开发小技巧

编程小技巧

1. 将List<Object> 转换为 List<String>

List<User> userList;// [{"name":"wang","age":23},{"name":"liu","age":13}]
List<String> nameList=userList.stream()
    .map(t->t.getName())
    .collect(Collectors.toList());// ["wang","li"]

2. 将List转换为Map

Map<Long,user> userInfo=user.stream()
    .collect(Collectors.toMap(user::getId,Function.identity()));

3. Java将JSON对象或JSON数组转list对象

https://blog.csdn.net/superPojo/article/details/108793355

4.lambda 遍历List＜String＞替换成另外一个值

estateMaps.stream().map(e-> domainChange(e)).collect(Collectors.toList())

5.Java对象类型集合单独取出对象中一个属性成为集合或数组

List<String> stateNameList = dictEntityList.stream().map(DictEntity::getName).collect(Collectors.toList());

pg序列自动生成 bigserial

@EnumValue

https://baomidou.com/pages/8390a4/

单元测试

• jacoco插件配置生成单元测试覆盖率报告：https://blog.csdn.net/weixin_38384296/article/details/122655287

PG删库提示“There are 2 other sessions using the database”信息

1. select pg_terminate_backend(pid) from (select pid from pg_stat_activity where datname = '数据库名' ) a;
2. 删库

分组 groupingBy

public class Groupingby {

    @Data
    @AllArgsConstructor
    public static class User {
        private String name;
        private Integer age;
        private String sex;
    }


    public static void main(String[] args) {
        // 初始化测试数据
        User user1 = new User("张三", 18, "男");
        User user2 = new User("李四", 19, "女");
        User user3 = new User("王五", 20, "男");
        User user4 = new User("赵六", 21, "女");
        User user5 = new User("钱七", 22, "男");
        List<User> users = Arrays.asList(user1, user2, user3, user4, user5);

        // 进行男女分组 <sex，List<User>>
        Map<String, List<User>> collect = users.stream().collect(Collectors.groupingBy(User::getSex));
        // 如果能保证名称唯一，可以快速生成map进行快速查找 <name,User>
        Map<String, User> collect1 = users.stream().collect(Collectors.toMap(User::getName, Function.identity()));

        // 进行男女计数 <sex，Integer>
        Map<String, Long> collect2 = users.stream().collect(Collectors.groupingBy(User::getSex, Collectors.counting()));
        // 进行男女分组并统计年龄总和 <sex，Integer>
        Map<String, Integer> collect3 = users.stream().collect(Collectors.groupingBy(User::getSex, Collectors.summingInt(User::getAge)));
        // 根据姓氏进行分组 <lastName,List<User>>
        Map<String, List<User>> collect4 = users.stream().collect(Collectors.groupingBy(user -> {
            return user.getName().substring(0, 1); // 自定义 Map key 逻辑
        }));

    }

}