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 *
030 * @since 3.0
031 */
032public class ConstantTransformer<I, O> implements Transformer<I, O>, Serializable {
033
034    /** Serial version UID */
035    private static final long serialVersionUID = 6374440726369055124L;
036
037    /** Returns null each time */
038    @SuppressWarnings("rawtypes")
039    public static final Transformer NULL_INSTANCE = new ConstantTransformer<>(null);
040
041    /** The closures to call in turn */
042    private final O iConstant;
043
044    /**
045     * Get a typed null instance.
046     *
047     * @param <I>  the input type
048     * @param <O>  the output type
049     * @return Transformer&lt;I, O&gt; that always returns null.
050     */
051    @SuppressWarnings("unchecked") // The null transformer works for all object types
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}