Function.java revision 0888a09821a98ac0680fad765217302858e70fa4 
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
17package com.google.common.base;
18
19import com.google.common.annotations.GwtCompatible;
20
21import 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
36public 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}
66