1   /*
2    * Copyright (C) 2007 The Guava Authors
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  
17  package com.google.common.base;
18  
19  import com.google.common.annotations.GwtCompatible;
20  
21  import javax.annotation.Nullable;
22  
23  /**
24   * Determines an output value based on an input value.
25   *
26   * <p>The {@link Functions} class provides common functions and related utilites.
27   *
28   * <p>See the Guava User Guide article on <a href=
29   * "http://code.google.com/p/guava-libraries/wiki/FunctionalExplained">the use of {@code
30   * Function}</a>.
31   *
32   * @author Kevin Bourrillion
33   * @since 2.0 (imported from Google Collections Library)
34   */
35  @GwtCompatible
36  public interface Function<F, T> {
37    /**
38     * Returns the result of applying this function to {@code input}. This method is <i>generally
39     * expected</i>, but not absolutely required, to have the following properties:
40     *
41     * <ul>
42     * <li>Its execution does not cause any observable side effects.
43     * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal
44     *     Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a),
45     *     function.apply(b))}.
46     * </ul>
47     *
48     * @throws NullPointerException if {@code input} is null and this function does not accept null
49     *     arguments
50     */
51    @Nullable T apply(@Nullable F input);
52  
53    /**
54     * Indicates whether another object is equal to this function.
55     *
56     * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.
57     * However, an implementation may also choose to return {@code true} whenever {@code object} is a
58     * {@link Function} that it considers <i>interchangeable</i> with this one. "Interchangeable"
59     * <i>typically</i> means that {@code Objects.equal(this.apply(f), that.apply(f))} is true for all
60     * {@code f} of type {@code F}. Note that a {@code false} result from this method does not imply
61     * that the functions are known <i>not</i> to be interchangeable.
62     */
63    @Override
64    boolean equals(@Nullable Object object);
65  }