Java8 函数式编程

From: https://lingcoder.gitee.io/onjava8/#/book/13-Functional-Programming

函数式编程语言操纵代码片段就像操作数据一样容易。 虽然 Java 不是函数式语言，但 Java 8 Lambda 表达式和方法引用 (Method References) 允许你以函数式编程。

Lambda表达式

Lambda 表达式是使用最小可能语法编写的函数定义：

1. Lambda 表达式产生函数，而不是类。 在 JVM（Java Virtual Machine，Java 虚拟机）上，一切都是一个类，因此在幕后执行各种操作使 Lambda 看起来像函数 —— 但作为程序员，你可以高兴地假装它们“只是函数”。

2. Lambda 语法尽可能少，这正是为了使 Lambda 易于编写和使用。

方法引用

Java 8 方法引用没有历史包袱。方法引用组成：类名或对象名，后面跟 :: [^4]，然后跟方法名称。

// functional/MethodReferences.java

import java.util.*;

interface Callable { // [1]
  void call(String s);
}

class Describe {
  void show(String msg) { // [2]
    System.out.println(msg);
  }
}

public class MethodReferences {
  static void hello(String name) { // [3]
    System.out.println("Hello, " + name);
  }
  static class Description {
    String about;
    Description(String desc) { about = desc; }
    void help(String msg) { // [4]
      System.out.println(about + " " + msg);
    }
  }
  static class Helper {
    static void assist(String msg) { // [5]
      System.out.println(msg);
    }
  }
  public static void main(String[] args) {
    Describe d = new Describe();
    Callable c = d::show; // [6]
    c.call("call()"); // [7]

    c = MethodReferences::hello; // [8]
    c.call("Bob");

    c = new Description("valuable")::help; // [9]
    c.call("information");

    c = Helper::assist; // [10]
    c.call("Help!");
  }
}

函数式接口

方法引用和 Lambda 表达式必须被赋值，同时编译器需要识别类型信息以确保类型正确。

Java 8 引入了 java.util.function 包。它包含一组接口，这些接口是 Lambda 表达式和方法引用的目标类型。 每个接口只包含一个抽象方法，称为函数式方法。

在编写接口时，可以使用 @FunctionalInterface 注解强制执行此“函数式方法”模式：

// functional/FunctionalAnnotation.java

@FunctionalInterface
interface Functional {
  String goodbye(String arg);
}

interface FunctionalNoAnn {
  String goodbye(String arg);
}

/*
@FunctionalInterface
interface NotFunctional {
  String goodbye(String arg);
  String hello(String arg);
}
产生错误信息:
NotFunctional is not a functional interface
multiple non-overriding abstract methods
found in interface NotFunctional
*/

public class FunctionalAnnotation {
  public String goodbye(String arg) {
    return "Goodbye, " + arg;
  }
  public static void main(String[] args) {
    FunctionalAnnotation fa =
      new FunctionalAnnotation();
    Functional f = fa::goodbye;
    FunctionalNoAnn fna = fa::goodbye;
    // Functional fac = fa; // Incompatible
    Functional fl = a -> "Goodbye, " + a;
    FunctionalNoAnn fnal = a -> "Goodbye, " + a;
  }
}

@FunctionalInterface 注解是可选的; Java 在 main() 中把 Functional 和 FunctionalNoAnn 都当作函数式接口。 在 NotFunctional 的定义中可看到@FunctionalInterface 的作用：接口中如果有多个方法则会产生编译期错误。

Function源码：

/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package java.util.function;

import java.util.Objects;

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object)}.
 *
 * @param <T> the type of the input to the function
 * @param <R> the type of the result of the function
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Function<T, R> {

    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

    /**
     * Returns a composed function that first applies the {@code before}
     * function to its input, and then applies this function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of input to the {@code before} function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }

    /**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }
}