Lambda常用写法

1、排序

import lombok.Data;
import java.util.*;
import java.util.stream.Collectors;

public class StreamStudy {

    public static void main(String[] args) {
        User u1 = new User("陈一",32);
        User u2 = new User("陈二",23);
        User u3 = new User("陈三",33);
        List<User> users = new ArrayList<>();
        users.add(u1);
        users.add(u2);
        users.add(u3);
        List<User> collect1 = users.stream().sorted(Comparator.comparing(User::getAge)).collect(Collectors.toList());
        System.out.println(collect1);
        List<User> collect2 = users.stream().sorted(Comparator.comparing(User::getAge).reversed()).collect(Collectors.toList());
        System.out.println(collect2);
    }
}

@Data
class User {
    public User() {}
    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }
    String name;
    int age;
}

 2、集合操作

import lombok.Data;
import java.util.*;
import java.util.stream.Collectors;

public class StreamStudy {

    public static void main(String[] args) {
        User user1 = new User(5, "aa");
        User user2 = new User(2, "ba");
        User user3 = new User(3, "ac");
        User user4 = new User(1, "aad");
        User user5 = new User(4, "asd");
        List<User> userList = Arrays.asList(user1, user2, user3, user4, user5);

        //获取所有ID集合
        List<Integer> idList = userList.stream().map(User::getId).collect(Collectors.toList());
        System.out.println(idList);

        //根据ID排序
        List<User> userList1 = userList.stream().sorted(Comparator.comparingInt(User::getId)).collect(Collectors.toList());
        System.out.println(userList1);

        //分组
        Map<String, Long> map = userList.stream().collect(Collectors.groupingBy(User::getName, Collectors.counting()));
        System.out.println(map);

        //获取id大于2的
        List<User> userList2 = userList.stream().filter(user -> user.getId() > 2).collect(Collectors.toList());
        System.out.println(userList2);

        //获取最大
        Integer id = userList.stream().map(User::getId).max(Integer::compareTo).get();
        System.out.println(id);

        //获取最小
        Integer id1 = userList.stream().map(User::getId).min(Integer::compareTo).get();
        System.out.println(id1);

        //获取数量
        Long id3 = userList.stream().count();
        System.out.println(id3);

        //求和
        int sum = userList.stream().mapToInt(User::getId).sum();
        System.out.println(sum);

        //获取平均值
        double avg = userList.stream().mapToInt(User::getId).average().getAsDouble();
        System.out.println(avg);

        //如果存在获取任意一个对象
        User user = userList.stream().filter(us -> us.getId() > 2).findAny().get();
        System.out.println(user);

        //获取第一个对象
        User userFirst = userList.stream().findFirst().get();
        System.out.println(userFirst);

        //将名字全转换为大写
        List<String> upName = userList.stream().map(User::getName).map(String::toUpperCase).collect(Collectors.toList());
        System.out.println(upName);

        //获取忽略第一个并取前几条数据
        List<User> users = userList.stream().skip(1).limit(2).collect(Collectors.toList());
        System.out.println(users);

        //去重
        List<User> userDis = userList.stream().distinct().collect(Collectors.toList());
        System.out.println(userDis);
    }
}

@Data
class User {

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

    int id;
    String name;
}

 3、并行流

将List转为Map

Map<Long, DeviceOnlineDto> onlinemap = onlinelist.parallelStream().collect(Collectors.toMap(DeviceOnlineDto::getDeviceid, e -> e));