1 /* 2 * Copyright (C) 2007 The Guava Authors 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except 5 * in compliance with the License. You may obtain a copy of the License at 6 * 7 * http://www.apache.org/licenses/LICENSE-2.0 8 * 9 * Unless required by applicable law or agreed to in writing, software distributed under the License 10 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express 11 * or implied. See the License for the specific language governing permissions and limitations under 12 * the License. 13 */ 14 15 package com.google.common.base; 16 17 import com.google.common.annotations.GwtCompatible; 18 import javax.annotation.CheckForNull; 19 import org.checkerframework.checker.nullness.qual.Nullable; 20 21 /** 22 * Determines an output value based on an input value; a pre-Java-8 version of {@link 23 * java.util.function.Function java.util.function.Function}. 24 * 25 * <p>The {@link Functions} class provides common functions and related utilities. 26 * 27 * <p>See the Guava User Guide article on <a 28 * href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code Function}</a>. 29 * 30 * <h3>For Java 8+ users</h3> 31 * 32 * <p>This interface is now a legacy type. Use {@code java.util.function.Function} (or the 33 * appropriate primitive specialization such as {@code ToIntFunction}) instead whenever possible. 34 * Otherwise, at least reduce <i>explicit</i> dependencies on this type by using lambda expressions 35 * or method references instead of classes, leaving your code easier to migrate in the future. 36 * 37 * <p>To use an existing function (say, named {@code function}) in a context where the <i>other 38 * type</i> of function is expected, use the method reference {@code function::apply}. A future 39 * version of {@code com.google.common.base.Function} will be made to <i>extend</i> {@code 40 * java.util.function.Function}, making conversion code necessary only in one direction. At that 41 * time, this interface will be officially discouraged. 42 * 43 * @author Kevin Bourrillion 44 * @since 2.0 45 */ 46 @GwtCompatible 47 @ElementTypesAreNonnullByDefault 48 public interface Function<F extends @Nullable Object, T extends @Nullable Object> { 49 /** 50 * Returns the result of applying this function to {@code input}. This method is <i>generally 51 * expected</i>, but not absolutely required, to have the following properties: 52 * 53 * <ul> 54 * <li>Its execution does not cause any observable side effects. 55 * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal 56 * Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a), 57 * function.apply(b))}. 58 * </ul> 59 * 60 * @throws NullPointerException if {@code input} is null and this function does not accept null 61 * arguments 62 */ 63 @ParametricNullness apply(@arametricNullness F input)64 T apply(@ParametricNullness F input); 65 66 /** 67 * <i>May</i> return {@code true} if {@code object} is a {@code Function} that behaves identically 68 * to this function. 69 * 70 * <p><b>Warning: do not depend</b> on the behavior of this method. 71 * 72 * <p>Historically, {@code Function} instances in this library have implemented this method to 73 * recognize certain cases where distinct {@code Function} instances would in fact behave 74 * identically. However, as code migrates to {@code java.util.function}, that behavior will 75 * disappear. It is best not to depend on it. 76 */ 77 @Override equals(@heckForNull Object object)78 boolean equals(@CheckForNull Object object); 79 } 80