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 org.checkerframework.checker.nullness.compatqual.NullableDecl;
020
021/**
022 * Legacy version of {@link java.util.function.Function java.util.function.Function}.
023 *
024 * <p>The {@link Functions} class provides common functions and related utilities.
025 *
026 * <p>As this interface extends {@code java.util.function.Function}, an instance of this type can be
027 * used as a {@code java.util.function.Function} directly. To use a {@code
028 * java.util.function.Function} in a context where a {@code com.google.common.base.Function} is
029 * needed, use {@code function::apply}.
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>See the Guava User Guide article on <a
037 * href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code Function}</a>.
038 *
039 * @author Kevin Bourrillion
040 * @since 2.0
041 */
042@GwtCompatible
043@FunctionalInterface
044public interface Function<F, T> extends java.util.function.Function<F, T> {
045  @Override
046  @NullableDecl
047  @CanIgnoreReturnValue // TODO(kevinb): remove this
048  T apply(@NullableDecl F input);
049
050  /**
051   * <i>May</i> return {@code true} if {@code object} is a {@code Function} that behaves identically
052   * to this function.
053   *
054   * <p><b>Warning: do not depend</b> on the behavior of this method.
055   *
056   * <p>Historically, {@code Function} instances in this library have implemented this method to
057   * recognize certain cases where distinct {@code Function} instances would in fact behave
058   * identically. However, as code migrates to {@code java.util.function}, that behavior will
059   * disappear. It is best not to depend on it.
060   */
061  @Override
062  boolean equals(@NullableDecl Object object);
063}