the first mapping's key
  • v1 – the first mapping's value
  • k2 – the second mapping's key
  • v2 – the second mapping's value
  • k3 – the third mapping's key
  • v3 – the third mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing three mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the first mapping's key
     * @param v1 the first mapping's value
     * @param k2 the second mapping's key
     * @param v2 the second mapping's value
     * @param k3 the third mapping's key
     * @param v3 the third mapping's value
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if there are any duplicate keys
     * @throws NullPointerException if any key or value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) {
        return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3);
    }

    
Returns an unmodifiable map containing four mappings.
See Unmodifiable Maps for details.

Params:
  • k1 – the first mapping's key
  • v1 – the first mapping's value
  • k2 – the second mapping's key
  • v2 – the second mapping's value
  • k3 – the third mapping's key
  • v3 – the third mapping's value
  • k4 – the fourth mapping's key
  • v4 – the fourth mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing four mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the first mapping's key
     * @param v1 the first mapping's value
     * @param k2 the second mapping's key
     * @param v2 the second mapping's value
     * @param k3 the third mapping's key
     * @param v3 the third mapping's value
     * @param k4 the fourth mapping's key
     * @param v4 the fourth mapping's value
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if there are any duplicate keys
     * @throws NullPointerException if any key or value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) {
        return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4);
    }

    
Returns an unmodifiable map containing five mappings.
See Unmodifiable Maps for details.

Params:
  • k1 – the first mapping's key
  • v1 – the first mapping's value
  • k2 – the second mapping's key
  • v2 – the second mapping's value
  • k3 – the third mapping's key
  • v3 – the third mapping's value
  • k4 – the fourth mapping's key
  • v4 – the fourth mapping's value
  • k5 – the fifth mapping's key
  • v5 – the fifth mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing five mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the first mapping's key
     * @param v1 the first mapping's value
     * @param k2 the second mapping's key
     * @param v2 the second mapping's value
     * @param k3 the third mapping's key
     * @param v3 the third mapping's value
     * @param k4 the fourth mapping's key
     * @param v4 the fourth mapping's value
     * @param k5 the fifth mapping's key
     * @param v5 the fifth mapping's value
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if there are any duplicate keys
     * @throws NullPointerException if any key or value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) {
        return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5);
    }

    
Returns an unmodifiable map containing six mappings.
See Unmodifiable Maps for details.

Params:
  • k1 – the first mapping's key
  • v1 – the first mapping's value
  • k2 – the second mapping's key
  • v2 – the second mapping's value
  • k3 – the third mapping's key
  • v3 – the third mapping's value
  • k4 – the fourth mapping's key
  • v4 – the fourth mapping's value
  • k5 – the fifth mapping's key
  • v5 – the fifth mapping's value
  • k6 – the sixth mapping's key
  • v6 – the sixth mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing six mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the first mapping's key
     * @param v1 the first mapping's value
     * @param k2 the second mapping's key
     * @param v2 the second mapping's value
     * @param k3 the third mapping's key
     * @param v3 the third mapping's value
     * @param k4 the fourth mapping's key
     * @param v4 the fourth mapping's value
     * @param k5 the fifth mapping's key
     * @param v5 the fifth mapping's value
     * @param k6 the sixth mapping's key
     * @param v6 the sixth mapping's value
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if there are any duplicate keys
     * @throws NullPointerException if any key or value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                               K k6, V v6) {
        return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                               k6, v6);
    }

    
Returns an unmodifiable map containing seven mappings.
See Unmodifiable Maps for details.

