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/* Collection.java -- Interface that represents a collection of objects

   Copyright (C) 1998, 2001, 2004, 2005 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

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any later version.

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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

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02110-1301 USA.

Linking this library statically or dynamically with other modules is

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As a special exception, the copyright holders of this library give you

permission to link this library with independent modules to produce an

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terms of your choice, provided that you also meet, for each linked

independent module, the terms and conditions of the license of that

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this exception to your version of the library, but you are not

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exception statement from your version. */

package java.util;

/**

 * Interface that represents a collection of objects. This interface is the

 * root of the collection hierarchy, and does not provide any guarantees about

 * the order of its elements or whether or not duplicate elements are

 * permitted.

 * <p>

 * All methods of this interface that are defined to modify the collection are

 * defined as <dfn>optional</dfn>. An optional operation may throw an

 * UnsupportedOperationException if the data backing this collection does not

 * support such a modification. This may mean that the data structure is

 * immutable, or that it is read-only but may change ("unmodifiable"), or

 * that it is modifiable but of fixed size (such as an array), or any number

 * of other combinations.

 * <p>

 * A class that wishes to implement this interface should consider subclassing

 * AbstractCollection, which provides basic implementations of most of the

 * methods of this interface. Classes that are prepared to make guarantees

 * about ordering or about absence of duplicate elements should consider

 * implementing List or Set respectively, both of which are subinterfaces of

 * Collection.

 * <p>

 * A general-purpose implementation of the Collection interface should in most

 * cases provide at least two constructors: One which takes no arguments and

 * creates an empty collection, and one which takes a Collection as an argument

 * and returns a collection containing the same elements (that is, creates a

 * copy of the argument using its own implementation).

 *

 * @author Original author unknown

 * @author Eric Blake (ebb9@email.byu.edu)

 * @author Tom Tromey (tromey@redhat.com)

 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)

 * @see List

 * @see Set

 * @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

 * @status updated to 1.4

 */

public interface Collection<E> extends Iterable<E>

{

  /**

   * Add an element to this collection.

   *

   * @param o the object to add.

   * @return true if the collection was modified as a result of this action.

   * @throws UnsupportedOperationException if this collection does not

   *   support the add operation.

   * @throws ClassCastException if o cannot be added to this collection due

   *   to its type.

   * @throws NullPointerException if o is null and this collection doesn't

   *   support the addition of null values.

   * @throws IllegalArgumentException if o cannot be added to this

   *   collection for some other reason.

   */

  boolean add(E o);

  /**

   * Add the contents of a given collection to this collection.

   *

   * @param c the collection to add.

   * @return true if the collection was modified as a result of this action.

   * @throws UnsupportedOperationException if this collection does not

   *   support the addAll operation.

   * @throws ClassCastException if some element of c cannot be added to this

   *   collection due to its type.

   * @throws NullPointerException if some element of c is null and this

   *   collection does not support the addition of null values.

   * @throws NullPointerException if c itself is null.

   * @throws IllegalArgumentException if some element of c cannot be added

   *   to this collection for some other reason.

   */

  boolean addAll(Collection<? extends E> c);

  /**

   * Clear the collection, such that a subsequent call to isEmpty() would

   * return true.

   *

   * @throws UnsupportedOperationException if this collection does not

   *   support the clear operation.

   */

  void clear();

  /**

   * Test whether this collection contains a given object as one of its

   * elements.

   *

   * @param o the element to look for.

   * @return true if this collection contains at least one element e such that

   *   <code>o == null ? e == null : o.equals(e)</code>.

   * @throws ClassCastException if the type of o is not a valid type for this

   *   collection.

   * @throws NullPointerException if o is null and this collection doesn't

   *   support null values.

   */

  boolean contains(Object o);

  /**

   * Test whether this collection contains every element in a given collection.

   *

   * @param c the collection to test for.

   * @return true if for every element o in c, contains(o) would return true.

   * @throws ClassCastException if the type of any element in c is not a valid

   *   type for this collection.

   * @throws NullPointerException if some element of c is null and this

   *   collection does not support null values.

   * @throws NullPointerException if c itself is null.

   */

  boolean containsAll(Collection<?> c);

  /**

   * Test whether this collection is equal to some object. The Collection

   * interface does not explicitly require any behaviour from this method, and

   * it may be left to the default implementation provided by Object. The Set

   * and List interfaces do, however, require specific behaviour from this

   * method.

   * <p>

   * If an implementation of Collection, which is not also an implementation of

   * Set or List, should choose to implement this method, it should take care

   * to obey the contract of the equals method of Object. In particular, care

   * should be taken to return false when o is a Set or a List, in order to

   * preserve the symmetry of the relation.

   *

   * @param o the object to compare to this collection.

   * @return true if the o is equal to this collection.

   */

  boolean equals(Object o);

  /**

   * Obtain a hash code for this collection. The Collection interface does not

   * explicitly require any behaviour from this method, and it may be left to

   * the default implementation provided by Object. The Set and List interfaces

   * do, however, require specific behaviour from this method.

   * <p>

   * If an implementation of Collection, which is not also an implementation of

   * Set or List, should choose to implement this method, it should take care

   * to obey the contract of the hashCode method of Object. Note that this

   * method renders it impossible to correctly implement both Set and List, as

   * the required implementations are mutually exclusive.

   *

   * @return a hash code for this collection.

   */

  int hashCode();

  /**

   * Test whether this collection is empty, that is, if size() == 0.

   *

   * @return true if this collection contains no elements.

   */

  boolean isEmpty();

  /**

   * Obtain an Iterator over this collection.

   *

   * @return an Iterator over the elements of this collection, in any order.

   */

  Iterator<E> iterator();

  /**

   * Remove a single occurrence of an object from this collection. That is,

   * remove an element e, if one exists, such that <code>o == null ? e == null

   *   : o.equals(e)</code>.

   *

   * @param o the object to remove.

   * @return true if the collection changed as a result of this call, that is,

   *   if the collection contained at least one occurrence of o.

   * @throws UnsupportedOperationException if this collection does not

   *   support the remove operation.

   * @throws ClassCastException if the type of o is not a valid type

   *   for this collection.

   * @throws NullPointerException if o is null and the collection doesn't

   *   support null values.

   */

  boolean remove(Object o);

  /**

   * Remove all elements of a given collection from this collection. That is,

   * remove every element e such that c.contains(e).

   *

   * @param c The collection of objects to be removed.

   * @return true if this collection was modified as a result of this call.

   * @throws UnsupportedOperationException if this collection does not

   *   support the removeAll operation.

   * @throws ClassCastException if the type of any element in c is not a valid

   *   type for this collection.

   * @throws NullPointerException if some element of c is null and this

   *   collection does not support removing null values.

   * @throws NullPointerException if c itself is null.

   */

  boolean removeAll(Collection<?> c);

  /**

   * Remove all elements of this collection that are not contained in a given

   * collection. That is, remove every element e such that !c.contains(e).

   *

   * @param c The collection of objects to be retained.

   * @return true if this collection was modified as a result of this call.

   * @throws UnsupportedOperationException if this collection does not

   *   support the retainAll operation.

   * @throws ClassCastException if the type of any element in c is not a valid

   *   type for this collection.

   * @throws NullPointerException if some element of c is null and this

   *   collection does not support retaining null values.

   * @throws NullPointerException if c itself is null.

   */

  boolean retainAll(Collection<?> c);

  /**

   * Get the number of elements in this collection.

   *

   * @return the number of elements in the collection.

   */

  int size();

  /**

   * Copy the current contents of this collection into an array.

   *

   * @return an array of type Object[] and length equal to the size of this

   *   collection, containing the elements currently in this collection, in

   *   any order.

   */

  Object[] toArray();

  /**

   * Copy the current contents of this collection into an array. If the array

   * passed as an argument has length less than the size of this collection, an

   * array of the same run-time type as a, and length equal to the size of this

   * collection, is allocated using Reflection. Otherwise, a itself is used.

   * The elements of this collection are copied into it, and if there is space

   * in the array, the following element is set to null. The resultant array is

   * returned.

   * Note: The fact that the following element is set to null is only useful

   * if it is known that this collection does not contain any null elements.

   *

   * @param a the array to copy this collection into.

   * @return an array containing the elements currently in this collection, in

   *   any order.

   * @throws ArrayStoreException if the type of any element of the

   *   collection is not a subtype of the element type of a.

   */

  <T> T[] toArray(T[] a);

}