Lambda学习 -- 从List的对象获取一个属性转换新的集合

Posted 2022-12-15 一枪尽骚丶魂

需求描述

现在有这样一个需求，有一个用户对象集合，需要从里面获取用户Id转换成一个新的集合。

    @Data
    public class User 

        /**
         * 用户Id
         */
        private Integer id;

        /**
         * 姓名
         */
        private String username;

        /**
         * 手机号
         */
        private String phone;
    

将List<User> userList转换为List<Integer> userIdList

实现方式

Lambda实现

    @Test
    public void main() 

        List<User> userList = Lists.newArrayList();
        for (int i = 0; i < 5; i++) 
            User user = new User();
            user.setId(i);
            user.setUsername("user_" + i);
            user.setPhone("1341341349" + i);

            userList.add(user);
        

        log.info(userList.toString());

        List<Integer> userIds = userList.stream().map(u -> u.getId()).collect(Collectors.toList());

        log.info(userIds.toString());
    

输出结果

10:04:51.481 [main] INFO com.csun.cmny.utils.LambdaTest - [LambdaTest.User(id=0, username=user_0, phone=13413413490), LambdaTest.User(id=1, username=user_1, phone=13413413491), LambdaTest.User(id=2, username=user_2, phone=13413413492), LambdaTest.User(id=3, username=user_3, phone=13413413493), LambdaTest.User(id=4, username=user_4, phone=13413413494)]
10:04:51.557 [main] INFO com.csun.cmny.utils.LambdaTest - [0, 1, 2, 3, 4]

常规方式实现

    @Test
    public void test() 

        List<User> userList = Lists.newArrayList();
        for (int i = 5; i < 10; i++) 
            User user = new User();
            user.setId(i);
            user.setUsername("user_" + i);
            user.setPhone("1341341349" + i);

            userList.add(user);
        

        log.info(userList.toString());

        List<Integer> userIds = Lists.newArrayList();
        for (User user : userList) 
            Integer id = user.getId();
            userIds.add(id);
        

        log.info(userIds.toString());
    

输出结果

10:16:33.884 [main] INFO com.csun.cmny.utils.LambdaTest - [LambdaTest.User(id=5, username=user_5, phone=13413413495), LambdaTest.User(id=6, username=user_6, phone=13413413496), LambdaTest.User(id=7, username=user_7, phone=13413413497), LambdaTest.User(id=8, username=user_8, phone=13413413498), LambdaTest.User(id=9, username=user_9, phone=13413413499)]
10:16:33.888 [main] INFO com.csun.cmny.utils.LambdaTest - [5, 6, 7, 8, 9]

参考地址：java8新特性： lambda表达式：直接获得某个list/array/对象里面的字段集合