Params:
  • k1 – the first mapping's key
  • v1 – the first mapping's value
  • k2 – the second mapping's key
  • v2 – the second mapping's value
  • k3 – the third mapping's key
  • v3 – the third mapping's value
  • k4 – the fourth mapping's key
  • v4 – the fourth mapping's value
  • k5 – the fifth mapping's key
  • v5 – the fifth mapping's value
  • k6 – the sixth mapping's key
  • v6 – the sixth mapping's value
  • k7 – the seventh mapping's key
  • v7 – the seventh mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing seven mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the first mapping's key
     * @param v1 the first mapping's value
     * @param k2 the second mapping's key
     * @param v2 the second mapping's value
     * @param k3 the third mapping's key
     * @param v3 the third mapping's value
     * @param k4 the fourth mapping's key
     * @param v4 the fourth mapping's value
     * @param k5 the fifth mapping's key
     * @param v5 the fifth mapping's value
     * @param k6 the sixth mapping's key
     * @param v6 the sixth mapping's value
     * @param k7 the seventh mapping's key
     * @param v7 the seventh mapping's value
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if there are any duplicate keys
     * @throws NullPointerException if any key or value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                               K k6, V v6, K k7, V v7) {
        return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                               k6, v6, k7, v7);
    }

    
Returns an unmodifiable map containing eight mappings.
See Unmodifiable Maps for details.

Params:
  • k1 – the first mapping's key
  • v1 – the first mapping's value
  • k2 – the second mapping's key
  • v2 – the second mapping's value
  • k3 – the third mapping's key
  • v3 – the third mapping's value
  • k4 – the fourth mapping's key
  • v4 – the fourth mapping's value
  • k5 – the fifth mapping's key
  • v5 – the fifth mapping's value
  • k6 – the sixth mapping's key
  • v6 – the sixth mapping's value
  • k7 – the seventh mapping's key
  • v7 – the seventh mapping's value
  • k8 – the eighth mapping's key
  • v8 – the eighth mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing eight mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the first mapping's key
     * @param v1 the first mapping's value
     * @param k2 the second mapping's key
     * @param v2 the second mapping's value
     * @param k3 the third mapping's key
     * @param v3 the third mapping's value
     * @param k4 the fourth mapping's key
     * @param v4 the fourth mapping's value
     * @param k5 the fifth mapping's key
     * @param v5 the fifth mapping's value
     * @param k6 the sixth mapping's key
     * @param v6 the sixth mapping's value
     * @param k7 the seventh mapping's key
     * @param v7 the seventh mapping's value
     * @param k8 the eighth mapping's key
     * @param v8 the eighth mapping's value
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if there are any duplicate keys
     * @throws NullPointerException if any key or value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                               K k6, V v6, K k7, V v7, K k8, V v8) {
        return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                               k6, v6, k7, v7, k8, v8);
    }

    
Returns an unmodifiable map containing nine mappings.
See Unmodifiable Maps for details.

Params:
  • k1 – the first mapping's key
  • v1 – the first mapping's value
  • k2 – the second mapping's key
  • v2 – the second mapping's value
  • k3 – the third mapping's key
  • v3 – the third mapping's value
  • k4 – the fourth mapping's key
  • v4 – the fourth mapping's value
  • k5 – the fifth mapping's key
  • v5 – the fifth mapping's value
  • k6 – the sixth mapping's key
  • v6 – the sixth mapping's value
  • k7 – the seventh mapping's key
  • v7 – the seventh mapping's value
  • k8 – the eighth mapping's key
  • v8 – the eighth mapping's value
  • k9 – the ninth mapping's key
  • v9 – the ninth mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing nine mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the first mapping's key
     * @param v1 the first mapping's value
     * @param k2 the second mapping's key
     * @param v2 the second mapping's value
     * @param k3 the third mapping's key
     * @param v3 the third mapping's value
     * @param k4 the fourth mapping's key
     * @param v4 the fourth mapping's value
     * @param k5 the fifth mapping's key
     * @param v5 the fifth mapping's value
     * @param k6 the sixth mapping's key
     * @param v6 the sixth mapping's value
     * @param k7 the seventh mapping's key
     * @param v7 the seventh mapping's value
     * @param k8 the eighth mapping's key
     * @param v8 the eighth mapping's value
     * @param k9 the ninth mapping's key
     * @param v9 the ninth mapping's value
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if there are any duplicate keys
     * @throws NullPointerException if any key or value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                               K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9) {
        return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                               k6, v6, k7, v7, k8, v8, k9, v9);
    }

    
Returns an unmodifiable map containing ten mappings.
See Unmodifiable Maps for details.

