1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.collections.functors;
18
19 import java.io.Serializable;
20
21 import org.apache.commons.collections.Transformer;
22
23 /**
24 * Transformer implementation that returns the same constant each time.
25 * <p>
26 * No check is made that the object is immutable. In general, only immutable
27 * objects should use the constant factory. Mutable objects should
28 * use the prototype factory.
29 *
30 * @since 3.0
31 * @version $Id: ConstantTransformer.java 1435965 2013-01-20 21:13:18Z tn $
32 */
33 public class ConstantTransformer<I, O> implements Transformer<I, O>, Serializable {
34
35 /** Serial version UID */
36 private static final long serialVersionUID = 6374440726369055124L;
37
38 /** Returns null each time */
39 public static final Transformer<Object, Object> NULL_INSTANCE = new ConstantTransformer<Object, Object>(null);
40
41 /** The closures to call in turn */
42 private final O iConstant;
43
44 /**
45 * Get a typed null instance.
46 *
47 * @param <I> the input type
48 * @param <O> the output type
49 * @return Transformer<I, O> that always returns null.
50 */
51 @SuppressWarnings("unchecked")
52 public static <I, O> Transformer<I, O> nullTransformer() {
53 return (Transformer<I, O>) NULL_INSTANCE;
54 }
55
56 /**
57 * Transformer method that performs validation.
58 *
59 * @param <I> the input type
60 * @param <O> the output type
61 * @param constantToReturn the constant object to return each time in the factory
62 * @return the <code>constant</code> factory.
63 */
64 public static <I, O> Transformer<I, O> constantTransformer(final O constantToReturn) {
65 if (constantToReturn == null) {
66 return nullTransformer();
67 }
68 return new ConstantTransformer<I, O>(constantToReturn);
69 }
70
71 /**
72 * Constructor that performs no validation.
73 * Use <code>getInstance</code> if you want that.
74 *
75 * @param constantToReturn the constant to return each time
76 */
77 public ConstantTransformer(final O constantToReturn) {
78 super();
79 iConstant = constantToReturn;
80 }
81
82 /**
83 * Transforms the input by ignoring it and returning the stored constant instead.
84 *
85 * @param input the input object which is ignored
86 * @return the stored constant
87 */
88 public O transform(final I input) {
89 return iConstant;
90 }
91
92 /**
93 * Gets the constant.
94 *
95 * @return the constant
96 * @since 3.1
97 */
98 public O getConstant() {
99 return iConstant;
100 }
101
102 /**
103 * {@inheritDoc}
104 */
105 @Override
106 public boolean equals(final Object obj) {
107 if (obj == this) {
108 return true;
109 }
110 if (obj instanceof ConstantTransformer == false) {
111 return false;
112 }
113 final Object otherConstant = ((ConstantTransformer<?, ?>) obj).getConstant();
114 return otherConstant == getConstant() || otherConstant != null && otherConstant.equals(getConstant());
115 }
116
117 /**
118 * {@inheritDoc}
119 */
120 @Override
121 public int hashCode() {
122 int result = "ConstantTransformer".hashCode() << 2;
123 if (getConstant() != null) {
124 result |= getConstant().hashCode();
125 }
126 return result;
127 }
128 }