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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-M1

/*
 * 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.functors;

import java.io.Serializable;
import java.util.Map;

import org.apache.commons.collections4.Transformer;

/**
 * Transformer implementation that returns the value held in a specified map
 * using the input parameter as a key.
 *
 * @since 3.0
 */
public final class MapTransformer implements Transformer, Serializable {

    /** Serial version UID */
    private static final long serialVersionUID = 862391807045468939L;

    /** The map of data to lookup in */
    private final Map iMap;

    /**
     * Factory to create the transformer.
     * 
     * If the map is null, a transformer that always returns null is returned.
     *
     * @param   the input type
     * @param   the output type
     * @param map the map, not cloned
     * @return the transformer
     */
    public static  Transformer mapTransformer(final Map map) {
        if (map == null) {
            return ConstantTransformer.nullTransformer();
        }
        return new MapTransformer<>(map);
    }

    /**
     * Constructor that performs no validation.
     * Use mapTransformer if you want that.
     *
     * @param map  the map to use for lookup, not cloned
     */
    private MapTransformer(final Map map) {
        super();
        iMap = map;
    }

    /**
     * Transforms the input to result by looking it up in a Map.
     *
     * @param input  the input object to transform
     * @return the transformed result
     */
    @Override
    public O transform(final I input) {
        return iMap.get(input);
    }

    /**
     * Gets the map to lookup in.
     *
     * @return the map
     * @since 3.1
     */
    public Map getMap() {
        return iMap;
    }

}