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.Nullable;
020
021/**
022 * Legacy version of {@link java.util.function.Function}.
023 *
024 * <p>The {@link Functions} class provides common functions and related utilities.
025 *
026 * <p>As this interface extends {@link java.util.function.Function}, an instance of this type
027 * can be used as a {@code java.util.function.Function} directly.  To use a
028 * {@code java.util.function.Function} in a context where a
029 * {@code com.google.common.base.Function} is 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
037 * <a href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code
038 * Function}</a>.
039 *
040 * @author Kevin Bourrillion
041 * @since 2.0
042 */
043@GwtCompatible
044@FunctionalInterface
045public interface Function<F, T> extends java.util.function.Function<F, T> {
046  @Override
047  @Nullable
048  @CanIgnoreReturnValue // TODO(kevinb): remove this
049  T apply(@Nullable F input);
050
051  /**
052   * <i>May</i> return {@code true} if {@code object} is a {@code Function} that behaves identically
053   * to this function.
054   *
055   * <p><b>Warning: do not depend</b> on the behavior of this method.
056   *
057   * <p>Historically, {@code Function} instances in this library have implemented this method to
058   * recognize certain cases where distinct {@code Function} instances would in fact behave
059   * identically. However, as code migrates to {@code java.util.function}, that behavior will
060   * disappear. It is best not to depend on it.
061   */
062  @Override
063  boolean equals(@Nullable Object object);
064}