The "destructive" methods contained in this interface, that is, the - * methods that modify the collection on which they operate, are specified to - * throw {@code UnsupportedOperationException} if this collection does not - * support the operation. If this is the case, these methods may, but are not - * required to, throw an {@code 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, + *
Certain methods are specified to be + * optional. If a collection implementation doesn't implement a + * particular operation, it should define the corresponding method to throw + * {@code UnsupportedOperationException}. Such methods are marked "optional + * operation" in method specifications of the collections interfaces. + * + *
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 * {@code NullPointerException} or {@code ClassCastException}. Attempting @@ -111,6 +107,83 @@ * methods. Implementations may optionally handle the self-referential scenario, * however most current implementations do not do so. * + *
View collections. + * Most collections manage storage for elements they contain. By contrast, view + * collections themselves do not store elements, but instead they rely on a + * backing collection to store the actual elements. Operations that are not handled + * by the view collection itself are delegated to the backing collection. Examples of + * view collections include the wrapper collections returned by methods such as + * {@link Collections#checkedCollection Collections.checkedCollection}, + * {@link Collections#synchronizedCollection Collections.synchronizedCollection}, and + * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}. + * Other examples of view collections include collections that provide a + * different representation of the same elements, for example, as + * provided by {@link List#subList List.subList}, + * {@link NavigableSet#subSet NavigableSet.subSet}, or + * {@link Map#entrySet Map.entrySet}. + * Any changes made to the backing collection are visible in the view collection. + * Correspondingly, any changes made to the view collection — if changes + * are permitted — are written through to the backing collection. + * Although they technically aren't collections, instances of + * {@link Iterator} and {@link ListIterator} can also allow modifications + * to be written through to the backing collection, and in some cases, + * modifications to the backing collection will be visible to the Iterator + * during iteration. + * + *
Unmodifiable collections. + * Certain methods of this interface are considered "destructive" and are called + * "mutator" methods in that they modify the group of objects contained within + * the collection on which they operate. They can be specified to throw + * {@code UnsupportedOperationException} if this collection implementation + * does not support the operation. Such methods should (but are not required + * to) throw an {@code UnsupportedOperationException} if the invocation would + * have no effect on the collection. For example, consider a collection that + * does not support the {@link #add add} operation. What will happen if the + * {@link #addAll addAll} method is invoked on this collection, with an empty + * collection as the argument? The addition of zero elements has no effect, + * so it is permissible for this collection simply to do nothing and not to throw + * an exception. However, it is recommended that such cases throw an exception + * unconditionally, as throwing only in certain cases can lead to + * programming errors. + * + *
An unmodifiable collection is a collection, all of whose + * mutator methods (as defined above) are specified to throw + * {@code UnsupportedOperationException}. Such a collection thus cannot be + * modified by calling any methods on it. For a collection to be properly + * unmodifiable, any view collections derived from it must also be unmodifiable. + * For example, if a List is unmodifiable, the List returned by + * {@link List#subList List.subList} is also unmodifiable. + * + *
An unmodifiable collection is not necessarily immutable. If the + * contained elements are mutable, the entire collection is clearly + * mutable, even though it might be unmodifiable. For example, consider + * two unmodifiable lists containing mutable elements. The result of calling + * {@code list1.equals(list2)} might differ from one call to the next if + * the elements had been mutated, even though both lists are unmodifiable. + * However, if an unmodifiable collection contains all immutable elements, + * it can be considered effectively immutable. + * + *
Unmodifiable view collections. + * An unmodifiable view is a collection that is unmodifiable and that is + * a view onto a backing collection. Mutator methods throw + * {@code UnsupportedOperationException}, as described above, while + * reading and querying methods are delegated to the backing collection. + * The effect is to provide read-only access to the backing collection. + * This is useful for a component to provide users with read access to + * an internal collection, while preventing them from modifying such + * collections unexpectedly. Examples of unmodifiable view collections + * are those returned by the + * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}, + * {@link Collections#unmodifiableList Collections.unmodifiableList}, and + * related methods. + * + *
Note that changes to the backing collection might still be possible, + * and if they occur, they are visible through the unmodifiable view. Thus, + * an unmodifiable view collection is not necessarily immutable. However, + * if the backing collection of an unmodifiable view is effectively immutable, + * or if the only reference to the backing collection is through an + * unmodifiable view, the view can be considered effectively immutable. + * *
This interface is a member of the * * Java Collections Framework.