https://openjdk.java.net/ 
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
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package java.util;

import jdk.internal.access.SharedSecrets;


A specialized Map implementation for use with enum type keys. All of the keys in an enum map must come from a single enum type that is specified, explicitly or implicitly, when the map is created. Enum maps are represented internally as arrays. This representation is extremely compact and efficient. 
Enum maps are maintained in the natural order of their keys (the order in which the enum constants are declared). This is reflected in the iterators returned by the collections views (keySet(), entrySet(), and values()). 
Iterators returned by the collection views are weakly consistent: they will never throw ConcurrentModificationException and they may or may not show the effects of any modifications to the map that occur while the iteration is in progress. 
Null keys are not permitted. Attempts to insert a null key will throw NullPointerException. Attempts to test for the presence of a null key or to remove one will, however, function properly. Null values are permitted. 
Like most collection implementations EnumMap is not synchronized. If multiple threads access an enum map concurrently, and at least one of the threads modifies the map, it should be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the enum map. If no such object exists, the map should be "wrapped" using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access: 
    Map<EnumKey, V> m
        = Collections.synchronizedMap(new EnumMap<EnumKey, V>(...));



Implementation note: All basic operations execute in constant time. They are likely (though not guaranteed) to be faster than their HashMap counterparts. 
This class is a member of the

Java Collections Framework.

Author:Josh Bloch
See Also:
Since:1.5
/**
 * A specialized {@link Map} implementation for use with enum type keys.  All
 * of the keys in an enum map must come from a single enum type that is
 * specified, explicitly or implicitly, when the map is created.  Enum maps
 * are represented internally as arrays.  This representation is extremely
 * compact and efficient.
 *
 * <p>Enum maps are maintained in the <i>natural order</i> of their keys
 * (the order in which the enum constants are declared).  This is reflected
 * in the iterators returned by the collections views ({@link #keySet()},
 * {@link #entrySet()}, and {@link #values()}).
 *
 * <p>Iterators returned by the collection views are <i>weakly consistent</i>:
 * they will never throw {@link ConcurrentModificationException} and they may
 * or may not show the effects of any modifications to the map that occur while
 * the iteration is in progress.
 *
 * <p>Null keys are not permitted.  Attempts to insert a null key will
 * throw {@link NullPointerException}.  Attempts to test for the
 * presence of a null key or to remove one will, however, function properly.
 * Null values are permitted.
 *
 * <P>Like most collection implementations {@code EnumMap} is not
 * synchronized. If multiple threads access an enum map concurrently, and at
 * least one of the threads modifies the map, it should be synchronized
 * externally.  This is typically accomplished by synchronizing on some
 * object that naturally encapsulates the enum map.  If no such object exists,
 * the map should be "wrapped" using the {@link Collections#synchronizedMap}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access:
 *
 * <pre>
 *     Map&lt;EnumKey, V&gt; m
 *         = Collections.synchronizedMap(new EnumMap&lt;EnumKey, V&gt;(...));
 * </pre>
 *
 * <p>Implementation note: All basic operations execute in constant time.
 * They are likely (though not guaranteed) to be faster than their
 * {@link HashMap} counterparts.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @author Josh Bloch
 * @see EnumSet
 * @since 1.5
 */
public class EnumMap<K extends Enum<K>, V> extends AbstractMap<K, V>
    implements java.io.Serializable, Cloneable
{
    
The Class object for the enum type of all the keys of this map. 
@serial
/**
     * The {@code Class} object for the enum type of all the keys of this map.
     *
     * @serial
     */
    private final Class<K> keyType;

    
All of the values comprising K.  (Cached for performance.)

/**
     * All of the values comprising K.  (Cached for performance.)
     */
    private transient K[] keyUniverse;

    
Array representation of this map.  The ith element is the value
to which universe[i] is currently mapped, or null if it isn't
mapped to anything, or NULL if it's mapped to null.

/**
     * Array representation of this map.  The ith element is the value
     * to which universe[i] is currently mapped, or null if it isn't
     * mapped to anything, or NULL if it's mapped to null.
     */
    private transient Object[] vals;

    
The number of mappings in this map.

