Java8集合流操作

一、集合流操作：
多的不说直接贴（过滤、排序、聚合以及汇总demo）代码：

/**
 * Created by wangk on 2019-03-10.
 */
@Data
public class User {
    private int id;
    private String name;
    private String classNo;
    private int age;

    public User(int id, String name, String classNo, int age) {
        this.id = id;
        this.name = name;
        this.classNo = classNo;
        this.age = age;
    }
}

class StreamTest{
    public static void main(String[] args) {
        User user1 = new User(1, "张三", "no1", 27);
        User user2 = new User(2, "李四", "no2", 15);
        User user3 = new User(3, "王五", "no1", 24);
        User user4 = new User(4, "赵六", "no3", 15);
        User user5 = new User(5, "李七", "no1", 29);
        User user6 = new User(6, "钱八", "no3", 18);
        User user7 = new User(7, "孙九", "no1", 25);
        User user8 = new User(8, "周十", "no2", 19);
        List<User> list = new ArrayList<>();
        list.add(user1);
        list.add(user2);
        list.add(user3);
        list.add(user4);
        list.add(user5);
        list.add(user6);
        list.add(user7);
        list.add(user8);
        
       //List转map：把list里面的元素拿出来作为map的value，自己定义key
        //a->a, (k1,k2)->k1: 如果有重复的key就返回第一个
        Map<Integer, User> map1 = list.stream().collect(Collectors.toMap(User::getId, a->a, (k1,k2)->k1 ));
        System.out.println("map1");
        System.out.println(map1);

        //过滤Filter
        List<User> map2 = list.stream().filter(a -> a.getAge()>20).collect(Collectors.toList());
        System.out.println("map2");
        System.out.println(map2);

        //排序sort
        List<User> map3 = list.stream().sorted(new Comparator<User>() {
            @Override
            public int compare(User o1, User o2) {
                return o1.getAge()-o2.getAge();
            }
        }).collect(Collectors.toList());
        System.out.println("map3");
        System.out.println(map3);

        //求最大最小值
        Optional<User> user = list.stream().max(new Comparator<User>() {
            @Override
            public int compare(User o1, User o2) {
                int n1 = Integer.parseInt(o1.getClassNo().substring(2, 3));
                int n2 = Integer.parseInt(o2.getClassNo().substring(2, 3));
                return n2-n1;
            }
        });
        System.out.println("user");
        System.out.println(user);

        //遍历修改数据
        list.stream().forEach(p -> {
            if ("张三".equals(p.getName())) {
                p.setClassNo("no3");
            }
        });
        System.out.println("list");
        System.out.println(list);


        //分组
        Map<String, List<User>> map6 =  list.stream().collect(Collectors.groupingBy(User::getClassNo));
        System.out.println("map6");
        System.out.println(map6);

      
        //groupingBy三个参数值是：1、分组字段 2、结合类型（默认是HashMap） 3、Coolections.toList()
        //因为HashMap是无序的，所以这里我们使用LinkHashMap
        //步骤：过滤 -> 排序 -> 分组
        LinkedHashMap<String, List<User>> map = list.stream().filter(a -> a.getAge()<29)
                .sorted(new Comparator<User>() {
                    @Override
                    public int compare(User o1, User o2) {
                        int n1 = Integer.parseInt(o1.getClassNo().substring(2, 3));
                        int n2 = Integer.parseInt(o2.getClassNo().substring(2, 3));
                        return n2-n1;
                    }
                })
                .collect(Collectors.groupingBy(
                        User::getClassNo,
                        LinkedHashMap::new,
                        Collectors.toList()));

        System.out.println("map");
        System.out.println(map);
    }
}

Stream常用方法：

Collectors 类的静态工厂方法: