001/* 002 * Copyright (C) 2007 The Guava Authors 003 * 004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except 005 * in compliance with the License. You may obtain a copy of the License at 006 * 007 * http://www.apache.org/licenses/LICENSE-2.0 008 * 009 * Unless required by applicable law or agreed to in writing, software distributed under the License 010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express 011 * or implied. See the License for the specific language governing permissions and limitations under 012 * the License. 013 */ 014 015package com.google.common.base; 016 017import com.google.common.annotations.GwtCompatible; 018import com.google.errorprone.annotations.CanIgnoreReturnValue; 019import javax.annotation.CheckForNull; 020import org.checkerframework.checker.nullness.qual.Nullable; 021 022/** 023 * Determines an output value based on an input value; a pre-Java-8 version of {@link 024 * java.util.function.Function java.util.function.Function}. 025 * 026 * <p>The {@link Functions} class provides common functions and related utilities. 027 * 028 * <p>See the Guava User Guide article on <a 029 * href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code Function}</a>. 030 * 031 * <h3>For Java 8+ users</h3> 032 * 033 * <p>This interface is now a legacy type. Use {@code java.util.function.Function} (or the 034 * appropriate primitive specialization such as {@code ToIntFunction}) instead whenever possible. 035 * Otherwise, at least reduce <i>explicit</i> dependencies on this type by using lambda expressions 036 * or method references instead of classes, leaving your code easier to migrate in the future. 037 * 038 * <p>To use an existing function (say, named {@code function}) in a context where the <i>other 039 * type</i> of function is expected, use the method reference {@code function::apply}. A future 040 * version of {@code com.google.common.base.Function} will be made to <i>extend</i> {@code 041 * java.util.function.Function}, making conversion code necessary only in one direction. At that 042 * time, this interface will be officially discouraged. 043 * 044 * @author Kevin Bourrillion 045 * @since 2.0 046 */ 047@GwtCompatible 048@ElementTypesAreNonnullByDefault 049public interface Function<F extends @Nullable Object, T extends @Nullable Object> { 050 /** 051 * Returns the result of applying this function to {@code input}. This method is <i>generally 052 * expected</i>, but not absolutely required, to have the following properties: 053 * 054 * <ul> 055 * <li>Its execution does not cause any observable side effects. 056 * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal 057 * Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a), 058 * function.apply(b))}. 059 * </ul> 060 * 061 * @throws NullPointerException if {@code input} is null and this function does not accept null 062 * arguments 063 */ 064 @CanIgnoreReturnValue // TODO(kevinb): remove this 065 @ParametricNullness 066 T apply(@ParametricNullness F input); 067 068 /** 069 * <i>May</i> return {@code true} if {@code object} is a {@code Function} that behaves identically 070 * to this function. 071 * 072 * <p><b>Warning: do not depend</b> on the behavior of this method. 073 * 074 * <p>Historically, {@code Function} instances in this library have implemented this method to 075 * recognize certain cases where distinct {@code Function} instances would in fact behave 076 * identically. However, as code migrates to {@code java.util.function}, that behavior will 077 * disappear. It is best not to depend on it. 078 */ 079 @Override 080 boolean equals(@CheckForNull Object object); 081}