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 org.checkerframework.checker.nullness.qual.Nullable;
019
020/**
021 * Determines an output value based on an input value; a pre-Java-8 version of {@link
022 * java.util.function.Function java.util.function.Function}.
023 *
024 * <p>The {@link Functions} class provides common functions and related utilities.
025 *
026 * <p>See the Guava User Guide article on <a
027 * href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code Function}</a>.
028 *
029 * <h3>For Java 8+ users</h3>
030 *
031 * <p>This interface is now a legacy type. Use {@code java.util.function.Function} (or the
032 * appropriate primitive specialization such as {@code ToIntFunction}) instead whenever possible.
033 * Otherwise, at least reduce <i>explicit</i> dependencies on this type by using lambda expressions
034 * or method references instead of classes, leaving your code easier to migrate in the future.
035 *
036 * <p>To use an existing function (say, named {@code function}) in a context where the <i>other
037 * type</i> of function is expected, use the method reference {@code function::apply}. A future
038 * version of {@code com.google.common.base.Function} will be made to <i>extend</i> {@code
039 * java.util.function.Function}, making conversion code necessary only in one direction. At that
040 * time, this interface will be officially discouraged.
041 *
042 * @author Kevin Bourrillion
043 * @since 2.0
044 */
045@GwtCompatible
046public interface Function<F extends @Nullable Object, T extends @Nullable Object> {
047  /**
048   * Returns the result of applying this function to {@code input}. This method is <i>generally
049   * expected</i>, but not absolutely required, to have the following properties:
050   *
051   * <ul>
052   *   <li>Its execution does not cause any observable side effects.
053   *   <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal
054   *       Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a),
055   *       function.apply(b))}.
056   * </ul>
057   *
058   * @throws NullPointerException if {@code input} is null and this function does not accept null
059   *     arguments
060   */
061  @ParametricNullness
062  T apply(@ParametricNullness F input);
063
064  /**
065   * <i>May</i> return {@code true} if {@code object} is a {@code Function} that behaves identically
066   * to this function.
067   *
068   * <p><b>Warning: do not depend</b> on the behavior of this method.
069   *
070   * <p>Historically, {@code Function} instances in this library have implemented this method to
071   * recognize certain cases where distinct {@code Function} instances would in fact behave
072   * identically. However, as code migrates to {@code java.util.function}, that behavior will
073   * disappear. It is best not to depend on it.
074   */
075  @Override
076  boolean equals(@Nullable Object object);
077}