List Stream常用方法总结

最新推荐文章于 2024-07-20 12:43:53 发布

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本文演示了Java8 Stream API的使用，包括filter、limit、skip、distinct等操作，以及排序和统计方法。示例代码展示了如何筛选、切片、去重、排序用户列表，并获取ID的最小值、最大值和总数。

package com.ai.apac.userservice.server;

import com.ai.apac.userservice.data.User;
import lombok.extern.slf4j.Slf4j;

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

@Slf4j
public class UserService {
    public static void main(String[] args) {
        User user0 = new User(1L,"bbb","bbb");
        User user1 = new User(1L,"aaa","bbb");
        User user2 = new User(2L,"bbb","bbb");
        User user3 = new User(3L,"ccc","bbb");
        User user4 = new User(4L,"ccc","bbb");
        User user5 = new User(5L,"ccc","bbb");

        List<User> userList = new ArrayList<>();
        userList.add(user0);
        userList.add(user1);

        userList.add(user4);
        userList.add(user5);
        userList.add(user5);
        userList.add(user2);
        userList.add(user3);
        if (user0.getId().compareTo(user5.getId()) == 0) {
            System.out.println("aaa");
        }

        log.debug("\n size={},\n userList = {}",userList.size(),userList.toString());
    //1、筛选与切片
        //filter：过滤流中的某些元素
        List<User> useFilter = userList.stream().filter(user -> user.getUsername().equals("aaa")).collect(Collectors.toList());
        log.debug("useFilter :\n size={},\n useFilter = {}",useFilter.size(),useFilter.toString());
        //limit(n)：获取n个元素
        List<User> useLimit = userList.stream().filter(user -> user.getUsername().equals("aaa")).limit(1).collect(Collectors.toList());
        log.debug("useFilter :\n size={},\n useLimit = {}",useLimit.size(),useLimit.toString());
        //skip(n)：跳过n元素，配合limit(n)可实现分页
        List<User> useSkip = userList.stream().skip(2L).limit(3).collect(Collectors.toList());
        log.debug("useFilter :\n size={},\n useLimit = {}",useSkip.size(),useSkip.toString());
        //distinct：通过流中元素的 hashCode() 和 equals() 去除重复元素
        List<User> userDistinct = userList.stream().distinct().collect(Collectors.toList());
        log.debug("useFilter :\n size={},\n useLimit = {}",userDistinct.size(),userDistinct.toString());

        //按照id排序
        List<User> userSort = userList.stream().sorted((o1,o2)->o1.getId().compareTo(o2.getId())).collect(Collectors.toList());
        log.debug("usersort =",userSort);
        //按照id排序，如果id相同按照名称排序
        List<User> userSort2 = userList.stream().sorted((o1,o2)->{
            if (0 !=o1.getId().compareTo(o2.getId())) {
                return o1.getId().compareTo(o2.getId());
            }else {
                return o1.getUsername().compareTo(o2.getUsername());
            }
        }).collect(Collectors.toList());
        log.debug("usersort =",userSort2);
        //获取id中最大，最小，总数
        LongSummaryStatistics intSummaryStatistics = userList.stream().mapToLong(User::getId).summaryStatistics();
        System.out.println(intSummaryStatistics.getMin());
        System.out.println(intSummaryStatistics.getMax());
        System.out.println(intSummaryStatistics.getCount());


    }
}