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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * under the terms of the GNU General Public License version 2 only, as
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 * particular file as subject to the "Classpath" exception as provided
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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 * 2 along with this work; if not, write to the Free Software Foundation,
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 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.util;


The root interface in the collection hierarchy.  A collection
represents a group of objects, known as its elements.  Some
collections allow duplicate elements and others do not.  Some are ordered
and others unordered.  The JDK does not provide any direct
implementations of this interface: it provides implementations of more
specific subinterfaces like Set and List.  This interface
is typically used to pass collections around and manipulate them where
maximum generality is desired.

Bags or multisets (unordered collections that may contain
duplicate elements) should implement this interface directly.

All general-purpose Collection implementation classes (which
typically implement Collection indirectly through one of its
subinterfaces) should provide two "standard" constructors: a void (no
arguments) constructor, which creates an empty collection, and a
constructor with a single argument of type Collection, which
creates a new collection with the same elements as its argument.  In
effect, the latter constructor allows the user to copy any collection,
producing an equivalent collection of the desired implementation type.
There is no way to enforce this convention (as interfaces cannot contain
constructors) but all of the general-purpose Collection
implementations in the Java platform libraries comply.

The "destructive" methods contained in this interface, that is, the
methods that modify the collection on which they operate, are specified to
throw UnsupportedOperationException if this collection 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 collection. For example, invoking the addAll(Collection) method on an unmodifiable collection may, but is not required to, throw the exception if the collection to be added is empty. 
Some collection implementations have restrictions on the elements that
they may contain.  For example, some implementations prohibit null elements,
and some have restrictions on the types of their elements.  Attempting to
add an ineligible element throws an unchecked exception, typically
NullPointerException or ClassCastException.  Attempting
to query the presence of an ineligible element 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 element whose completion would not result in
the insertion of an ineligible element into the collection 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.

It is up to each collection to determine its own synchronization
policy.  In the absence of a stronger guarantee by the
implementation, undefined behavior may result from the invocation
of any method on a collection that is being mutated by another
thread; this includes direct invocations, passing the collection to
a method that might perform invocations, and using an existing
iterator to examine the collection.

Many methods in Collections Framework interfaces are defined in terms of the equals method. For example, the specification for the contains(Object o) method says: "returns true if and only if this collection
contains at least one element e such that
(o==null ? e==null : o.equals(e))."  This specification should
not be construed to imply that invoking Collection.contains
with a non-null argument o will cause o.equals(e) to be
invoked for any element e.  Implementations are free to implement
optimizations whereby the equals invocation is avoided, for example, by first comparing the hash codes of the two elements. (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. 
This interface is a member of the

Java Collections Framework.

Author: Josh Bloch,  Neal Gafter
See Also:
Since:1.2
/**
 * The root interface in the <i>collection hierarchy</i>.  A collection
 * represents a group of objects, known as its <i>elements</i>.  Some
 * collections allow duplicate elements and others do not.  Some are ordered
 * and others unordered.  The JDK does not provide any <i>direct</i>
 * implementations of this interface: it provides implementations of more
 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>.  This interface
 * is typically used to pass collections around and manipulate them where
 * maximum generality is desired.
 *
 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
 * duplicate elements) should implement this interface directly.
 *
 * <p>All general-purpose <tt>Collection</tt> implementation classes (which
 * typically implement <tt>Collection</tt> indirectly through one of its
 * subinterfaces) should provide two "standard" constructors: a void (no
 * arguments) constructor, which creates an empty collection, and a
 * constructor with a single argument of type <tt>Collection</tt>, which
 * creates a new collection with the same elements as its argument.  In
 * effect, the latter constructor allows the user to copy any collection,
 * producing an equivalent collection of the desired implementation type.
 * There is no way to enforce this convention (as interfaces cannot contain
 * constructors) but all of the general-purpose <tt>Collection</tt>
 * implementations in the Java platform libraries comply.
 *
 * <p>The "destructive" methods contained in this interface, that is, the
 * methods that modify the collection on which they operate, are specified to
 * throw <tt>UnsupportedOperationException</tt> if this collection does not
 * support the operation.  If this is the case, these methods may, but are not
 * required to, throw an <tt>UnsupportedOperationException</tt> if the
 * invocation would have no effect on the collection.  For example, invoking
 * the {@link #addAll(Collection)} method on an unmodifiable collection may,
 * but is not required to, throw the exception if the collection to be added
 * is empty.
 *
 * <p>Some collection implementations have restrictions on the elements that
 * they may contain.  For example, some implementations prohibit null elements,
 * and some have restrictions on the types of their elements.  Attempting to
 * add an ineligible element throws an unchecked exception, typically
 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting
 * to query the presence of an ineligible element 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 element whose completion would not result in
 * the insertion of an ineligible element into the collection 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>It is up to each collection to determine its own synchronization
 * policy.  In the absence of a stronger guarantee by the
 * implementation, undefined behavior may result from the invocation
 * of any method on a collection that is being mutated by another
 * thread; this includes direct invocations, passing the collection to
 * a method that might perform invocations, and using an existing
 * iterator to examine the collection.
 *
 * <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 #contains(Object) contains(Object o)}
 * method says: "returns <tt>true</tt> if and only if this collection
 * contains at least one element <tt>e</tt> such that
 * <tt>(o==null ? e==null : o.equals(e))</tt>."  This specification should
 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
 * invoked for any element <tt>e</tt>.  Implementations are free to implement
 * optimizations whereby the <tt>equals</tt> invocation is avoided, for
 * example, by first comparing the hash codes of the two elements.  (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>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Set
 * @see     List
 * @see     Map
 * @see     SortedSet
 * @see     SortedMap
 * @see     HashSet
 * @see     TreeSet
 * @see     ArrayList
 * @see     LinkedList
 * @see     Vector
 * @see     Collections
 * @see     Arrays
 * @see     AbstractCollection
 * @since 1.2
 */