/**
     * The number of mappings in this map.
     */
    private transient int size = 0;

    
Distinguished non-null value for representing null values.

/**
     * Distinguished non-null value for representing null values.
     */
    private static final Object NULL = new Object() {
        public int hashCode() {
            return 0;
        }

        public String toString() {
            return "java.util.EnumMap.NULL";
        }
    };

    private Object maskNull(Object value) {
        return (value == null ? NULL : value);
    }

    @SuppressWarnings("unchecked")
    private V unmaskNull(Object value) {
        return (V)(value == NULL ? null : value);
    }

    
Creates an empty enum map with the specified key type.

Params:
  • keyType – the class object of the key type for this enum map
Throws:
/**
     * Creates an empty enum map with the specified key type.
     *
     * @param keyType the class object of the key type for this enum map
     * @throws NullPointerException if {@code keyType} is null
     */
    public EnumMap(Class<K> keyType) {
        this.keyType = keyType;
        keyUniverse = getKeyUniverse(keyType);
        vals = new Object[keyUniverse.length];
    }

    
Creates an enum map with the same key type as the specified enum
map, initially containing the same mappings (if any).

Params:
  • m – the enum map from which to initialize this enum map
Throws:
/**
     * Creates an enum map with the same key type as the specified enum
     * map, initially containing the same mappings (if any).
     *
     * @param m the enum map from which to initialize this enum map
     * @throws NullPointerException if {@code m} is null
     */
    public EnumMap(EnumMap<K, ? extends V> m) {
        keyType = m.keyType;
        keyUniverse = m.keyUniverse;
        vals = m.vals.clone();
        size = m.size;
    }

    
Creates an enum map initialized from the specified map. If the specified map is an EnumMap instance, this constructor behaves identically to EnumMap(EnumMap). Otherwise, the specified map must contain at least one mapping (in order to determine the new enum map's key type). 
Params:
  • m – the map from which to initialize this enum map
Throws:
/**
     * Creates an enum map initialized from the specified map.  If the
     * specified map is an {@code EnumMap} instance, this constructor behaves
     * identically to {@link #EnumMap(EnumMap)}.  Otherwise, the specified map
     * must contain at least one mapping (in order to determine the new
     * enum map's key type).
     *
     * @param m the map from which to initialize this enum map
     * @throws IllegalArgumentException if {@code m} is not an
     *     {@code EnumMap} instance and contains no mappings
     * @throws NullPointerException if {@code m} is null
     */
    public EnumMap(Map<K, ? extends V> m) {
        if (m instanceof EnumMap) {
            EnumMap<K, ? extends V> em = (EnumMap<K, ? extends V>) m;
            keyType = em.keyType;
            keyUniverse = em.keyUniverse;
            vals = em.vals.clone();
            size = em.size;
        } else {
            if (m.isEmpty())
                throw new IllegalArgumentException("Specified map is empty");
            keyType = m.keySet().iterator().next().getDeclaringClass();
            keyUniverse = getKeyUniverse(keyType);
            vals = new Object[keyUniverse.length];
            putAll(m);
        }
    }

    // Query Operations

    
Returns the number of key-value mappings in this map.

Returns:the number of key-value mappings in this map
/**
     * Returns the number of key-value mappings in this map.
     *
     * @return the number of key-value mappings in this map
     */
    public int size() {
        return size;
    }

    
Returns true if this map maps one or more keys to the specified value. 
Params:
  • value – the value whose presence in this map is to be tested
Returns:true if this map maps one or more keys to this value
/**
     * Returns {@code true} if this map maps one or more keys to the
     * specified value.
     *
     * @param value the value whose presence in this map is to be tested
     * @return {@code true} if this map maps one or more keys to this value
     */
    public boolean containsValue(Object value) {
        value = maskNull(value);

        for (Object val : vals)
            if (value.equals(val))
                return true;

        return false;
    }

    
Returns true if this map contains a mapping for the specified key. 
Params:
  • key – the key whose presence in this map is to be tested
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.
     *
     * @param key the key whose presence in this map is to be tested
     * @return {@code true} if this map contains a mapping for the specified
     *            key
     */
    public boolean containsKey(Object key) {
        return isValidKey(key) && vals[((Enum<?>)key).ordinal()] != null;
    }

    private boolean containsMapping(Object key, Object value) {
        return isValidKey(key) &&
            maskNull(value).equals(vals[((Enum<?>)key).ordinal()]);
    }

    
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 (key == k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.) 
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. 
/**
     * 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 (key == k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * <p>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.
     */
    public V get(Object key) {
        return (isValidKey(key) ?
                unmaskNull(vals[((Enum<?>)key).ordinal()]) : null);
    }

    // Modification Operations

    
Associates the specified value with the specified key in this map.
If the map previously contained a mapping for this key, the old
value is replaced.

