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 {@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. + * + *
+ * 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 {@link Object#equals(Object) equals} method. For example, + * the specification for the {@link #contains(Object) 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 + * {@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. + * + *
This interface is a member of the
+ *
+ * Java Collections Framework.
+ *
+ * @param 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.
+ *
+ * @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 {@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.
+ *
+ * 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:
+ *
+ *
+ *
+ * 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.
+ *
+ * @param e element whose presence in this collection is to be ensured
+ * @return true if this collection changed as a result of the
+ * call
+ * @throws UnsupportedOperationException if the add 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).
+ *
+ * @param o element to be removed from this collection, if present
+ * @return true 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 remove 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.
+ *
+ * @param c collection to be checked for containment in this collection
+ * @return true 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.)
+ *
+ * @param c collection containing elements to be added to this collection
+ * @return true if this collection changed as a result of the call
+ * @throws UnsupportedOperationException if the addAll 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 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.
+ *
+ * @param c collection containing elements to be removed from this collection
+ * @return true if this collection changed as a result of the
+ * call
+ * @throws UnsupportedOperationException if the removeAll 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.
+ *
+ * @param c collection containing elements to be retained in this collection
+ * @return true if this collection changed as a result of the call
+ * @throws UnsupportedOperationException if the retainAll 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
+ */
+ 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.)
+ *
+ * @param o object to be compared for equality with this collection
+ * @return true 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.hashCode method.
+ * In particular, c1.equals(c2) implies that
+ * c1.hashCode()==c2.hashCode().
+ *
+ * @return the hash code value for this collection
+ *
+ * @see Object#hashCode()
+ * @see Object#equals(Object)
+ */
+ int hashCode();
+}
+ * String[] y = x.toArray(new String[0]);
+ *
+ * Note that toArray(new Object[0]) is identical in function to
+ * toArray().
+ *
+ * @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
+ */
+