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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;

import java.util.Set;

/**
 * Defines a map that holds a set of values against each key.
 * 
 * A {@code SetValuedMap} is a Map with slightly different semantics:
 * 

 *   Putting a value into the map will add the value to a {@link Set} at that key.
 *   Getting a value will return a {@link Set}, holding all the values put to that key.
 * 
 *
 * @since 4.1
 * @version $Id: SetValuedMap.java 1685299 2015-06-13 18:27:11Z tn $
 */
public interface SetValuedMap extends MultiValuedMap {

    /**
     * Gets the set of values associated with the specified key.
     * 
     * Implementations typically return an empty {@code Set} if no values
     * have been mapped to the key.
     * 

     *
     * @param key  the key to retrieve
     * @return the {@code Set} of values, implementations should return an
     *   empty {@code Set} for no mapping
     * @throws NullPointerException if the key is null and null keys are invalid
     */
    @Override
    Set get(K key);

    /**
     * Removes all values associated with the specified key.
     * 
     * The returned set may be modifiable, but updates will not be
     * propagated to this set-valued map. In case no mapping was stored for the
     * specified key, an empty, unmodifiable set will be returned.
     *
     * @param key  the key to remove values from
     * @return the {@code Set} of values removed, implementations should
     *   return null for no mapping found, but may return an empty collection
     * @throws UnsupportedOperationException if the map is unmodifiable
     * @throws NullPointerException if the key is null and null keys are invalid
     */
    @Override
    Set remove(Object key);
}