1 /*
   2  * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.util.function;
  26 
  27 import java.util.Objects;
  28 
  29 /**
  30  * Apply a function to the input argument, yielding an appropriate result.  A
  31  * function may variously provide a mapping between types, object instances or
  32  * keys and values or any other form of transformation upon the input.
  33  *
  34  * @param <T> the type of the input to the {@code apply} operation
  35  * @param <R> the type of the result of the {@code apply} operation
  36  *
  37  * @since 1.8
  38  */
  39 @FunctionalInterface
  40 public interface Function<T, R> {
  41 
  42     /**
  43      * Compute the result of applying the function to the input argument
  44      *
  45      * @param t the input object
  46      * @return the function result
  47      */
  48     R apply(T t);
  49 
  50     /**
  51      * Returns a new function which applies the provided function followed by
  52      * this function.  If either function throws an exception, it is relayed
  53      * to the caller.
  54      *
  55      * @param <V> type of input objects to the combined function. May be the
  56      * same type as {@code <T>} or {@code <R>}
  57      * @param before an additional function to be applied before this function
  58      * is applied
  59      * @return a function which performs the provided function followed by this
  60      * function
  61      * @throws NullPointerException if before is null
  62      */
  63     default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
  64         Objects.requireNonNull(before);
  65         return (V v) -> apply(before.apply(v));
  66     }
  67 
  68     /**
  69      * Returns a new function which applies this function followed by the
  70      * provided function.  If either function throws an exception, it is relayed
  71      * to the caller.
  72      *
  73      * @param <V> type of output objects to the combined function. May be the
  74      * same type as {@code <T>} or {@code <R>}
  75      * @param after an additional function to be applied after this function is
  76      * applied
  77      * @return a function which performs this function followed by the provided
  78      * function
  79      * @throws NullPointerException if after is null
  80      */
  81     default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
  82         Objects.requireNonNull(after);
  83         return (T t) -> after.apply(apply(t));
  84     }
  85 
  86     /**
  87      * Returns a {@code Function} whose {@code apply} method returns its input.
  88      *
  89      * @param <T> the type of the input and output objects to the function
  90      * @return a {@code Function} whose {@code apply} method returns its input
  91      */
  92     static <T> Function<T, T> identity() {
  93         return t -> t;
  94     }
  95 }