Android
java.util.concurrent
public class

java.util.concurrent.CopyOnWriteArraySet<E>

java.lang.Object
java.util.AbstractCollection<E> Collection<E>
java.util.AbstractSet<E> Set<E>
java.util.concurrent.CopyOnWriteArraySet<E> Serializable

A Set that uses CopyOnWriteArrayList for all of its operations. Thus, it shares the same basic properties:

  • It is best suited for applications in which set sizes generally stay small, read-only operations vastly outnumber mutative operations, and you need to prevent interference among threads during traversal.
  • Mutative operations(add, set, remove, etc) are expensive since they usually entail copying the entire underlying array.
  • Iterators do not support the mutative remove operation
  • Traversal via iterators is very fast and cannot ever encounter interference from other threads. Iterators rely on unchanging snapshots of the array at the time the iterators were constructed.

Sample Usage. Probably the main application of copy-on-write sets are classes that maintain sets of Handler objects that must be multicasted to upon an update command. This is a classic case where you do not want to be holding a lock while sending a message, and where traversals normally vastly overwhelm additions.

 class Handler { void handle(); ... }

 class X {
    private final CopyOnWriteArraySet<Handler> handlers = new CopyOnWriteArraySet<Handler>();
    public void addHandler(Handler h) { handlers.add(h); }

    private long internalState;
    private synchronized void changeState() { internalState = ...; }

    public void update() {
       changeState();
       Iterator it = handlers.iterator();
       while (it.hasNext())
          it.next().handle();
    }
 }
 

Summary

Public Constructors

            CopyOnWriteArraySet()
Creates an empty set.
            CopyOnWriteArraySet(Collection<? extends E> c)
Creates a set containing all of the elements of the specified Collection.

Public Methods

          boolean  add(E o)
If the specified element is not contained within this collection, and addition of this element succeeds, then true will be returned.
          boolean  addAll(Collection<? extends E> c)
Adds the objects in the specified Collection to this Collection.
          void  clear()
Removes all the elements in this collection.
          boolean  contains(Object o)
Searches this Collection for the specified object.
          boolean  containsAll(Collection<?> c)
Searches this Collection for all objects in the specified Collection.
          boolean  isEmpty()
Returns true if the collection has no element, otherwise false.
          Iterator<E>  iterator()
Returns an Iterator on the elements of this Collection.
          boolean  remove(Object o)
Removes the first occurrence of the specified object from this Collection.
          boolean  removeAll(Collection<?> c)
Removes all occurrences in this Collection of each object in the specified Collection.
          boolean  retainAll(Collection<?> c)
Removes all objects from this Collection that are not contained in the specified Collection.
          int  size()
Returns the number of elements in this Collection.
        <T>  T[]  toArray(T[] a)
Returns an array containing all elements contained in this Collection.
          Object[]  toArray()
Returns a new array containing all elements contained in this Collection.
Methods inherited from class java.util.AbstractSet
Methods inherited from class java.util.AbstractCollection
Methods inherited from class java.lang.Object
Methods inherited from interface java.lang.Iterable
Methods inherited from interface java.util.Collection
Methods inherited from interface java.util.Set

Details

Public Constructors

public CopyOnWriteArraySet()

Creates an empty set.

public CopyOnWriteArraySet(Collection<? extends E> c)

Creates a set containing all of the elements of the specified Collection.

Parameters

c the collection

Public Methods

public boolean add(E o)

If the specified element is not contained within this collection, and addition of this element succeeds, then true will be returned. If the specified element is already contained within this collection, or duplication is not permitted, false will be returned. Different implementations may add specific limitations on this method to filter permitted elements. For example, in some implementation, null element may be denied, and NullPointerException will be thrown out. These limitations should be explicitly documented by specific collection implementation. Add operation is not supported in this implementation, and UnsupportedOperationException will always be thrown out.

Parameters

o the element to be added.

Returns

  • true if the collection is changed successfully after invoking this method. Otherwise, false.

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

Adds the objects in the specified Collection to this Collection.

Parameters

c the Collection of objects

Returns

  • true if this Collection is modified, false otherwise

public void clear()

Removes all the elements in this collection. This collection will be cleared up after this operation. The operation iterates over the collection, removes every element using Iterator.remove method. UnsupportedOperationException will be thrown out if the iterator returned by this collection does not implement the remove method and the collection is not zero length.

public boolean contains(Object o)

Searches this Collection for the specified object.

Parameters

o the object to search for

Returns

  • true if object is an element of this Collection, false otherwise

public boolean containsAll(Collection<?> c)

Searches this Collection for all objects in the specified Collection.

Parameters

c the Collection of objects

Returns

  • true if all objects in the specified Collection are elements of this Collection, false otherwise

public boolean isEmpty()

Returns true if the collection has no element, otherwise false.

Returns

  • true if the collection has no element.

public Iterator<E> iterator()

Returns an Iterator on the elements of this Collection. A subclass must implement the abstract methods iterator() and size().

Returns

  • an Iterator on the elements of this Collection

public boolean remove(Object o)

Removes the first occurrence of the specified object from this Collection. This operation traverses over the collection, looking for the specified object. Once the object is found, the object will be removed from the collection using the iterator's remove method. This collection will throw an UnsupportedOperationException if the iterator returned does not implement remove method, and the specified object is in this collection.

Parameters

o the object to remove

Returns

  • true if this Collection is modified, false otherwise

public boolean removeAll(Collection<?> c)

Removes all occurrences in this Collection of each object in the specified Collection.

Parameters

c the Collection of objects to remove

Returns

  • true if this Collection is modified, false otherwise

public boolean retainAll(Collection<?> c)

Removes all objects from this Collection that are not contained in the specified Collection. This operation traverses over the collection itself, to verify whether any element is contained in the specified collection. The object will be removed from the collection itself using the iterator's remove method if it is not contained in the specified collection. This collection will throw an UnsupportedOperationException if the iterator returned does not implement remove method, and the collection itself does contain elements which do not exist in the specified collection.

Parameters

c the Collection of objects to retain

Returns

  • true if this Collection is modified, false otherwise

public int size()

Returns the number of elements in this Collection.

Returns

  • the number of elements in this Collection

public T[] toArray(T[] a)

Returns an array containing all elements contained in this Collection. If the specified array is large enough to hold the elements, the specified array is used, otherwise an array of the same type is created. If the specified array is used and is larger than this Collection, the array element following the collection elements is set to null.

Parameters

a the array

Returns

  • an array of the elements from this Collection

public Object[] toArray()

Returns a new array containing all elements contained in this Collection. All the elements in the array will not be referenced by the collection. The elements in the returned array will be sorted to the same order as those returned by the iterator of this collection itself if the collection guarantees the order.

Returns

  • an array of the elements from this Collection
Copyright 2007 Google Inc. Build 0.9_r1-98467 - 14 Aug 2008 18:48