java8新特性-StreamAPI

StreamAPI

参考https://blog.csdn.net/y_k_y/article/details/84633001，仅自己学习记录，可劝删
Stream是用函数式编程在集合类上进行复杂操作的工具

一、流的生成方法

1.Collection接口的stream()

2.静态的stream.of 

3.Arrays.stream

4.Stream.generate()方法生成无限流

...

二、流的中间操作

筛选

1 .filter()

        ArrayList<Apple> apples = new ArrayList<>();
        apples.add(new Apple(23,"lin"));
        apples.add(new Apple(42,"z"));
        apples.add(new Apple(44,"zhu"));
        apples.add(new Apple(40,"xiao"));
        apples.add(new Apple(17,"lin"));
        apples.add(new Apple(31,"z"));
        apples.add(new Apple(25,"xiao"));
        apples.add(new Apple(8,"z"));
        apples.add(new Apple(35,null));
        List<Apple> appleList = apples.stream().filter(x -> x.getName() != null).collect(Collectors.toList());
        appleList.forEach(System.out::println);

limit(n) 获取n个元素

skip(n) 跳过n元素 

distinct():通过流元素的hashCode(）和equals()去除重复元素

2.映射

map() ：转换元素的值　会映射到每一个元素 生成新元素　

map映射到每个元素对应的结果

        List<Integer> integerList = apples.stream().map(Apple::getWeight).collect(Collectors.toList());
        integerList.forEach(System.out::println);

3.排序

sorted()：自然排序和自定义排序

public class Test {
    public static void main(String[] args) {
        ArrayList<Apple> apples = new ArrayList<>();
        apples.add(new Apple(23,"lin"));
        apples.add(new Apple(42,"z"));
        apples.add(new Apple(44,"zhu"));
        apples.add(new Apple(40,"xiao"));
        apples.add(new Apple(17,"lin"));
        apples.add(new Apple(31,"z"));
        apples.add(new Apple(25,"xiao"));
        apples.add(new Apple(8,"z"));
        List<Integer> list = apples.stream().map(Apple::getWeight).collect(Collectors.toList());
        //自然排序
        List<Integer> appleList = list.stream().sorted().collect(Collectors.toList());
        appleList.forEach(System.out::println);
        //自定义排序
        List<Apple> apples1 = apples.stream().sorted(Comparator.comparing(Apple::getWeight)).collect(Collectors.toList());
        apples1.forEach(System.out::println);
    }
}

 4.消费
peek：如同于map，能得到流中的每一个元素。但map接收的是一个Function表达式，有返回值；而peek接收的是Consumer表达式，没有返回值

三、Collectors

Stream.collect() 是java 8 stream api中的终止方法。Collector接口实现对流中的元素重新包装

分组

public class Test {
    public static void main(String[] args) {
        ArrayList<Apple> apples = new ArrayList<>();
        apples.add(new Apple(23,"lin"));
        apples.add(new Apple(42,"z"));
        apples.add(new Apple(44,"zhu"));
        apples.add(new Apple(40,"xiao"));
        apples.add(new Apple(17,"lin"));
        apples.add(new Apple(31,"z"));
        apples.add(new Apple(25,"xiao"));
        apples.add(new Apple(8,"z"));
        Map<String, List<Apple>> map = apples.stream().collect(Collectors.groupingBy(Apple::getName));
        map.forEach((key,value)->{
            System.out.println("key:"+key+",value"+value);
        });
    }
}

 joining() 连接字符串流中的元素

Collectors.joining（）

        ArrayList<String> strings = new ArrayList<>();
        strings.add("s");
        strings.add("c");
        strings.add("x");
        String collect = strings.stream().collect(Collectors.joining(",", "(", ")"));
        System.out.println(collect);//(s,c,x)
    }