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1   /*
2    *  Copyright 2001-2004 The Apache Software Foundation
3    *
4    *  Licensed under the Apache License, Version 2.0 (the "License");
5    *  you may not use this file except in compliance with the License.
6    *  You may obtain a copy of the License at
7    *
8    *      http://www.apache.org/licenses/LICENSE-2.0
9    *
10   *  Unless required by applicable law or agreed to in writing, software
11   *  distributed under the License is distributed on an "AS IS" BASIS,
12   *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13   *  See the License for the specific language governing permissions and
14   *  limitations under the License.
15   */
16  package net.sf.collections15.functors.transformer;
17  
18  import net.sf.collections15.Transformer;
19  
20  import java.io.Serializable;
21  
22  /***
23   * <code>Transformer</code> implementation that returns the same constant each
24   * time.
25   * <p/>
26   * No check is made that the object is immutable. In general, only immutable
27   * objects should use the <code>ConstantTransformer</code>. Mutable objects
28   * should use the {@link FactoryTransformer}, seeded with a {@link
29   * net.sf.collections15.functors.factory.PrototypeFactory PrototypeFactory}
30   *
31   * @author Stephen Colebourne
32   * @author Chris Lambrou (port to Java 5.0)
33   * @since Collections15 1.0
34   */
35  public class ConstantTransformer <I, O> implements Transformer<I, O>, Serializable
36  {
37  
38      static final long serialVersionUID = 6266774234958279132L;
39  
40      /***
41       * The constant returned by the transform.
42       */
43      private final O constant;
44  
45      /***
46       * Returns an instance whose {@link #transform} method always returns the
47       * specified constant.
48       *
49       * @param constant The constant to be returned each time the {@link
50       *                 #transform} method is invoked. May be <code>null</code>.
51       *
52       * @return An instance whose {@link #transform} method always returns the
53       *         specified constant.
54       */
55      public static <I, O> ConstantTransformer<I, O> getInstance(O constant)
56      {
57          return new ConstantTransformer<I, O>(constant);
58      }
59  
60      /***
61       * Creates a new instance whose {@link #transform} method always returns the
62       * specified constant.
63       *
64       * @param constant The constant to be returned each time the {@link
65       *                 #transform} method is invoked. May be <code>null</code>.
66       */
67      protected ConstantTransformer(O constant)
68      {
69          this.constant = constant;
70      }
71  
72      /***
73       * Transforms the input by ignoring it and returning the stored constant
74       * instead.
75       *
76       * @param input The input object which is ignored.
77       *
78       * @return The stored constant.
79       */
80      public O transform(I input)
81      {
82          return constant;
83      }
84  
85      /***
86       * Returns the constant used by this <code>Transformer</code>.
87       *
88       * @return The constant used by this <code>Transformer</code>.
89       *
90       * @since Collections15 1.0
91       */
92      public O getConstant()
93      {
94          return constant;
95      }
96  
97  }