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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.multimap;

import java.util.Collections;
import java.util.Map;
import java.util.Set;

import org.apache.commons.collections4.SetUtils;
import org.apache.commons.collections4.SetValuedMap;

/**
 * Abstract implementation of the {@link SetValuedMap} interface to simplify the
 * creation of subclass implementations.
 * 
 * Subclasses specify a Map implementation to use as the internal storage and
 * the Set implementation to use as values.
 * 
 *
 * @param  the type of the keys in this map
 * @param  the type of the values in this map
 * @since 4.1
 */
public abstract class AbstractSetValuedMap extends AbstractMultiValuedMap
    implements SetValuedMap {

    /**
     * Constructor needed for subclass serialisation.
     */
    protected AbstractSetValuedMap() {
        super();
    }

    /**
     * A constructor that wraps, not copies
     *
     * @param map  the map to wrap, must not be null
     * @throws NullPointerException if the map is null
     */
    protected AbstractSetValuedMap(final Map> map) {
        super(map);
    }

    // -----------------------------------------------------------------------
    @Override
    @SuppressWarnings("unchecked")
    protected Map> getMap() {
        return (Map>) super.getMap();
    }

    /**
     * Creates a new value collection using the provided factory.
     * @return a new list
     */
    @Override
    protected abstract Set createCollection();

    // -----------------------------------------------------------------------
    /**
     * Gets the set of values associated with the specified key. This would
     * return an empty set in case the mapping is not present
     *
     * @param key  the key to retrieve
     * @return the Set of values, will return an empty
     *   Set for no mapping
     */
    @Override
    public Set get(final K key) {
        return wrappedCollection(key);
    }

    @Override
    Set wrappedCollection(final K key) {
        return new WrappedSet(key);
    }

    /**
     * Removes all values associated with the specified key.
     * 
     * A subsequent get(Object) would return an empty set.
     *
     * @param key the key to remove values from
     * @return the Set of values removed, will return an empty,
     *   unmodifiable set for no mapping found.
     */
    @Override
    public Set remove(final Object key) {
        return SetUtils.emptyIfNull(getMap().remove(key));
    }

    // -----------------------------------------------------------------------
    /**
     * Wrapped set to handle add and remove on the collection returned by
     * {@code get(Object)}.
     */
    private class WrappedSet extends WrappedCollection implements Set {

        public WrappedSet(final K key) {
            super(key);
        }

        @Override
        public boolean equals(final Object other) {
            final Set set = (Set) getMapping();
            if (set == null) {
                return Collections.emptySet().equals(other);
            }
            if (!(other instanceof Set)) {
                return false;
            }
            final Set otherSet = (Set) other;
            return SetUtils.isEqualSet(set, otherSet);
        }

        @Override
        public int hashCode() {
            final Set set = (Set) getMapping();
            return SetUtils.hashCodeForSet(set);
        }

    }

}