path: root/java/util/Collection.java

/* Collection.java -- Interface that represents a collection of objects
   Copyright (C) 1998, 2001, 2004, 2005 Free Software Foundation, Inc.

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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);
}