Java 8 Lambda演练

在工作中，我做了关于Java 8项目lambda的演示，当然还有一些简单的代码来说明其中的一些要点。 Java 8的总体原因是：

更简洁的代码（适用于只有一种方法和集合的类）。 “我们希望代码的读者在到达lambda表达式的“实质”之前，必须尽可能地减少语法。 – Brian Goetz（http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-4.html）
能够传递功能，而不仅仅是数据
更好地支持多核处理

所有示例都可以在从此处下载的以下Java 8版本上运行：

openjdk version "1.8.0-ea"
OpenJDK Runtime Environment (build 1.8.0-ea-lambda-nightly-h3876-20130403-b84-b00)
OpenJDK 64-Bit Server VM (build 25.0-b21, mixed mode)

最简单的情况：

public class ThreadA {public static void main(String[] args) {new Thread(new Runnable() {@Overridepublic void run() {System.err.println("Hello from anonymous class");}}).start();}}

public class ThreadB {public static void main(String[] args) {new Thread(() -> {System.err.println("Hello from lambda");}).start();}}

非正式地注意语法

()|x|(x,..,z) -> expr|stmt

箭头是新的运算符。并注意第二段代码与第一段代码相比更为简洁。

集合：

首先让我介绍一个简单的领域和一些帮助者

public class Something {private double amount;public Something(double amount) {this.amount = amount;}public double getAmount() {return amount;}public String toString() {return "Amount: " + amount;}
}public class Helper {public static List<Something> someThings() {List<Something> things = new ArrayList<>();things.add(new Something(99.9));things.add(new Something(199.9));things.add(new Something(299.9));things.add(new Something(399.9));things.add(new Something(1199.9));return things;}}public interface Doer<T> {void doSomething(T t);}

让我们对Java 7样式进行一些过滤和排序：

public class CollectionA {public static void main(String... args) {List<Something> things = Helper.someThings();System.err.println("Filter");List<Something> filtered = filter(things);System.err.println(filtered);System.err.println("Sum");double sum = sum(filtered);System.err.println(sum);}public static List<Something> filter(List<Something> things) {List<Something> filtered = new ArrayList<>();for (Something s : things) {if (s.getAmount() > 100.00) {if (s.getAmount() < 1000.00) {filtered.add(s);}}}return filtered;}public static double sum(List<Something> things) {double d = 0.0;for (Something s : things) {d += s.getAmount();}return d;}}

现在是Java 8样式– 流式传输：

import java.util.stream.Collectors;public class CollectionB {public static void main(String... args) {List<Something> things = Helper.someThings();System.err.println("Filter lambda");List<Something> filtered = things.stream().parallel().filter( t -> t.getAmount() > 100.00 && t.getAmount() < 1000.00).collect(Collectors.toList());System.err.println(filtered);System.err.println("Sum lambda");double sum = filtered.stream().mapToDouble(t -> t.getAmount()).sum();System.err.println(sum);}}

导入java.util.function。*接口和方法参考

public class CollectionC {public static void main(String... args) {List<Something> things = Helper.someThings();System.err.println("Do something");doSomething(things, new Doer<Something>() {@Overridepublic void doSomething(Something t) {System.err.println(t);}});}public static void doSomething(List<Something> things, Doer<Something> doer) {for (Something s : things) {doer.doSomething(s);}}}

将我们的Doer界面替换为标准的Consumer界面（以前称为Block）

import java.util.function.Consumer;public class CollectionD {public static void main(String... args) {List<Something> things = Helper.someThings();System.err.println("Do something functional interfaces");consumeSomething(things, new Consumer<Something>() {@Overridepublic void accept(Something t) {System.err.println(t);}});System.err.println("Do something functional interfaces, using lambda");consumeSomething(things, (t) -> System.err.println(t));System.err.println("Do something functional interfaces, using lambda method reference (new operator ::) ");consumeSomething(things, System.err::println);System.err.println("Do something functional interfaces, using stream");things.stream().forEach(new Consumer<Something>() {@Overridepublic void accept(Something t) {System.err.println(t);}});System.err.println("Do something functional interfaces, using stream and method reference");things.stream().forEach(System.err::println);}public static void doSomething(List<Something> things, Doer<Something> doer) {for (Something s : things) {doer.doSomething(s);}}public static void consumeSomething(List<Something> things, Consumer<Something> consumer) {for (Something s : things) {consumer.accept(s);}}}

import java.util.List;
import java.util.NoSuchElementException;
import java.util.Optional;
import java.util.stream.Collectors;public class Various {public static void main(String... args) {List<Something> things = Helper.someThings();//MapSystem.err.println(things.stream().map((Something t) -> t.getAmount()).collect(Collectors.toList()));//Reducedouble d = things.stream().reduce(new Something(0.0), (Something t, Something u) -> new Something(t.getAmount() + u.getAmount())).getAmount();System.err.println(d);//Reduce againSystem.err.println(things.stream().reduce((Something t, Something u) -> new Something(t.getAmount() + u.getAmount())).get());//Map/reduceSystem.err.println(things.stream().map((Something t) -> t.getAmount()).reduce(0.0, (x, y) -> x + y));//LazyOptional<Something> findFirst = things.stream().filter(t -> t.getAmount() > 1000).findFirst();System.err.println(findFirst.get());//Lazy no valueOptional<Something> findFirstNotThere = things.stream().filter(t -> t.getAmount() > 2000).findFirst();try {System.err.println(findFirstNotThere.get());} catch (NoSuchElementException e) {System.err.println("Optional was not null, but its value was");}//Optional one step deeperthings.stream().filter(t -> t.getAmount() > 1000).findFirst().ifPresent(t -> System.err.println("Here I am"));}}

参考： Kim Saabye Pedersen的博客博客中的JCG合作伙伴 Kim Saabye Pedersen的Java 8 lambda演练。