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;
020import java.util.Objects;
021
022import org.apache.commons.collections4.Transformer;
023
024/**
025 * Transformer implementation that returns the same constant each time.
026 * <p>
027 * No check is made that the object is immutable. In general, only immutable
028 * objects should use the constant factory. Mutable objects should
029 * use the prototype factory.
030 * </p>
031 *
032 * @param <T> the type of the input to the function.
033 * @param <R> the type of the result of the function.
034 * @since 3.0
035 */
036public class ConstantTransformer<T, R> implements Transformer<T, R>, Serializable {
037
038    /** Serial version UID */
039    private static final long serialVersionUID = 6374440726369055124L;
040
041    /** Returns null each time */
042    @SuppressWarnings("rawtypes")
043    public static final Transformer NULL_INSTANCE = new ConstantTransformer<>(null);
044
045    /**
046     * Transformer method that performs validation.
047     *
048     * @param <I>  the input type
049     * @param <O>  the output type
050     * @param constantToReturn  the constant object to return each time in the factory
051     * @return the {@code constant} factory.
052     */
053    public static <I, O> Transformer<I, O> constantTransformer(final O constantToReturn) {
054        if (constantToReturn == null) {
055            return nullTransformer();
056        }
057        return new ConstantTransformer<>(constantToReturn);
058    }
059
060    /**
061     * Gets a typed null instance.
062     *
063     * @param <I>  the input type
064     * @param <O>  the output type
065     * @return Transformer&lt;I, O&gt; that always returns null.
066     */
067    public static <I, O> Transformer<I, O> nullTransformer() {
068        return NULL_INSTANCE;
069    }
070
071    /** The closures to call in turn */
072    private final R iConstant;
073
074    /**
075     * Constructor that performs no validation.
076     * Use {@code constantTransformer} if you want that.
077     *
078     * @param constantToReturn  the constant to return each time
079     */
080    public ConstantTransformer(final R constantToReturn) {
081        iConstant = constantToReturn;
082    }
083
084    /**
085     * {@inheritDoc}
086     */
087    @Override
088    public boolean equals(final Object obj) {
089        if (obj == this) {
090            return true;
091        }
092        if (!(obj instanceof ConstantTransformer)) {
093            return false;
094        }
095        final Object otherConstant = ((ConstantTransformer<?, ?>) obj).getConstant();
096        return Objects.equals(otherConstant, getConstant());
097    }
098
099    /**
100     * Gets the constant.
101     *
102     * @return the constant
103     * @since 3.1
104     */
105    public R getConstant() {
106        return iConstant;
107    }
108
109    /**
110     * {@inheritDoc}
111     */
112    @Override
113    public int hashCode() {
114        int result = "ConstantTransformer".hashCode() << 2;
115        if (getConstant() != null) {
116            result |= getConstant().hashCode();
117        }
118        return result;
119    }
120
121    /**
122     * Transforms the input by ignoring it and returning the stored constant instead.
123     *
124     * @param input  the input object which is ignored
125     * @return the stored constant
126     */
127    @Override
128    public R transform(final T input) {
129        return iConstant;
130    }
131}