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
- */
-