Java example source code file (CopyOnWriteArraySet.java)

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Java - Java tags/keywords

abstractset, copyonwritearraylist, copyonwritearrayset, iterator, object, set, spliterator, suppresswarnings, util

The CopyOnWriteArraySet.java Java example source code

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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.Collection;
import java.util.Set;
import java.util.AbstractSet;
import java.util.Iterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Predicate;
import java.util.function.Consumer;

/**
 * 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>Sample Usage. 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>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @see CopyOnWriteArrayList
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this collection
 */
public class CopyOnWriteArraySet<E> extends AbstractSet
        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) {
        if (c.getClass() == CopyOnWriteArraySet.class) {
            @SuppressWarnings("unchecked") CopyOnWriteArraySet<E> cc =
                (CopyOnWriteArraySet<E>)c;
            al = new CopyOnWriteArrayList<E>(cc.al);
        }
        else {
            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)).
     *
     * @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 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]);}

* * 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)), * 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)). * 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 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 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 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 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 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 Set#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; } public boolean removeIf(Predicate<? super E> filter) { return al.removeIf(filter); } public void forEach(Consumer<? super E> action) { al.forEach(action); } /** * Returns a {@link Spliterator} over the elements in this set in the order * in which these elements were added. * * <p>The {@code Spliterator} reports {@link Spliterator#IMMUTABLE}, * {@link Spliterator#DISTINCT}, {@link Spliterator#SIZED}, and * {@link Spliterator#SUBSIZED}. * * <p>The spliterator provides a snapshot of the state of the set * when the spliterator was constructed. No synchronization is needed while * operating on the spliterator. * * @return a {@code Spliterator} over the elements in this set * @since 1.8 */ public Spliterator<E> spliterator() { return Spliterators.spliterator (al.getArray(), Spliterator.IMMUTABLE | Spliterator.DISTINCT); } /** * Tests for equality, coping with nulls. */ private static boolean eq(Object o1, Object o2) { return (o1 == null) ? o2 == null : o1.equals(o2); } }