    001    /*
    002     * Copyright (C) 2007 The Guava Authors
    003     *
    004     * Licensed under the Apache License, Version 2.0 (the "License");
    005     * you may not use this file except in compliance with the License.
    006     * You may obtain a copy of the License at
    007     *
    008     * http://www.apache.org/licenses/LICENSE-2.0
    009     *
    010     * Unless required by applicable law or agreed to in writing, software
    011     * distributed under the License is distributed on an "AS IS" BASIS,
    012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    013     * See the License for the specific language governing permissions and
    014     * limitations under the License.
    015     */
    016
    017    package com.google.common.base;
    018
    019    import com.google.common.annotations.GwtCompatible;
    020
    021    import javax.annotation.Nullable;
    022
    023    /**
    024     * Determines an output value based on an input value.
    025     *
    026     * <p>See the Guava User Guide article on <a href=
    027     * "http://code.google.com/p/guava-libraries/wiki/FunctionalExplained">the use of {@code
    028     * Function}</a>.
    029     *
    030     * @author Kevin Bourrillion
    031     * @since 2.0 (imported from Google Collections Library)
    032     */
    033    @GwtCompatible
    034    public interface Function<F, T> {
    035      /**
    036       * Returns the result of applying this function to {@code input}. This method is <i>generally
    037       * expected</i>, but not absolutely required, to have the following properties:
    038       *
    039       * <ul>
    040       * <li>Its execution does not cause any observable side effects.
    041       * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal
    042       *     Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a),
    043       *     function.apply(b))}.
    044       * </ul>
    045       *
    046       * @throws NullPointerException if {@code input} is null and this function does not accept null
    047       *     arguments
    048       */
    049      @Nullable T apply(@Nullable F input);
    050
    051      /**
    052       * Indicates whether another object is equal to this function.
    053       *
    054       * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.
    055       * However, an implementation may also choose to return {@code true} whenever {@code object} is a
    056       * {@link Function} that it considers <i>interchangeable</i> with this one. "Interchangeable"
    057       * <i>typically</i> means that {@code Objects.equal(this.apply(f), that.apply(f))} is true for all
    058       * {@code f} of type {@code F}. Note that a {@code false} result from this method does not imply
    059       * that the functions are known <i>not</i> to be interchangeable.
    060       */
    061      @Override
    062      boolean equals(@Nullable Object object);
    063    }