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.collections4.functors;
18
19 import java.io.Serializable;
20
21 import org.apache.commons.collections4.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 * </p>
30 *
31 * @since 3.0
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 @SuppressWarnings("rawtypes")
40 public static final Transformer NULL_INSTANCE = new ConstantTransformer<>(null);
41
42 /**
43 * Transformer method that performs validation.
44 *
45 * @param <I> the input type
46 * @param <O> the output type
47 * @param constantToReturn the constant object to return each time in the factory
48 * @return the {@code constant} factory.
49 */
50 public static <I, O> Transformer<I, O> constantTransformer(final O constantToReturn) {
51 if (constantToReturn == null) {
52 return nullTransformer();
53 }
54 return new ConstantTransformer<>(constantToReturn);
55 }
56
57 /**
58 * Gets a typed null instance.
59 *
60 * @param <I> the input type
61 * @param <O> the output type
62 * @return Transformer<I, O> that always returns null.
63 */
64 public static <I, O> Transformer<I, O> nullTransformer() {
65 return NULL_INSTANCE;
66 }
67
68 /** The closures to call in turn */
69 private final O iConstant;
70
71 /**
72 * Constructor that performs no validation.
73 * Use {@code constantTransformer} if you want that.
74 *
75 * @param constantToReturn the constant to return each time
76 */
77 public ConstantTransformer(final O constantToReturn) {
78 iConstant = constantToReturn;
79 }
80
81 /**
82 * {@inheritDoc}
83 */
84 @Override
85 public boolean equals(final Object obj) {
86 if (obj == this) {
87 return true;
88 }
89 if (!(obj instanceof ConstantTransformer)) {
90 return false;
91 }
92 final Object otherConstant = ((ConstantTransformer<?, ?>) obj).getConstant();
93 return otherConstant == getConstant() || otherConstant != null && otherConstant.equals(getConstant());
94 }
95
96 /**
97 * Gets the constant.
98 *
99 * @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 }