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;
019
020import javax.annotation.Nullable;
021
022/**
023 * Determines a true or false value for a given input; a pre-Java-8 version of {@code
024 * java.util.function.Predicate}.
025 *
026 * <p>The {@link Predicates} class provides common predicates and related utilities.
027 *
028 * <p>See the Guava User Guide article on
029 * <a href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code
030 * Predicate}</a>.
031 *
032 * <h3>For Java 8+ users</h3>
033 *
034 * <p>This interface is now a legacy type. Use {@code java.util.function.Predicate} (or the
035 * appropriate primitive specialization such as {@code IntPredicate}) instead whenever possible.
036 * Otherwise, at least reduce <i>explicit</i> dependencies on this type by using lambda expressions
037 * or method references instead of classes, leaving your code easier to migrate in the future.
038 *
039 * <p>To use a reference of this type (say, named {@code guavaPredicate}) in a context where {@code
040 * java.util.function.Predicate} is expected, use the method reference {@code
041 * guavaPredicate::apply}. For the other direction, use {@code javaUtilPredicate::test}. A future
042 * version of this interface will be made to <i>extend</i> {@code java.util.function.Predicate}, so
043 * that conversion will be necessary in only one direction. At that time, this interface will be
044 * officially discouraged.
045 *
046 * @author Kevin Bourrillion
047 * @since 2.0
048 */
049@GwtCompatible
050public interface Predicate<T> {
051  /**
052   * Returns the result of applying this predicate to {@code input} (Java 8 users, see notes in the
053   * class documentation above). This method is <i>generally expected</i>, but not absolutely
054   * required, to have the following properties:
055   *
056   * <ul>
057   * <li>Its execution does not cause any observable side effects.
058   * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal
059   *     Objects.equal}{@code (a, b)} implies that {@code predicate.apply(a) ==
060   *     predicate.apply(b))}.
061   * </ul>
062   *
063   * @throws NullPointerException if {@code input} is null and this predicate does not accept null
064   *     arguments
065   */
066  @CanIgnoreReturnValue
067  boolean apply(@Nullable T input);
068
069  /**
070   * Indicates whether another object is equal to this predicate.
071   *
072   * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.
073   * However, an implementation may also choose to return {@code true} whenever {@code object} is a
074   * {@link Predicate} that it considers <i>interchangeable</i> with this one. "Interchangeable"
075   * <i>typically</i> means that {@code this.apply(t) == that.apply(t)} for all {@code t} of type
076   * {@code T}). Note that a {@code false} result from this method does not imply that the
077   * predicates are known <i>not</i> to be interchangeable.
078   */
079  @Override
080  boolean equals(@Nullable Object object);
081}