    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    /**
    043     * Transformer method that performs validation.
    044     *
    045     * @param <I>  the input type
    046     * @param <O>  the output type
    047     * @param constantToReturn  the constant object to return each time in the factory
    048     * @return the {@code constant} factory.
    049     */
    050    public static <I, O> Transformer<I, O> constantTransformer(final O constantToReturn) {
    051        if (constantToReturn == null) {
    052            return nullTransformer();
    053        }
    054        return new ConstantTransformer<>(constantToReturn);
    055    }
    056
    057    /**
    058     * Gets a typed null instance.
    059     *
    060     * @param <I>  the input type
    061     * @param <O>  the output type
    062     * @return Transformer&lt;I, O&gt; that always returns null.
    063     */
    064    public static <I, O> Transformer<I, O> nullTransformer() {
    065        return NULL_INSTANCE;
    066    }
    067
    068    /** The closures to call in turn */
    069    private final O iConstant;
    070
    071    /**
    072     * Constructor that performs no validation.
    073     * Use {@code constantTransformer} if you want that.
    074     *
    075     * @param constantToReturn  the constant to return each time
    076     */
    077    public ConstantTransformer(final O constantToReturn) {
    078        iConstant = constantToReturn;
    079    }
    080
    081    /**
    082     * {@inheritDoc}
    083     */
    084    @Override
    085    public boolean equals(final Object obj) {
    086        if (obj == this) {
    087            return true;
    088        }
    089        if (!(obj instanceof ConstantTransformer)) {
    090            return false;
    091        }
    092        final Object otherConstant = ((ConstantTransformer<?, ?>) obj).getConstant();
    093        return otherConstant == getConstant() || otherConstant != null && otherConstant.equals(getConstant());
    094    }
    095
    096    /**
    097     * Gets the constant.
    098     *
    099     * @return the constant
    100     * @since 3.1
    101     */
    102    public O getConstant() {
    103        return iConstant;
    104    }
    105
    106    /**
    107     * {@inheritDoc}
    108     */
    109    @Override
    110    public int hashCode() {
    111        int result = "ConstantTransformer".hashCode() << 2;
    112        if (getConstant() != null) {
    113            result |= getConstant().hashCode();
    114        }
    115        return result;
    116    }
    117
    118    /**
    119     * Transforms the input by ignoring it and returning the stored constant instead.
    120     *
    121     * @param input  the input object which is ignored
    122     * @return the stored constant
    123     */
    124    @Override
    125    public O transform(final I input) {
    126        return iConstant;
    127    }
    128}