001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.base;
018
019import com.google.common.annotations.GwtCompatible;
020
021import javax.annotation.Nullable;
022
023/**
024 * Determines an output value based on an input value.
025 *
026 * <p>The {@link Functions} class provides common functions and related utilites.
027 *
028 * <p>See the Guava User Guide article on <a href=
029 * "https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code
030 * Function}</a>.
031 *
032 * @author Kevin Bourrillion
033 * @since 2.0
034 */
035@GwtCompatible
036public interface Function<F, T> {
037  /**
038   * Returns the result of applying this function to {@code input}. This method is <i>generally
039   * expected</i>, but not absolutely required, to have the following properties:
040   *
041   * <ul>
042   * <li>Its execution does not cause any observable side effects.
043   * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal
044   *     Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a),
045   *     function.apply(b))}.
046   * </ul>
047   *
048   * @throws NullPointerException if {@code input} is null and this function does not accept null
049   *     arguments
050   */
051  @Nullable
052  T apply(@Nullable F input);
053
054  /**
055   * Indicates whether another object is equal to this function.
056   *
057   * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.
058   * However, an implementation may also choose to return {@code true} whenever {@code object} is a
059   * {@link Function} that it considers <i>interchangeable</i> with this one. "Interchangeable"
060   * <i>typically</i> means that {@code Objects.equal(this.apply(f), that.apply(f))} is true for all
061   * {@code f} of type {@code F}. Note that a {@code false} result from this method does not imply
062   * that the functions are known <i>not</i> to be interchangeable.
063   */
064  @Override
065  boolean equals(@Nullable Object object);
066}