/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.util.*;
// BEGIN android-note
// removed link to collections framework docs
// END android-note
/**
* A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList}
* for all of its operations. Thus, it shares the same basic properties:
* <ul>
* <li>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.
* <li>It is thread-safe.
* <li>Mutative operations ({@code add}, {@code set}, {@code remove}, etc.)
* are expensive since they usually entail copying the entire underlying
* array.
* <li>Iterators do not support the mutative {@code remove} operation.
* <li>Traversal via iterators is fast and cannot encounter
* interference from other threads. Iterators rely on
* unchanging snapshots of the array at the time the iterators were
* constructed.
* </ul>
*
* <p><b>Sample Usage.</b> The following code sketch uses a
* copy-on-write set to maintain a set of Handler objects that
* perform some action upon state updates.
*
* <pre> {@code
* 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();
* for (Handler handler : handlers)
* handler.handle();
* }
* }}</pre>
*
* @see CopyOnWriteArrayList
* @since 1.5
* @author Doug Lea
* @param <E> the type of elements held in this collection
*/
public class CopyOnWriteArraySet<E> extends AbstractSet<E>
implements java.io.Serializable {
private static final long serialVersionUID = 5457747651344034263L;
private final CopyOnWriteArrayList<E> al;
/**
* Creates an empty set.
*/
public CopyOnWriteArraySet() {
al = new CopyOnWriteArrayList<E>();
}
/**
* Creates a set containing all of the elements of the specified
* collection.
*
* @param c the collection of elements to initially contain
* @throws NullPointerException if the specified collection is null
*/
public CopyOnWriteArraySet(Collection<? extends E> c) {
al = new CopyOnWriteArrayList<E>();
al.addAllAbsent(c);
}
/**
* Returns the number of elements in this set.
*
* @return the number of elements in this set
*/
public int size() {
return al.size();
}
/**
* Returns {@code true} if this set contains no elements.
*
* @return {@code true} if this set contains no elements
*/
public boolean isEmpty() {
return al.isEmpty();
}
/**
* Returns {@code true} if this set contains the specified element.
* More formally, returns {@code true} if and only if this set
* contains an element {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @param o element whose presence in this set is to be tested
* @return {@code true} if this set contains the specified element
*/
public boolean contains(Object o) {
return al.contains(o);
}
/**
* Returns an array containing all of the elements in this set.
* If this set makes any guarantees as to what order its elements
* are returned by its iterator, this method must return the
* elements in the same order.
*
* <p>The returned array will be "safe" in that no references to it
* are maintained by this set. (In other words, this method must
* allocate a new array even if this set is backed by an array).
* The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all the elements in this set
*/
public Object[] toArray() {
return al.toArray();
}
/**
* Returns an array containing all of the elements in this set; the
* runtime type of the returned array is that of the specified array.
* If the set 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 set.
*
* <p>If this set fits in the specified array with room to spare
* (i.e., the array has more elements than this set), the element in
* the array immediately following the end of the set is set to
* {@code null}. (This is useful in determining the length of this
* set <i>only</i> if the caller knows that this set does not contain
* any null elements.)
*
* <p>If this set makes any guarantees as to what order its elements
* are returned by its iterator, this method must return the elements
* in the same order.
*
* <p>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.
*
* <p>Suppose {@code x} is a set known to contain only strings.
* The following code can be used to dump the set into a newly allocated
* array of {@code String}:
*
* <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @param a the array into which the elements of this set 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 the elements in this set
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in this
* set
* @throws NullPointerException if the specified array is null
*/
public <T> T[] toArray(T[] a) {
return al.toArray(a);
}
/**
* Removes all of the elements from this set.
* The set will be empty after this call returns.
*/
public void clear() {
al.clear();
}
/**
* Removes the specified element from this set if it is present.
* More formally, removes an element {@code e} such that
* <tt>(o==null ? e==null : o.equals(e))</tt>,
* if this set contains such an element. Returns {@code true} if
* this set contained the element (or equivalently, if this set
* changed as a result of the call). (This set will not contain the
* element once the call returns.)
*
* @param o object to be removed from this set, if present
* @return {@code true} if this set contained the specified element
*/
public boolean remove(Object o) {
return al.remove(o);
}
/**
* Adds the specified element to this set if it is not already present.
* More formally, adds the specified element {@code e} to this set if
* the set contains no element {@code e2} such that
* <tt>(e==null ? e2==null : e.equals(e2))</tt>.
* If this set already contains the element, the call leaves the set
* unchanged and returns {@code false}.
