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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     * @author Kevin Bourrillion
027     * @since 2.0 (imported from Google Collections Library)
028     */
029    @GwtCompatible
030    public interface Function<F, T> {
031      /**
032       * Returns the result of applying this function to {@code input}. This method is <i>generally
033       * expected</i>, but not absolutely required, to have the following properties:
034       *
035       * <ul>
036       * <li>Its execution does not cause any observable side effects.
037       * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal
038       *     Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a),
039       *     function.apply(b))}.
040       * </ul>
041       *
042       * @throws NullPointerException if {@code input} is null and this function does not accept null
043       *     arguments
044       */
045      T apply(@Nullable F input);
046    
047      /**
048       * Indicates whether another object is equal to this function.
049       *
050       * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}.
051       * However, an implementation may also choose to return {@code true} whenever {@code object} is a
052       * {@link Function} that it considers <i>interchangeable</i> with this one. "Interchangeable"
053       * <i>typically</i> means that {@code Objects.equal(this.apply(f), that.apply(f))} is true for all
054       * {@code f} of type {@code F}. Note that a {@code false} result from this method does not imply
055       * that the functions are known <i>not</i> to be interchangeable.
056       */
057      @Override
058      boolean equals(@Nullable Object object);
059    }