    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.collections4.functors;
    018
    019import java.io.Serializable;
    020
    021import org.apache.commons.collections4.Transformer;
    022
    023/**
    024 * Transformer implementation that returns the same constant each time.
    025 * <p>
    026 * No check is made that the object is immutable. In general, only immutable
    027 * objects should use the constant factory. Mutable objects should
    028 * use the prototype factory.
    029 * </p>
    030 *
    031 * @since 3.0
    032 */
    033public class ConstantTransformer<I, O> implements Transformer<I, O>, Serializable {
    034
    035    /** Serial version UID */
    036    private static final long serialVersionUID = 6374440726369055124L;
    037
    038    /** Returns null each time */
    039    @SuppressWarnings("rawtypes")
    040    public static final Transformer NULL_INSTANCE = new ConstantTransformer<>(null);
    041
    042    /** The closures to call in turn */
    043    private final O iConstant;
    044
    045    /**
    046     * Get a typed null instance.
    047     *
    048     * @param <I>  the input type
    049     * @param <O>  the output type
    050     * @return Transformer&lt;I, O&gt; that always returns null.
    051     */
    052    public static <I, O> Transformer<I, O> nullTransformer() {
    053        return NULL_INSTANCE;
    054    }
    055
    056    /**
    057     * Transformer method that performs validation.
    058     *
    059     * @param <I>  the input type
    060     * @param <O>  the output type
    061     * @param constantToReturn  the constant object to return each time in the factory
    062     * @return the <code>constant</code> factory.
    063     */
    064    public static <I, O> Transformer<I, O> constantTransformer(final O constantToReturn) {
    065        if (constantToReturn == null) {
    066            return nullTransformer();
    067        }
    068        return new ConstantTransformer<>(constantToReturn);
    069    }
    070
    071    /**
    072     * Constructor that performs no validation.
    073     * Use <code>constantTransformer</code> if you want that.
    074     *
    075     * @param constantToReturn  the constant to return each time
    076     */
    077    public ConstantTransformer(final O constantToReturn) {
    078        super();
    079        iConstant = constantToReturn;
    080    }
    081
    082    /**
    083     * Transforms the input by ignoring it and returning the stored constant instead.
    084     *
    085     * @param input  the input object which is ignored
    086     * @return the stored constant
    087     */
    088    @Override
    089    public O transform(final I input) {
    090        return iConstant;
    091    }
    092
    093    /**
    094     * Gets the constant.
    095     *
    096     * @return the constant
    097     * @since 3.1
    098     */
    099    public O getConstant() {
    100        return iConstant;
    101    }
    102
    103    /**
    104     * {@inheritDoc}
    105     */
    106    @Override
    107    public boolean equals(final Object obj) {
    108        if (obj == this) {
    109            return true;
    110        }
    111        if (obj instanceof ConstantTransformer == false) {
    112            return false;
    113        }
    114        final Object otherConstant = ((ConstantTransformer<?, ?>) obj).getConstant();
    115        return otherConstant == getConstant() || otherConstant != null && otherConstant.equals(getConstant());
    116    }
    117
    118    /**
    119     * {@inheritDoc}
    120     */
    121    @Override
    122    public int hashCode() {
    123        int result = "ConstantTransformer".hashCode() << 2;
    124        if (getConstant() != null) {
    125            result |= getConstant().hashCode();
    126        }
    127        return result;
    128    }
    129}