Lambda学习 -- 从List的对象获取一个属性转换新的集合

柔情只为你懂 2022-03-09 02:51 565阅读 0赞

## 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][java8_ lambda_list_array]

[java8_ lambda_list_array]: https://www.cnblogs.com/achengmu/p/9974550.html