guava/src/com/google/common/base/Predicate.java

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.base;

import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Legacy version of {@link java.util.function.Predicate java.util.function.Predicate}. Determines a
 * true or false value for a given input.
 *
 * <p>As this interface extends {@code java.util.function.Predicate}, an instance of this type may
 * be used as a {@code Predicate} directly. To use a {@code java.util.function.Predicate} where a
 * {@code com.google.common.base.Predicate} is expected, use the method reference {@code
 * predicate::test}.
 *
 * <p>This interface is now a legacy type. Use {@code java.util.function.Predicate} (or the
 * appropriate primitive specialization such as {@code IntPredicate}) instead whenever possible.
 * Otherwise, at least reduce <i>explicit</i> dependencies on this type by using lambda expressions
 * or method references instead of classes, leaving your code easier to migrate in the future.
 *
 * <p>The {@link Predicates} class provides common predicates and related utilities.
 *
 * <p>See the Guava User Guide article on <a
 * href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code Predicate}</a>.
 *
 * @author Kevin Bourrillion
 * @since 2.0
 */
@FunctionalInterface
@GwtCompatible
public interface Predicate<T> extends java.util.function.Predicate<T> {
  /**
   * Returns the result of applying this predicate to {@code input} (Java 8 users, see notes in the
   * class documentation above). This method is <i>generally expected</i>, but not absolutely
   * required, to have the following properties:
   *
   * <ul>
   *   <li>Its execution does not cause any observable side effects.
   *   <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal
   *       Objects.equal}{@code (a, b)} implies that {@code predicate.apply(a) ==
   *       predicate.apply(b))}.
   * </ul>
   *
   * @throws NullPointerException if {@code input} is null and this predicate does not accept null
   *     arguments
   */
  @CanIgnoreReturnValue
  boolean apply(@Nullable T input);

  /**
   * Indicates whether another object is equal to this predicate.
   *
   * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.
   * However, an implementation may also choose to return {@code true} whenever {@code object} is a
   * {@link Predicate} that it considers <i>interchangeable</i> with this one. "Interchangeable"
   * <i>typically</i> means that {@code this.apply(t) == that.apply(t)} for all {@code t} of type
   * {@code T}). Note that a {@code false} result from this method does not imply that the
   * predicates are known <i>not</i> to be interchangeable.
   */
  @Override
  boolean equals(@Nullable Object object);

  @Override
  default boolean test(@Nullable T input) {
    return apply(input);
  }
}