*
* @param e element to be added to this set
* @return {@code true} if this set did not already contain the specified
* element
*/
public boolean add(E e) {
return al.addIfAbsent(e);
}
/**
* Returns {@code true} if this set contains all of the elements of the
* specified collection. If the specified collection is also a set, this
* method returns {@code true} if it is a <i>subset</i> of this set.
*
* @param c collection to be checked for containment in this set
* @return {@code true} if this set contains all of the elements of the
* specified collection
* @throws NullPointerException if the specified collection is null
* @see #contains(Object)
*/
public boolean containsAll(Collection<?> c) {
return al.containsAll(c);
}
/**
* Adds all of the elements in the specified collection to this set if
* they're not already present. If the specified collection is also a
* set, the {@code addAll} operation effectively modifies this set so
* that its value is the <i>union</i> of the two sets. The behavior of
* this operation is undefined if the specified collection is modified
* while the operation is in progress.
*
* @param c collection containing elements to be added to this set
* @return {@code true} if this set changed as a result of the call
* @throws NullPointerException if the specified collection is null
* @see #add(Object)
*/
public boolean addAll(Collection<? extends E> c) {
return al.addAllAbsent(c) > 0;
}
/**
* Removes from this set all of its elements that are contained in the
* specified collection. If the specified collection is also a set,
* this operation effectively modifies this set so that its value is the
* <i>asymmetric set difference</i> of the two sets.
*
* @param c collection containing elements to be removed from this set
* @return {@code true} if this set changed as a result of the call
* @throws ClassCastException if the class of an element of this set
* is incompatible with the specified collection (optional)
* @throws NullPointerException if this set contains a null element and the
* specified collection does not permit null elements (optional),
* or if the specified collection is null
* @see #remove(Object)
*/
public boolean removeAll(Collection<?> c) {
return al.removeAll(c);
}
/**
* Retains only the elements in this set that are contained in the
* specified collection. In other words, removes from this set all of
* its elements that are not contained in the specified collection. If
* the specified collection is also a set, this operation effectively
* modifies this set so that its value is the <i>intersection</i> of the
* two sets.
*
* @param c collection containing elements to be retained in this set
* @return {@code true} if this set changed as a result of the call
* @throws ClassCastException if the class of an element of this set
* is incompatible with the specified collection (optional)
* @throws NullPointerException if this set contains a null element and the
* specified collection does not permit null elements (optional),
* or if the specified collection is null
* @see #remove(Object)
*/
public boolean retainAll(Collection<?> c) {
return al.retainAll(c);
}
/**
* Returns an iterator over the elements contained in this set
* in the order in which these elements were added.
*
* <p>The returned iterator provides a snapshot of the state of the set
* when the iterator was constructed. No synchronization is needed while
* traversing the iterator. The iterator does <em>NOT</em> support the
* {@code remove} method.
*
* @return an iterator over the elements in this set
*/
public Iterator<E> iterator() {
return al.iterator();
}
/**
* Compares the specified object with this set for equality.
* Returns {@code true} if the specified object is the same object
* as this object, or if it is also a {@link Set} and the elements
* returned by an {@linkplain List#iterator() iterator} over the
* specified set are the same as the elements returned by an
* iterator over this set. More formally, the two iterators are
* considered to return the same elements if they return the same
* number of elements and for every element {@code e1} returned by
* the iterator over the specified set, there is an element
* {@code e2} returned by the iterator over this set such that
* {@code (e1==null ? e2==null : e1.equals(e2))}.
*
* @param o object to be compared for equality with this set
* @return {@code true} if the specified object is equal to this set
*/
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Set))
return false;
Set<?> set = (Set<?>)(o);
Iterator<?> it = set.iterator();
// Uses O(n^2) algorithm that is only appropriate
// for small sets, which CopyOnWriteArraySets should be.
// Use a single snapshot of underlying array
Object[] elements = al.getArray();
int len = elements.length;
// Mark matched elements to avoid re-checking
boolean[] matched = new boolean[len];
int k = 0;
outer: while (it.hasNext()) {
if (++k > len)
return false;
Object x = it.next();
for (int i = 0; i < len; ++i) {
if (!matched[i] && eq(x, elements[i])) {
matched[i] = true;
continue outer;
}
}
return false;
}
return k == len;
}
/**
* Tests for equality, coping with nulls.
*/
private static boolean eq(Object o1, Object o2) {
return (o1 == null) ? o2 == null : o1.equals(o2);
}
}