java8 in action 学习笔记 一

为什么学习Java8

• ## java8 的变化 ##
  
  1. 流处理
     -Java8 在java.util.stream中添加了一个Stram API； Stream 就是一系列T类型的项目。可以看做是一种迭代器。Stream API 很多方法可以链接成一个复杂的流水线；
     Java8可以透明的吧输入的不想管的部分拿到几个cpu上运行，不用自己创建Thread。
  2. 用行为参数化把代码传递给方法
     java8新增的另一个概念是通过API来传递代码的能力。
  3. 并行和共享的可变数据
• ##java中的函数 ##
  java8新增了函数——值的一种新形式，通过它Java8可以进行多核处理器上的并行编程。
  
  1. 方法和Lambda作为一等公民
     Java8的第一个新功能是方法引用。比如需要筛选一个目录中的所有隐藏文件，需要编写一个方法，然后给它一个File，判断是否隐藏。

File[] hiddenFiles = new File(".").listFiles(new FileFilter(){  public boolean accept(File file){      return file.isHidden();  }});

//Java8 File[] hiddenFiles = new File(".").listFiles(File::isHidden);

Lambdas——匿名函数
Java8 将函数作为值进行传递，包括lambdas，比如(int x) -> x+1; 表示调用是给定参数x，就返回x+1的值。

代码实例

/** * Created by tom on 17-7-1. */public class FilteringApples {    public static void main(String[] args) {        List<Apple>  inventory = Arrays.asList(new Apple(80,"green"),                new Apple(155, "green"),                new Apple(120, "red"));        List<Apple> greenApples = filterApples(inventory,FilteringApples::isGreenApple);        System.out.println(greenApples);        List<Apple> heavyApples = filterApples(inventory,FilteringApples::isHeavyApple);        System.out.println(heavyApples);        List<Apple> greenApples2 = filterApples(inventory, (Apple a)-> "green".equals(a.getColor()));        System.out.println(greenApples2);        List<Apple> heavyApples2 = filterApples(inventory, (Apple a)->a.getWeight()>150);        System.out.println(heavyApples2);        List<Apple> weirdApples = filterApples(inventory, (Apple a) -> a.getWeight() <= 80 &&                "green".equals(a.getColor()));        System.out.println(weirdApples);    }    public static List<Apple> filterGreenApple(List<Apple> inventory){        List<Apple> result = new ArrayList<>();        for (Apple apple:inventory             ) {            if("green".equals(apple.getColor())){                result.add(apple);            }        }        return  result;    }    public static List<Apple> filterHeavyApples(List<Apple> inventory){        List<Apple> result = new ArrayList<>();        for (Apple apple:inventory ) {            if (apple.getWeight()>150){                result.add(apple);            }        }        return result;    }    public static boolean isGreenApple(Apple apple){        return "green".equals(apple.getColor());    }    public static boolean isHeavyApple(Apple apple){        return  apple.getWeight()>150;    }    public static List<Apple> filterApples(List<Apple> inventory, Predicate<Apple> p) {        List<Apple> result = new ArrayList<>();        for (Apple apple : inventory) {            if (p.test(apple)) {                result.add(apple);            }        }        return result;    }    public static class Apple {        private int weight = 0;        private String color = "";        public int getWeight() {            return weight;        }        public void setWeight(int weight) {            this.weight = weight;        }        public Apple(int weight, String color) {            this.weight = weight;            this.color = color;        }        public String getColor() {            return color;        }        public void setColor(String color) {            this.color = color;        }        public String toString() {            return "Apple{" +                    "color='" + color + '\'' +                    ", weight= " + weight + '}';        }    }}