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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.
There is a newer version: 4.5.0-M2
/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.collections4.multiset;

import java.util.Set;

import org.apache.commons.collections4.MultiSet;
import org.apache.commons.collections4.collection.SynchronizedCollection;

/**
 * Decorates another {@link MultiSet} to synchronize its behaviour
 * for a multi-threaded environment.
 * 
 * Methods are synchronized, then forwarded to the decorated multiset.
 * Iterators must be separately synchronized around the loop.
 *
 * @since 4.1
 * @version $Id: SynchronizedMultiSet.java 1714424 2015-11-15 10:06:16Z tn $
 */
public class SynchronizedMultiSet extends SynchronizedCollection implements MultiSet {

    /** Serialization version */
    private static final long serialVersionUID = 20150629L;

    /**
     * Factory method to create a synchronized multiset.
     *
     * @param  the type of the elements in the multiset
     * @param multiset  the multiset to decorate, must not be null
     * @return a new synchronized MultiSet
     * @throws NullPointerException if multiset is null
     */
    public static  SynchronizedMultiSet synchronizedMultiSet(final MultiSet multiset) {
        return new SynchronizedMultiSet(multiset);
    }

    //-----------------------------------------------------------------------
    /**
     * Constructor that wraps (not copies).
     *
     * @param multiset  the multiset to decorate, must not be null
     * @throws NullPointerException if multiset is null
     */
    protected SynchronizedMultiSet(final MultiSet multiset) {
        super(multiset);
    }

    /**
     * Constructor that wraps (not copies).
     *
     * @param multiset  the multiset to decorate, must not be null
     * @param lock  the lock to use, must not be null
     * @throws NullPointerException if multiset or lock is null
     */
    protected SynchronizedMultiSet(final MultiSet multiset, final Object lock) {
        super(multiset, lock);
    }

    /**
     * Gets the multiset being decorated.
     *
     * @return the decorated multiset
     */
    @Override
    protected MultiSet decorated() {
        return (MultiSet) super.decorated();
    }

    @Override
    public boolean equals(final Object object) {
        if (object == this) {
            return true;
        }
        synchronized (lock) {
            return decorated().equals(object);
        }
    }

    @Override
    public int hashCode() {
        synchronized (lock) {
            return decorated().hashCode();
        }
    }

    //-----------------------------------------------------------------------

    @Override
    public int add(final E object, final int count) {
        synchronized (lock) {
            return decorated().add(object, count);
        }
    }

    @Override
    public int remove(final Object object, final int count) {
        synchronized (lock) {
            return decorated().remove(object, count);
        }
    }

    @Override
    public int getCount(final Object object) {
        synchronized (lock) {
            return decorated().getCount(object);
        }
    }

    @Override
    public int setCount(E object, int count) {
        synchronized (lock) {
            return decorated().setCount(object, count);
        }
    }

    @Override
    public Set uniqueSet() {
        synchronized (lock) {
            final Set set = decorated().uniqueSet();
            return new SynchronizedSet(set, lock);
        }
    }

    @Override
    public Set> entrySet() {
        synchronized (lock) {
            final Set> set = decorated().entrySet();
            return new SynchronizedSet>(set, lock);
        }
    }

    //-----------------------------------------------------------------------
    /**
     * Synchronized Set for the MultiSet class.
     */
    static class SynchronizedSet extends SynchronizedCollection implements Set {
        /** Serialization version */
        private static final long serialVersionUID = 20150629L;

        /**
         * Constructor.
         * @param set  the set to decorate
         * @param lock  the lock to use, shared with the multiset
         */
        SynchronizedSet(final Set set, final Object lock) {
            super(set, lock);
        }
    }

}