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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.
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/*
 * 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.Collection;
import java.util.Set;

/**
 * The "read" subset of the {@link java.util.Map} interface.
 *
 * @param  the type of the keys in this map
 * @param  the type of the values in this map
 *
 * @since 4.0
 * @see Put
 */
public interface Get {

    /**
     * @param key key whose presence in this map is to be tested
     * @return true if this map contains a mapping for the specified
     *         key
     * @see java.util.Map#containsKey(Object)
     */
    boolean containsKey(Object key);

    /**
     * @param value value whose presence in this map is to be tested
     * @return true if this map maps one or more keys to the
     *         specified value
     * @see java.util.Map#containsValue(Object)
     */
    boolean containsValue(Object value);

    /**
     * @return a set view of the mappings contained in this map
     * @see java.util.Map#entrySet()
     */
    Set> entrySet();

    /**
     * @param key the key whose associated value is to be returned
     * @return the value to which the specified key is mapped, or
     *         {@code null} if this map contains no mapping for the key
     * @see java.util.Map#get(Object)
     */
    V get(Object key);

    /**
     * @param key key whose mapping is to be removed from the map
     * @return the previous value associated with key, or
     *         null if there was no mapping for key.
     * @see java.util.Map#remove(Object)
     */
    V remove(Object key);

    /**
     * @return true if this map contains no key-value mappings
     * @see java.util.Map#isEmpty()
     */
    boolean isEmpty();

    /**
     * @return a set view of the keys contained in this map
     * @see java.util.Map#keySet()
     */
    Set keySet();

    /**
     * @return the number of key-value mappings in this map
     * @see java.util.Map#size()
     */
    int size();

    /**
     * @return a collection view of the values contained in this map
     * @see java.util.Map#values()
     */
    Collection values();

}