public interface Collection<E> extends Iterable<E> {
    // Query Operations

    
Returns the number of elements in this collection.  If this collection
contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.

Returns:the number of elements in this collection
/**
     * Returns the number of elements in this collection.  If this collection
     * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * @return the number of elements in this collection
     */
    int size();

    
Returns true if this collection contains no elements.

Returns:true if this collection contains no elements
/**
     * Returns <tt>true</tt> if this collection contains no elements.
     *
     * @return <tt>true</tt> if this collection contains no elements
     */
    boolean isEmpty();

    
Returns true if this collection contains the specified element.
More formally, returns true if and only if this collection
contains at least one element e such that
(o==null ? e==null : o.equals(e)).

Params:
  • o – element whose presence in this collection is to be tested
Throws:
  • ClassCastException – if the type of the specified element
        is incompatible with this collection (optional)
  • NullPointerException – if the specified element is null and this
        collection does not permit null elements (optional)
Returns:true if this collection contains the specified
        element
/**
     * Returns <tt>true</tt> if this collection contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this collection
     * contains at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this collection is to be tested
     * @return <tt>true</tt> if this collection contains the specified
     *         element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this collection (optional)
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements (optional)
     */
    boolean contains(Object o);

    
Returns an iterator over the elements in this collection.  There are no
guarantees concerning the order in which the elements are returned
(unless this collection is an instance of some class that provides a
guarantee).

Returns:an Iterator over the elements in this collection
/**
     * Returns an iterator over the elements in this collection.  There are no
     * guarantees concerning the order in which the elements are returned
     * (unless this collection is an instance of some class that provides a
     * guarantee).
     *
     * @return an <tt>Iterator</tt> over the elements in this collection
     */
    Iterator<E> iterator();

    
Returns an array containing all of the elements in this collection.
If this collection makes any guarantees as to what order its elements
are returned by its iterator, this method must return the elements in
the same order.

The returned array will be "safe" in that no references to it are
maintained by this collection.  (In other words, this method must
allocate a new array even if this collection is backed by an array).
The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based
APIs.

Returns:an array containing all of the elements in this collection
/**
     * Returns an array containing all of the elements in this collection.
     * If this collection makes any guarantees as to what order its elements
     * are returned by its iterator, this method must return the elements in
     * the same order.
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this collection.  (In other words, this method must
     * allocate a new array even if this collection is backed by an array).
     * The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this collection
     */
    Object[] toArray();

    
Returns an array containing all of the elements in this collection;
the runtime type of the returned array is that of the specified array.
If the collection fits in the specified array, it is returned therein.
Otherwise, a new array is allocated with the runtime type of the
specified array and the size of this collection.

If this collection fits in the specified array with room to spare
(i.e., the array has more elements than this collection), the element
in the array immediately following the end of the collection is set to
null.  (This is useful in determining the length of this
collection only if the caller knows that this collection does
not contain any null elements.)

If this collection makes any guarantees as to what order its elements
are returned by its iterator, this method must return the elements in
the same order.

Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs. 
Suppose x is a collection known to contain only strings.
The following code can be used to dump the collection into a newly
allocated array of String:

    String[] y = x.toArray(new String[0]);

Note that toArray(new Object[0]) is identical in function to
toArray().

Params:
  • a – the array into which the elements of this collection are to be
       stored, if it is big enough; otherwise, a new array of the same
       runtime type is allocated for this purpose.
Throws:
  • ArrayStoreException – if the runtime type of the specified array
        is not a supertype of the runtime type of every element in
        this collection
  • NullPointerException – if the specified array is null
Returns:an array containing all of the elements in this collection
/**
     * Returns an array containing all of the elements in this collection;
     * the runtime type of the returned array is that of the specified array.
     * If the collection fits in the specified array, it is returned therein.
     * Otherwise, a new array is allocated with the runtime type of the
     * specified array and the size of this collection.
     *
     * <p>If this collection fits in the specified array with room to spare
     * (i.e., the array has more elements than this collection), the element
     * in the array immediately following the end of the collection is set to
     * <tt>null</tt>.  (This is useful in determining the length of this
     * collection <i>only</i> if the caller knows that this collection does
     * not contain any <tt>null</tt> elements.)
     *
     * <p>If this collection makes any guarantees as to what order its elements
     * are returned by its iterator, this method must return the elements in
     * the same order.
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
     * The following code can be used to dump the collection into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of this collection are to be
     *        stored, if it is big enough; otherwise, a new array of the same
     *        runtime type is allocated for this purpose.
     * @return an array containing all of the elements in this collection
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this collection
     * @throws NullPointerException if the specified array is null
     */
    <T> T[] toArray(T[] a);

    // Modification Operations

    
Ensures that this collection contains the specified element (optional
operation).  Returns true if this collection changed as a
result of the call.  (Returns false if this collection does
not permit duplicates and already contains the specified element.)

Collections that support this operation may place limitations on what
elements may be added to this collection.  In particular, some
collections will refuse to add null elements, and others will
impose restrictions on the type of elements that may be added.
Collection classes should clearly specify in their documentation any
restrictions on what elements may be added.

If a collection refuses to add a particular element for any reason
other than that it already contains the element, it must throw
an exception (rather than returning false).  This preserves
the invariant that a collection always contains the specified element
after this call returns.

Params:
  • e – element whose presence in this collection is to be ensured
Throws:
Returns:true if this collection changed as a result of the
        call
/**
     * Ensures that this collection contains the specified element (optional
     * operation).  Returns <tt>true</tt> if this collection changed as a
     * result of the call.  (Returns <tt>false</tt> if this collection does
     * not permit duplicates and already contains the specified element.)<p>
     *
     * Collections that support this operation may place limitations on what
     * elements may be added to this collection.  In particular, some
     * collections will refuse to add <tt>null</tt> elements, and others will
     * impose restrictions on the type of elements that may be added.
     * Collection classes should clearly specify in their documentation any
     * restrictions on what elements may be added.<p>
     *
     * If a collection refuses to add a particular element for any reason
     * other than that it already contains the element, it <i>must</i> throw
     * an exception (rather than returning <tt>false</tt>).  This preserves
     * the invariant that a collection always contains the specified element
     * after this call returns.
     *
     * @param e element whose presence in this collection is to be ensured
     * @return <tt>true</tt> if this collection changed as a result of the
     *         call
     * @throws UnsupportedOperationException if the <tt>add</tt> operation
     *         is not supported by this collection
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this collection
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements
     * @throws IllegalArgumentException if some property of the element
     *         prevents it from being added to this collection
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to insertion restrictions
     */
    boolean add(E e);

    
Removes a single instance of the specified element from this
collection, if it is present (optional operation).  More formally,
removes an element e such that
(o==null ? e==null : o.equals(e)), if
this collection contains one or more such elements.  Returns
true if this collection contained the specified element (or
equivalently, if this collection changed as a result of the call).

Params:
  • o – element to be removed from this collection, if present
Throws:
Returns:true if an element was removed as a result of this call
/**
     * Removes a single instance of the specified element from this
     * collection, if it is present (optional operation).  More formally,
     * removes an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
     * this collection contains one or more such elements.  Returns
     * <tt>true</tt> if this collection contained the specified element (or
     * equivalently, if this collection changed as a result of the call).
     *
     * @param o element to be removed from this collection, if present
     * @return <tt>true</tt> if an element was removed as a result of this call
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this collection (optional)
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements (optional)
     * @throws UnsupportedOperationException if the <tt>remove</tt> operation
     *         is not supported by this collection
     */
    boolean remove(Object o);


    // Bulk Operations

    
Returns true if this collection contains all of the elements
in the specified collection.

