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.jspecify.annotations.Nullable;
019
020/**
021 * Legacy version of {@link java.util.function.Function java.util.function.Function}.
022 *
023 * <p>The {@link Functions} class provides common functions and related utilities.
024 *
025 * <p>As this interface extends {@code java.util.function.Function}, an instance of this type can be
026 * used as a {@code java.util.function.Function} directly. To use a {@code
027 * java.util.function.Function} in a context where a {@code com.google.common.base.Function} is
028 * needed, use {@code function::apply}.
029 *
030 * <p>This interface is now a legacy type. Use {@code java.util.function.Function} (or the
031 * appropriate primitive specialization such as {@code ToIntFunction}) instead whenever possible.
032 * Otherwise, at least reduce <i>explicit</i> dependencies on this type by using lambda expressions
033 * or method references instead of classes, leaving your code easier to migrate in the future.
034 *
035 * <p>See the Guava User Guide article on <a
036 * href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code Function}</a>.
037 *
038 * @author Kevin Bourrillion
039 * @since 2.0
040 */
041@GwtCompatible
042@FunctionalInterface
043public interface Function<F extends @Nullable Object, T extends @Nullable Object>
044    extends java.util.function.Function<F, T> {
045  @Override
046  @ParametricNullness
047  T apply(@ParametricNullness F input);
048
049  /**
050   * <i>May</i> return {@code true} if {@code object} is a {@code Function} that behaves identically
051   * to this function.
052   *
053   * <p><b>Warning: do not depend</b> on the behavior of this method.
054   *
055   * <p>Historically, {@code Function} instances in this library have implemented this method to
056   * recognize certain cases where distinct {@code Function} instances would in fact behave
057   * identically. However, as code migrates to {@code java.util.function}, that behavior will
058   * disappear. It is best not to depend on it.
059   */
060  @Override
061  boolean equals(@Nullable Object object);
062}