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 * Legacy version of {@link java.util.function.Function java.util.function.Function}.
024 *
025 * <p>The {@link Functions} class provides common functions and related utilities.
026 *
027 * <p>As this interface extends {@code java.util.function.Function}, an instance of this type can be
028 * used as a {@code java.util.function.Function} directly. To use a {@code
029 * java.util.function.Function} in a context where a {@code com.google.common.base.Function} is
030 * needed, use {@code function::apply}.
031 *
032 * <p>This interface is now a legacy type. Use {@code java.util.function.Function} (or the
033 * appropriate primitive specialization such as {@code ToIntFunction}) instead whenever possible.
034 * Otherwise, at least reduce <i>explicit</i> dependencies on this type by using lambda expressions
035 * or method references instead of classes, leaving your code easier to migrate in the future.
036 *
037 * <p>See the Guava User Guide article on <a
038 * href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code Function}</a>.
039 *
040 * @author Kevin Bourrillion
041 * @since 2.0
042 */
043@GwtCompatible
044@FunctionalInterface
045@ElementTypesAreNonnullByDefault
046public interface Function<F extends @Nullable Object, T extends @Nullable Object>
047    extends java.util.function.Function<F, T> {
048  @Override
049  @CanIgnoreReturnValue // TODO(kevinb): remove this
050  @ParametricNullness
051  T apply(@ParametricNullness F input);
052
053  /**
054   * <i>May</i> return {@code true} if {@code object} is a {@code Function} that behaves identically
055   * to this function.
056   *
057   * <p><b>Warning: do not depend</b> on the behavior of this method.
058   *
059   * <p>Historically, {@code Function} instances in this library have implemented this method to
060   * recognize certain cases where distinct {@code Function} instances would in fact behave
061   * identically. However, as code migrates to {@code java.util.function}, that behavior will
062   * disappear. It is best not to depend on it.
063   */
064  @Override
065  boolean equals(@CheckForNull Object object);
066}