Params:
  • c – collection to be checked for containment in this collection
Throws:
  • ClassCastException – if the types of one or more elements
        in the specified collection are incompatible with this
        collection (optional)
  • NullPointerException – if the specified collection contains one
        or more null elements and this collection does not permit null
        elements (optional), or if the specified collection is null
See Also:
Returns:true if this collection contains all of the elements
        in the specified collection
/**
     * Returns <tt>true</tt> if this collection contains all of the elements
     * in the specified collection.
     *
     * @param  c collection to be checked for containment in this collection
     * @return <tt>true</tt> if this collection contains all of the elements
     *         in the specified collection
     * @throws ClassCastException if the types of one or more elements
     *         in the specified collection are incompatible with this
     *         collection (optional)
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this collection does not permit null
     *         elements (optional), or if the specified collection is null
     * @see    #contains(Object)
     */
    boolean containsAll(Collection<?> c);

    
Adds all of the elements in the specified collection to this collection
(optional operation).  The behavior of this operation is undefined if
the specified collection is modified while the operation is in progress.
(This implies that the behavior of this call is undefined if the
specified collection is this collection, and this collection is
nonempty.)

Params:
  • c – collection containing elements to be added to this collection
Throws:
  • UnsupportedOperationException – if the addAll operation
        is not supported by this collection
  • ClassCastException – if the class of an element of the specified
        collection prevents it from being added to this collection
  • NullPointerException – if the specified collection contains a
        null element and this collection does not permit null elements,
        or if the specified collection is null
  • IllegalArgumentException – if some property of an element of the
        specified collection prevents it from being added to this
        collection
  • IllegalStateException – if not all the elements can be added at
        this time due to insertion restrictions
See Also:
Returns:true if this collection changed as a result of the call
/**
     * Adds all of the elements in the specified collection to this collection
     * (optional operation).  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in progress.
     * (This implies that the behavior of this call is undefined if the
     * specified collection is this collection, and this collection is
     * nonempty.)
     *
     * @param c collection containing elements to be added to this collection
     * @return <tt>true</tt> if this collection changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
     *         is not supported by this collection
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this collection
     * @throws NullPointerException if the specified collection contains a
     *         null element and this collection does not permit null elements,
     *         or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this
     *         collection
     * @throws IllegalStateException if not all the elements can be added at
     *         this time due to insertion restrictions
     * @see #add(Object)
     */
    boolean addAll(Collection<? extends E> c);

    
Removes all of this collection's elements that are also contained in the
specified collection (optional operation).  After this call returns,
this collection will contain no elements in common with the specified
collection.

Params:
  • c – collection containing elements to be removed from this collection
Throws:
  • UnsupportedOperationException – if the removeAll method
        is not supported by this collection
  • ClassCastException – if the types of one or more elements
        in this collection are incompatible with the specified
        collection (optional)
  • NullPointerException – if this collection contains one or more
        null elements and the specified collection does not support
        null elements (optional), or if the specified collection is null
See Also:
Returns:true if this collection changed as a result of the
        call
/**
     * Removes all of this collection's elements that are also contained in the
     * specified collection (optional operation).  After this call returns,
     * this collection will contain no elements in common with the specified
     * collection.
     *
     * @param c collection containing elements to be removed from this collection
     * @return <tt>true</tt> if this collection changed as a result of the
     *         call
     * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
     *         is not supported by this collection
     * @throws ClassCastException if the types of one or more elements
     *         in this collection are incompatible with the specified
     *         collection (optional)
     * @throws NullPointerException if this collection contains one or more
     *         null elements and the specified collection does not support
     *         null elements (optional), or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean removeAll(Collection<?> c);

    
Retains only the elements in this collection that are contained in the
specified collection (optional operation).  In other words, removes from
this collection all of its elements that are not contained in the
specified collection.

