Stream流的使用

1. 什么是Stream?

　　是对集合collection对象的功能增强

　　并不是集合元素,他不是数据结构,像一个高级的Iterator

　　提供串行和并行两种模式进行汇聚操作

2. Stream流的创建

　　Collection.stream()单线程

　　Collection.parallelStream()多线程,效率高

　　Stream.of(1,2,3,4,5)

　　Arrays.stream(intArray)

　　BufferedReader in = new BufferedReader()     in.lines()

　　Files.list(path)等等

3. Stream流的使用

　　sorted排序,limit取流数据的前几个数据

@Test
    public void test02() {
        List<Student> list = Lists.newArrayList(
                new Student("刘备", 18), new Student("关羽", 17),
                new Student("张飞", 16), new Student("赵云", 15),
                new Student("诸葛亮", 15), new Student("刘禅", 3));
        //sorted排序(升序),limit取流数据的前几个数据
        System.out.println(list.stream().sorted(new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getAge() - o2.getAge();
            }
        }).limit(4).collect(Collectors.toList()).toString());
    }

[Student(name=刘禅, age=3), Student(name=赵云, age=15), Student(name=诸葛亮, age=15), Student(name=张飞, age=16)]

　　filter 过滤

    @Test
    public void test03(){
        List<Student> list = Lists.newArrayList(
                new Student("刘备", 18), new Student("关羽", 17),
                new Student("张飞", 16), new Student("赵云", 15),
                new Student("诸葛亮", 15), new Student("刘禅", 3));
        System.out.println(list.stream().filter(new Predicate<Student>() {
            @Override
            public boolean test(Student student) {
                return student.getAge() >= 15 && student.getName().length() > 2;
            }
        }).sorted(new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o2.getAge() - o1.getAge();
            }
        }).collect(Collectors.toList()).toString());
    }

[Student(name=诸葛亮, age=15)]

　　map 元素转化  对象转字符串

@Test
    public void test04(){
        List<Student> list = Lists.newArrayList(
                new Student("刘备", 18), new Student("关羽", 17),
                new Student("张飞", 16), new Student("赵云", 15),
                new Student("诸葛亮", 15), new Student("刘禅", 3));
        System.out.println(list.stream().map(new Function<Student, Object>() {
            @Override
            public Object apply(Student student) {
                return student.getName();
            }
        }).collect(Collectors.toList()).toString());
    }

[刘备, 关羽, 张飞, 赵云, 诸葛亮, 刘禅]

　　distinct 去重

　　peek 查看流里面的每个元素

　　skip 跳过前n个元素