Params:
  • key – the key with which the specified value is to be associated
  • value – the value to be associated with the specified key
Throws:
Returns:the previous value associated with specified key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with the specified key.)
/**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for this key, the old
     * value is replaced.
     *
     * @param key the key with which the specified value is to be associated
     * @param value the value to be associated with the specified key
     *
     * @return the previous value associated with specified key, or
     *     {@code null} if there was no mapping for key.  (A {@code null}
     *     return can also indicate that the map previously associated
     *     {@code null} with the specified key.)
     * @throws NullPointerException if the specified key is null
     */
    public V put(K key, V value) {
        typeCheck(key);

        int index = key.ordinal();
        Object oldValue = vals[index];
        vals[index] = maskNull(value);
        if (oldValue == null)
            size++;
        return unmaskNull(oldValue);
    }

    
Removes the mapping for this key from this map if present.

Params:
  • key – the key whose mapping is to be removed from the map
Returns:the previous value associated with specified key, or null if there was no entry for key. (A null return can also indicate that the map previously associated null with the specified key.)
/**
     * Removes the mapping for this key from this map if present.
     *
     * @param key the key whose mapping is to be removed from the map
     * @return the previous value associated with specified key, or
     *     {@code null} if there was no entry for key.  (A {@code null}
     *     return can also indicate that the map previously associated
     *     {@code null} with the specified key.)
     */
    public V remove(Object key) {
        if (!isValidKey(key))
            return null;
        int index = ((Enum<?>)key).ordinal();
        Object oldValue = vals[index];
        vals[index] = null;
        if (oldValue != null)
            size--;
        return unmaskNull(oldValue);
    }

    private boolean removeMapping(Object key, Object value) {
        if (!isValidKey(key))
            return false;
        int index = ((Enum<?>)key).ordinal();
        if (maskNull(value).equals(vals[index])) {
            vals[index] = null;
            size--;
            return true;
        }
        return false;
    }

    
Returns true if key is of the proper type to be a key in this
enum map.

/**
     * Returns true if key is of the proper type to be a key in this
     * enum map.
     */
    private boolean isValidKey(Object key) {
        if (key == null)
            return false;

        // Cheaper than instanceof Enum followed by getDeclaringClass
        Class<?> keyClass = key.getClass();
        return keyClass == keyType || keyClass.getSuperclass() == keyType;
    }

    // Bulk Operations

    
Copies all of the mappings from the specified map to this map.
These mappings will replace any mappings that this map had for
any of the keys currently in the specified map.

Params:
  • m – the mappings to be stored in this map
Throws:
  • NullPointerException – the specified map is null, or if
    one or more keys in the specified map are null
/**
     * Copies all of the mappings from the specified map to this map.
     * These mappings will replace any mappings that this map had for
     * any of the keys currently in the specified map.
     *
     * @param m the mappings to be stored in this map
     * @throws NullPointerException the specified map is null, or if
     *     one or more keys in the specified map are null
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        if (m instanceof EnumMap) {
            EnumMap<?, ?> em = (EnumMap<?, ?>)m;
            if (em.keyType != keyType) {
                if (em.isEmpty())
                    return;
                throw new ClassCastException(em.keyType + " != " + keyType);
            }

            for (int i = 0; i < keyUniverse.length; i++) {
                Object emValue = em.vals[i];
                if (emValue != null) {
                    if (vals[i] == null)
                        size++;
                    vals[i] = emValue;
                }
            }
        } else {
            super.putAll(m);
        }
    }

    
Removes all mappings from this map.