Params:
  • k1 – the first mapping's key
  • v1 – the first mapping's value
  • k2 – the second mapping's key
  • v2 – the second mapping's value
  • k3 – the third mapping's key
  • v3 – the third mapping's value
  • k4 – the fourth mapping's key
  • v4 – the fourth mapping's value
  • k5 – the fifth mapping's key
  • v5 – the fifth mapping's value
  • k6 – the sixth mapping's key
  • v6 – the sixth mapping's value
  • k7 – the seventh mapping's key
  • v7 – the seventh mapping's value
  • k8 – the eighth mapping's key
  • v8 – the eighth mapping's value
  • k9 – the ninth mapping's key
  • v9 – the ninth mapping's value
  • k10 – the tenth mapping's key
  • v10 – the tenth mapping's value
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing ten mappings.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param k1 the first mapping's key
     * @param v1 the first mapping's value
     * @param k2 the second mapping's key
     * @param v2 the second mapping's value
     * @param k3 the third mapping's key
     * @param v3 the third mapping's value
     * @param k4 the fourth mapping's key
     * @param v4 the fourth mapping's value
     * @param k5 the fifth mapping's key
     * @param v5 the fifth mapping's value
     * @param k6 the sixth mapping's key
     * @param v6 the sixth mapping's value
     * @param k7 the seventh mapping's key
     * @param v7 the seventh mapping's value
     * @param k8 the eighth mapping's key
     * @param v8 the eighth mapping's value
     * @param k9 the ninth mapping's key
     * @param v9 the ninth mapping's value
     * @param k10 the tenth mapping's key
     * @param v10 the tenth mapping's value
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if there are any duplicate keys
     * @throws NullPointerException if any key or value is {@code null}
     *
     * @since 9
     */
    static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                               K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10) {
        return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                               k6, v6, k7, v7, k8, v8, k9, v9, k10, v10);
    }

    
Returns an unmodifiable map containing keys and values extracted from the given entries.
The entries themselves are not stored in the map.
See Unmodifiable Maps for details.

Params:
  • entries – Map.Entrys containing the keys and values from which the map is populated
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
See Also:
API Note: It is convenient to create the map entries using the Map.entry() method. For example, 

    import static java.util.Map.entry;
    Map<Integer,String> map = Map.ofEntries(
        entry(1, "a"),
        entry(2, "b"),
        entry(3, "c"),
        ...
        entry(26, "z"));

Returns:a Map containing the specified mappings
Since:9
/**
     * Returns an unmodifiable map containing keys and values extracted from the given entries.
     * The entries themselves are not stored in the map.
     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
     *
     * @apiNote
     * It is convenient to create the map entries using the {@link Map#entry Map.entry()} method.
     * For example,
     *
     * <pre>{@code
     *     import static java.util.Map.entry;
     *
     *     Map<Integer,String> map = Map.ofEntries(
     *         entry(1, "a"),
     *         entry(2, "b"),
     *         entry(3, "c"),
     *         ...
     *         entry(26, "z"));
     * }</pre>
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param entries {@code Map.Entry}s containing the keys and values from which the map is populated
     * @return a {@code Map} containing the specified mappings
     * @throws IllegalArgumentException if there are any duplicate keys
     * @throws NullPointerException if any entry, key, or value is {@code null}, or if
     *         the {@code entries} array is {@code null}
     *
     * @see Map#entry Map.entry()
     * @since 9
     */
    @SafeVarargs
    @SuppressWarnings("varargs")
    static <K, V> Map<K, V> ofEntries(Entry<? extends K, ? extends V>... entries) {
        if (entries.length == 0) { // implicit null check of entries array
            return ImmutableCollections.emptyMap();
        } else if (entries.length == 1) {
            // implicit null check of the array slot
            return new ImmutableCollections.Map1<>(entries[0].getKey(),
                    entries[0].getValue());
        } else {
            Object[] kva = new Object[entries.length << 1];
            int a = 0;
            for (Entry<? extends K, ? extends V> entry : entries) {
                // implicit null checks of each array slot
                kva[a++] = entry.getKey();
                kva[a++] = entry.getValue();
            }
            return new ImmutableCollections.MapN<>(kva);
        }
    }

    
Returns an unmodifiable Entry containing the given key and value. These entries are suitable for populating Map instances using the Map.ofEntries() method. The Entry instances created by this method have the following characteristics: 
    

  • They disallow null keys and values. Attempts to create them using a null key or value result in NullPointerException. 
  • They are unmodifiable. Calls to Entry.setValue() on a returned Entry result in UnsupportedOperationException. 
  • They are not serializable.

