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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;

/**
 * Defines a functor interface implemented by classes that transform one
 * object into another.
 * 
 * A Transformer converts the input object to the output object.
 * The input object should be left unchanged.
 * Transformers are typically used for type conversions, or extracting data
 * from an object.
 * 
 * 
 * Standard implementations of common transformers are provided by
 * {@link TransformerUtils}. These include method invocation, returning a constant,
 * cloning and returning the string value.
 * 
 *
 * @param  the input type to the transformer
 * @param  the output type from the transformer
 *
 * @since 1.0
 */
@FunctionalInterface
public interface Transformer {

    /**
     * Transforms the input object (leaving it unchanged) into some output object.
     *
     * @param input  the object to be transformed, should be left unchanged
     * @return a transformed object
     * @throws ClassCastException (runtime) if the input is the wrong class
     * @throws IllegalArgumentException (runtime) if the input is invalid
     * @throws FunctorException (runtime) if the transform cannot be completed
     */
    O transform(I input);

}