/**
     * Removes all mappings from this map.
     */
    public void clear() {
        Arrays.fill(vals, null);
        size = 0;
    }

    // Views

    
This field is initialized to contain an instance of the entry set
view the first time this view is requested.  The view is stateless,
so there's no reason to create more than one.

/**
     * This field is initialized to contain an instance of the entry set
     * view the first time this view is requested.  The view is stateless,
     * so there's no reason to create more than one.
     */
    private transient Set<Map.Entry<K,V>> entrySet;

    
Returns a Set view of the keys contained in this map. The returned set obeys the general contract outlined in Map.keySet(). The set's iterator will return the keys in their natural order (the order in which the enum constants are declared). 
Returns:a set view of the keys contained in this enum map
/**
     * Returns a {@link Set} view of the keys contained in this map.
     * The returned set obeys the general contract outlined in
     * {@link Map#keySet()}.  The set's iterator will return the keys
     * in their natural order (the order in which the enum constants
     * are declared).
     *
     * @return a set view of the keys contained in this enum map
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks == null) {
            ks = new KeySet();
            keySet = ks;
        }
        return ks;
    }

    private class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return new KeyIterator();
        }
        public int size() {
            return size;
        }
        public boolean contains(Object o) {
            return containsKey(o);
        }
        public boolean remove(Object o) {
            int oldSize = size;
            EnumMap.this.remove(o);
            return size != oldSize;
        }
        public void clear() {
            EnumMap.this.clear();
        }
    }

    
Returns a Collection view of the values contained in this map. The returned collection obeys the general contract outlined in Map.values(). The collection's iterator will return the values in the order their corresponding keys appear in map, which is their natural order (the order in which the enum constants are declared). 
Returns:a collection view of the values contained in this map
/**
     * Returns a {@link Collection} view of the values contained in this map.
     * The returned collection obeys the general contract outlined in
     * {@link Map#values()}.  The collection's iterator will return the
     * values in the order their corresponding keys appear in map,
     * which is their natural order (the order in which the enum constants
     * are declared).
     *
     * @return a collection view of the values contained in this map
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return vs;
    }

    private class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator();
        }
        public int size() {
            return size;
        }
        public boolean contains(Object o) {
            return containsValue(o);
        }
        public boolean remove(Object o) {
            o = maskNull(o);

            for (int i = 0; i < vals.length; i++) {
                if (o.equals(vals[i])) {
                    vals[i] = null;
                    size--;
                    return true;
                }
            }
            return false;
        }
        public void clear() {
            EnumMap.this.clear();
        }
    }

    
Returns a Set view of the mappings contained in this map. The returned set obeys the general contract outlined in Map.keySet(). The set's iterator will return the mappings in the order their keys appear in map, which is their natural order (the order in which the enum constants are declared). 
Returns:a set view of the mappings contained in this enum map
/**
     * Returns a {@link Set} view of the mappings contained in this map.
     * The returned set obeys the general contract outlined in
     * {@link Map#keySet()}.  The set's iterator will return the
     * mappings in the order their keys appear in map, which is their
     * natural order (the order in which the enum constants are declared).
     *
     * @return a set view of the mappings contained in this enum map
     */
    public Set<Map.Entry<K,V>> entrySet() {
        Set<Map.Entry<K,V>> es = entrySet;
        if (es != null)
            return es;
        else
            return entrySet = new EntrySet();
    }

    private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        public Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator();
        }

        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
            return containsMapping(entry.getKey(), entry.getValue());
        }
        public boolean remove(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
            return removeMapping(entry.getKey(), entry.getValue());
        }
        public int size() {
            return size;
        }
        public void clear() {
            EnumMap.this.clear();
        }
        public Object[] toArray() {
            return fillEntryArray(new Object[size]);
        }
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            int size = size();
            if (a.length < size)
                a = (T[])java.lang.reflect.Array
                    .newInstance(a.getClass().getComponentType(), size);
            if (a.length > size)
                a[size] = null;
            return (T[]) fillEntryArray(a);
        }
        private Object[] fillEntryArray(Object[] a) {
            int j = 0;
            for (int i = 0; i < vals.length; i++)
                if (vals[i] != null)
                    a[j++] = new AbstractMap.SimpleEntry<>(
                        keyUniverse[i], unmaskNull(vals[i]));
            return a;
        }
    }

    private abstract class EnumMapIterator<T> implements Iterator<T> {
        // Lower bound on index of next element to return
        int index = 0;

        // Index of last returned element, or -1 if none
        int lastReturnedIndex = -1;

        public boolean hasNext() {
            while (index < vals.length && vals[index] == null)
                index++;
            return index != vals.length;
        }

        public void remove() {
            checkLastReturnedIndex();

            if (vals[lastReturnedIndex] != null) {
                vals[lastReturnedIndex] = null;
                size--;
            }
            lastReturnedIndex = -1;
        }

        private void checkLastReturnedIndex() {
            if (lastReturnedIndex < 0)
                throw new IllegalStateException();
        }
    }

    private class KeyIterator extends EnumMapIterator<K> {
        public K next() {
            if (!hasNext())
                throw new NoSuchElementException();
            lastReturnedIndex = index++;
            return keyUniverse[lastReturnedIndex];
        }
    }

    private class ValueIterator extends EnumMapIterator<V> {
        public V next() {
            if (!hasNext())
                throw new NoSuchElementException();
            lastReturnedIndex = index++;
            return unmaskNull(vals[lastReturnedIndex]);
        }
    }

    private class EntryIterator extends EnumMapIterator<Map.Entry<K,V>> {
        private Entry lastReturnedEntry;

        public Map.Entry<K,V> next() {
            if (!hasNext())
                throw new NoSuchElementException();
            lastReturnedEntry = new Entry(index++);
            return lastReturnedEntry;
        }

        public void remove() {
            lastReturnedIndex =
                ((null == lastReturnedEntry) ? -1 : lastReturnedEntry.index);
            super.remove();
            lastReturnedEntry.index = lastReturnedIndex;
            lastReturnedEntry = null;
        }

        private class Entry implements Map.Entry<K,V> {
            private int index;

            private Entry(int index) {
                this.index = index;
            }

            public K getKey() {
                checkIndexForEntryUse();
                return keyUniverse[index];
            }

            public V getValue() {
                checkIndexForEntryUse();
                return unmaskNull(vals[index]);
            }

            public V setValue(V value) {
                checkIndexForEntryUse();
                V oldValue = unmaskNull(vals[index]);
                vals[index] = maskNull(value);
                return oldValue;
            }

            public boolean equals(Object o) {
                if (index < 0)
                    return o == this;

                if (!(o instanceof Map.Entry))
                    return false;

                Map.Entry<?,?> e = (Map.Entry<?,?>)o;
                V ourValue = unmaskNull(vals[index]);
                Object hisValue = e.getValue();
                return (e.getKey() == keyUniverse[index] &&
                        (ourValue == hisValue ||
                         (ourValue != null && ourValue.equals(hisValue))));
            }

            public int hashCode() {
                if (index < 0)
                    return super.hashCode();

                return entryHashCode(index);
            }

            public String toString() {
                if (index < 0)
                    return super.toString();

                return keyUniverse[index] + "="
                    + unmaskNull(vals[index]);
            }

            private void checkIndexForEntryUse() {
                if (index < 0)
                    throw new IllegalStateException("Entry was removed");
            }
        }
    }

    // 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, as specified in the Map.equals(Object) contract. 
Params:
  • o – the 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, as specified in the {@link
     * Map#equals(Object)} contract.
     *
     * @param o the object to be compared for equality with this map
     * @return {@code true} if the specified object is equal to this map
     */
    public boolean equals(Object o) {
        if (this == o)
            return true;
        if (o instanceof EnumMap)
            return equals((EnumMap<?,?>)o);
        if (!(o instanceof Map))
            return false;

        Map<?,?> m = (Map<?,?>)o;
        if (size != m.size())
            return false;

        for (int i = 0; i < keyUniverse.length; i++) {
            if (null != vals[i]) {
                K key = keyUniverse[i];
                V value = unmaskNull(vals[i]);
                if (null == value) {
                    if (!((null == m.get(key)) && m.containsKey(key)))
                       return false;
                } else {
                   if (!value.equals(m.get(key)))
                      return false;
                }
            }
        }

        return true;
    }

    private boolean equals(EnumMap<?,?> em) {
        if (em.size != size)
            return false;

        if (em.keyType != keyType)
            return size == 0;

        // Key types match, compare each value
        for (int i = 0; i < keyUniverse.length; i++) {
            Object ourValue =    vals[i];
            Object hisValue = em.vals[i];
            if (hisValue != ourValue &&
                (hisValue == null || !hisValue.equals(ourValue)))
                return false;
        }
        return true;
    }

    
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.