  • They are value-based. Callers should make no assumptions about the identity of the returned instances. This method is free to create new instances or reuse existing ones. Therefore, identity-sensitive operations on these instances (reference equality (==), identity hash code, and synchronization) are unreliable and should be avoided. 


Params:
  • k – the key
  • v – the value
Type parameters:
  • <K> – the key's type
  • <V> – the value's type
Throws:
See Also:
API Note: For a serializable Entry, see SimpleEntry or SimpleImmutableEntry.
Returns:an Entry containing the specified key and value
Since:9
/**
     * Returns an unmodifiable {@link Entry} containing the given key and value.
     * These entries are suitable for populating {@code Map} instances using the
     * {@link Map#ofEntries Map.ofEntries()} method.
     * The {@code Entry} instances created by this method have the following characteristics:
     *
     * <ul>
     * <li>They disallow {@code null} keys and values. Attempts to create them using a {@code null}
     * key or value result in {@code NullPointerException}.
     * <li>They are unmodifiable. Calls to {@link Entry#setValue Entry.setValue()}
     * on a returned {@code Entry} result in {@code UnsupportedOperationException}.
     * <li>They are not serializable.
     * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
     * Callers should make no assumptions about the identity of the returned instances.
     * This method is free to create new instances or reuse existing ones. Therefore,
     * identity-sensitive operations on these instances (reference equality ({@code ==}),
     * identity hash code, and synchronization) are unreliable and should be avoided.
     * </ul>
     *
     * @apiNote
     * For a serializable {@code Entry}, see {@link AbstractMap.SimpleEntry} or
     * {@link AbstractMap.SimpleImmutableEntry}.
     *
     * @param <K> the key's type
     * @param <V> the value's type
     * @param k the key
     * @param v the value
     * @return an {@code Entry} containing the specified key and value
     * @throws NullPointerException if the key or value is {@code null}
     *
     * @see Map#ofEntries Map.ofEntries()
     * @since 9
     */
    static <K, V> Entry<K, V> entry(K k, V v) {
        // KeyValueHolder checks for nulls
        return new KeyValueHolder<>(k, v);
    }

    
Returns an unmodifiable Map containing the entries
of the given Map. The given Map must not be null, and it must not contain any
null keys or values. If the given Map is subsequently modified, the returned
Map will not reflect such modifications.

Params:
  • map – a Map from which entries are drawn, must be non-null
Type parameters:
  • <K> – the Map's key type
  • <V> – the Map's value type
Throws:
Implementation Note:
If the given Map is an unmodifiable Map,
calling copyOf will generally not create a copy.
Returns:a Map containing the entries of the given Map
Since:10
/**
     * Returns an <a href="#unmodifiable">unmodifiable Map</a> containing the entries
     * of the given Map. The given Map must not be null, and it must not contain any
     * null keys or values. If the given Map is subsequently modified, the returned
     * Map will not reflect such modifications.
     *
     * @implNote
     * If the given Map is an <a href="#unmodifiable">unmodifiable Map</a>,
     * calling copyOf will generally not create a copy.
     *
     * @param <K> the {@code Map}'s key type
     * @param <V> the {@code Map}'s value type
     * @param map a {@code Map} from which entries are drawn, must be non-null
     * @return a {@code Map} containing the entries of the given {@code Map}
     * @throws NullPointerException if map is null, or if it contains any null keys or values
     * @since 10
     */
    @SuppressWarnings({"rawtypes","unchecked"})
    static <K, V> Map<K, V> copyOf(Map<? extends K, ? extends V> map) {
        if (map instanceof ImmutableCollections.AbstractImmutableMap) {
            return (Map<K,V>)map;
        } else {
            return (Map<K,V>)Map.ofEntries(map.entrySet().toArray(new Entry[0]));
        }
    }
}