Params:
  • c – collection containing elements to be retained in this collection
Throws:
  • UnsupportedOperationException – if the retainAll operation
        is not supported by this collection
  • ClassCastException – if the types of one or more elements
        in this collection are incompatible with the specified
        collection (optional)
  • NullPointerException – if this collection contains one or more
        null elements and the specified collection does not permit null
        elements (optional), or if the specified collection is null
See Also:
Returns:true if this collection changed as a result of the call
/**
     * Retains only the elements in this collection that are contained in the
     * specified collection (optional operation).  In other words, removes from
     * this collection all of its elements that are not contained in the
     * specified collection.
     *
     * @param c collection containing elements to be retained in this collection
     * @return <tt>true</tt> if this collection changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
     *         is not supported by this collection
     * @throws ClassCastException if the types of one or more elements
     *         in this collection are incompatible with the specified
     *         collection (optional)
     * @throws NullPointerException if this collection contains one or more
     *         null elements and the specified collection does not permit null
     *         elements (optional), or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean retainAll(Collection<?> c);

    
Removes all of the elements from this collection (optional operation).
The collection will be empty after this method returns.

Throws:
  • UnsupportedOperationException – if the clear operation
        is not supported by this collection
/**
     * Removes all of the elements from this collection (optional operation).
     * The collection will be empty after this method returns.
     *
     * @throws UnsupportedOperationException if the <tt>clear</tt> operation
     *         is not supported by this collection
     */
    void clear();


    // Comparison and hashing

    
Compares the specified object with this collection for equality. 

While the Collection interface adds no stipulations to the
general contract for the Object.equals, programmers who
implement the Collection interface "directly" (in other words,
create a class that is a Collection but is not a Set
or a List) must exercise care if they choose to override the
Object.equals.  It is not necessary to do so, and the simplest
course of action is to rely on Object's implementation, but
the implementor may wish to implement a "value comparison" in place of
the default "reference comparison."  (The List and
Set interfaces mandate such value comparisons.)

The general contract for the Object.equals method states that
equals must be symmetric (in other words, a.equals(b) if and
only if b.equals(a)).  The contracts for List.equals
and Set.equals state that lists are only equal to other lists,
and sets to other sets.  Thus, a custom equals method for a
collection class that implements neither the List nor
Set interface must return false when this collection
is compared to any list or set.  (By the same logic, it is not possible
to write a class that correctly implements both the Set and
List interfaces.)

Params:
  • o – object to be compared for equality with this collection
See Also:
Returns:true if the specified object is equal to this
collection
/**
     * Compares the specified object with this collection for equality. <p>
     *
     * While the <tt>Collection</tt> interface adds no stipulations to the
     * general contract for the <tt>Object.equals</tt>, programmers who
     * implement the <tt>Collection</tt> interface "directly" (in other words,
     * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
     * or a <tt>List</tt>) must exercise care if they choose to override the
     * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest
     * course of action is to rely on <tt>Object</tt>'s implementation, but
     * the implementor may wish to implement a "value comparison" in place of
     * the default "reference comparison."  (The <tt>List</tt> and
     * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
     *
     * The general contract for the <tt>Object.equals</tt> method states that
     * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
     * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>
     * and <tt>Set.equals</tt> state that lists are only equal to other lists,
     * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a
     * collection class that implements neither the <tt>List</tt> nor
     * <tt>Set</tt> interface must return <tt>false</tt> when this collection
     * is compared to any list or set.  (By the same logic, it is not possible
     * to write a class that correctly implements both the <tt>Set</tt> and
     * <tt>List</tt> interfaces.)
     *
     * @param o object to be compared for equality with this collection
     * @return <tt>true</tt> if the specified object is equal to this
     * collection
     *
     * @see Object#equals(Object)
     * @see Set#equals(Object)
     * @see List#equals(Object)
     */
    boolean equals(Object o);

    
Returns the hash code value for this collection.  While the
Collection interface adds no stipulations to the general
contract for the Object.hashCode method, programmers should
take note that any class that overrides the Object.equals
method must also override the Object.hashCode method in order
to satisfy the general contract for the Object.hashCodemethod.
In particular, c1.equals(c2) implies that
c1.hashCode()==c2.hashCode().

See Also:
Returns:the hash code value for this collection
/**
     * Returns the hash code value for this collection.  While the
     * <tt>Collection</tt> interface adds no stipulations to the general
     * contract for the <tt>Object.hashCode</tt> method, programmers should
     * take note that any class that overrides the <tt>Object.equals</tt>
     * method must also override the <tt>Object.hashCode</tt> method in order
     * to satisfy the general contract for the <tt>Object.hashCode</tt>method.
     * In particular, <tt>c1.equals(c2)</tt> implies that
     * <tt>c1.hashCode()==c2.hashCode()</tt>.
     *
     * @return the hash code value for this collection
     *
     * @see Object#hashCode()
     * @see Object#equals(Object)
     */
    int hashCode();
}