/**
     * 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.
     */
    public int hashCode() {
        int h = 0;

        for (int i = 0; i < keyUniverse.length; i++) {
            if (null != vals[i]) {
                h += entryHashCode(i);
            }
        }

        return h;
    }

    private int entryHashCode(int index) {
        return (keyUniverse[index].hashCode() ^ vals[index].hashCode());
    }

    
Returns a shallow copy of this enum map. The values themselves
are not cloned.

Returns:a shallow copy of this enum map
/**
     * Returns a shallow copy of this enum map. The values themselves
     * are not cloned.
     *
     * @return a shallow copy of this enum map
     */
    @SuppressWarnings("unchecked")
    public EnumMap<K, V> clone() {
        EnumMap<K, V> result = null;
        try {
            result = (EnumMap<K, V>) super.clone();
        } catch(CloneNotSupportedException e) {
            throw new AssertionError();
        }
        result.vals = result.vals.clone();
        result.entrySet = null;
        return result;
    }

    
Throws an exception if e is not of the correct type for this enum set.

/**
     * Throws an exception if e is not of the correct type for this enum set.
     */
    private void typeCheck(K key) {
        Class<?> keyClass = key.getClass();
        if (keyClass != keyType && keyClass.getSuperclass() != keyType)
            throw new ClassCastException(keyClass + " != " + keyType);
    }

    
Returns all of the values comprising K.
The result is uncloned, cached, and shared by all callers.

/**
     * Returns all of the values comprising K.
     * The result is uncloned, cached, and shared by all callers.
     */
    private static <K extends Enum<K>> K[] getKeyUniverse(Class<K> keyType) {
        return SharedSecrets.getJavaLangAccess()
                                        .getEnumConstantsShared(keyType);
    }

    @java.io.Serial
    private static final long serialVersionUID = 458661240069192865L;

    
Save the state of the EnumMap instance to a stream (i.e., serialize it). 
@serialDataThe size of the enum map (the number of key-value
            mappings) is emitted (int), followed by the key (Object)
            and value (Object) for each key-value mapping represented
            by the enum map.
/**
     * Save the state of the {@code EnumMap} instance to a stream (i.e.,
     * serialize it).
     *
     * @serialData The <i>size</i> of the enum map (the number of key-value
     *             mappings) is emitted (int), followed by the key (Object)
     *             and value (Object) for each key-value mapping represented
     *             by the enum map.
     */
    @java.io.Serial
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException
    {
        // Write out the key type and any hidden stuff
        s.defaultWriteObject();

        // Write out size (number of Mappings)
        s.writeInt(size);

        // Write out keys and values (alternating)
        int entriesToBeWritten = size;
        for (int i = 0; entriesToBeWritten > 0; i++) {
            if (null != vals[i]) {
                s.writeObject(keyUniverse[i]);
                s.writeObject(unmaskNull(vals[i]));
                entriesToBeWritten--;
            }
        }
    }

    
Reconstitute the EnumMap instance from a stream (i.e., deserialize it). 
/**
     * Reconstitute the {@code EnumMap} instance from a stream (i.e.,
     * deserialize it).
     */
    @SuppressWarnings("unchecked")
    @java.io.Serial
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException
    {
        // Read in the key type and any hidden stuff
        s.defaultReadObject();

        keyUniverse = getKeyUniverse(keyType);
        vals = new Object[keyUniverse.length];

        // Read in size (number of Mappings)
        int size = s.readInt();

        // Read the keys and values, and put the mappings in the HashMap
        for (int i = 0; i < size; i++) {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            put(key, value);
        }
    }
}