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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  import java.util.HashSet;
21  import java.util.Set;
22  
23  import org.apache.commons.collections4.Predicate;
24  
25  /**
26   * Predicate implementation that returns true the first time an object is
27   * passed into the predicate.
28   *
29   * @since 3.0
30   */
31  public final class UniquePredicate<T> implements Predicate<T>, Serializable {
32  
33      /** Serial version UID */
34      private static final long serialVersionUID = -3319417438027438040L;
35  
36      /** The set of previously seen objects */
37      private final Set<T> iSet = new HashSet<>();
38  
39      /**
40       * Factory to create the predicate.
41       *
42       * @param <T> the type that the predicate queries
43       * @return the predicate
44       * @throws IllegalArgumentException if the predicate is null
45       */
46      public static <T> Predicate<T> uniquePredicate() {
47          return new UniquePredicate<>();
48      }
49  
50      /**
51       * Constructor that performs no validation.
52       * Use <code>uniquePredicate</code> if you want that.
53       */
54      public UniquePredicate() {
55          super();
56      }
57  
58      /**
59       * Evaluates the predicate returning true if the input object hasn't been
60       * received yet.
61       *
62       * @param object  the input object
63       * @return true if this is the first time the object is seen
64       */
65      @Override
66      public boolean evaluate(final T object) {
67          return iSet.add(object);
68      }
69  
70  }