目录

一、groupingBy

1、groupingBy介绍

2、groupingBy使用

一、groupingBy

1、groupingBy介绍

roupingBy()是Stream API中最强大的收集器Collector之一，提供与SQL的GROUP BY子句类似的功能。

2、groupingBy使用

1、对整个字段进行分组查询：直接groupingBy那个字段就可以了

2、对字段的一部分进行分组查询：s->s.getId().substring(0, 3)，获取到这个字段后进行字符串截取。

3、对每个分组的数据个数进行统计：Collectors.groupingBy(UserEntity::getId,Collectors.counting())

4、对id字段进行分组并获取age的平均数：

Collectors.groupingBy(UserEntity::getId,Collectors.averagingInt(UserEntity::getAge)) 

5、对id字段进行分组并获取age的总值：

Collectors.groupingBy(UserEntity::getId, Collectors.summingInt(UserEntity::getAge))

public class Stream {

    public static void main(String[] args) {

        groupById(); // 对id字段进行分组查询
        groupByPartId(); // 对id字段的部分进行分组查询
        groupByIdGetCount(); // 对每个分组的数据个数进行统计
        groupByIdGetAgeAve(); // 对id字段进行分组并获取age的平均数
        groupByIdGetAgeSum(); // 对id字段进行分组并获取age的总值

    }

    public static void groupById(){
        UserEntity user1 = new UserEntity("1001", "zhangsan", 18, "123");
        UserEntity user2 = new UserEntity("1001", "lisi", 20, "123");
        UserEntity user3 = new UserEntity("1001", "wangwu", 15, "123");
        UserEntity user4 = new UserEntity("2001", "张三", 22, "123");
        UserEntity user5 = new UserEntity("2001", "李四", 21, "123");
        UserEntity user6 = new UserEntity("2001", "王五",23, "123");
        List list = new ArrayList<>();
        list.add(user1);
        list.add(user2);
        list.add(user3);
        list.add(user4);
        list.add(user5);
        list.add(user6);
        Map> map = list.stream().collect(Collectors.groupingBy(UserEntity::getId));
        map.forEach((key, value)->{
            System.out.println(key+":"+value);
            
        });
    }
    public static void groupByPartId(){
        UserEntity user1 = new UserEntity("1001", "zhangsan", 19, "123");
        UserEntity user2 = new UserEntity("1002", "lisi", 17, "123");
        UserEntity user3 = new UserEntity("1003", "wangwu", 18, "123");
        UserEntity user4 = new UserEntity("2001", "张三", 28, "123");
        UserEntity user5 = new UserEntity("2002", "李四", 23, "123");
        UserEntity user6 = new UserEntity("2003", "王五", 26, "123");

        List list = new ArrayList<>();
        list.add(user1);
        list.add(user2);
        list.add(user3);
        list.add(user4);
        list.add(user5);
        list.add(user6);
        Map> map = list.stream().collect(Collectors.groupingBy(s->s.getId().substring(0, 3)));
        map.forEach((key, value)->{
            System.out.println(key+":"+value);
            
        });
    }

    private static void groupByIdGetCount() {
        UserEntity user1 = new UserEntity("1001", "zhangsan", 18, "123");
        UserEntity user2 = new UserEntity("1001", "lisi", 20, "123");
        UserEntity user3 = new UserEntity("1001", "wangwu", 15, "123");
        UserEntity user4 = new UserEntity("2001", "张三", 22, "123");
        UserEntity user5 = new UserEntity("2001", "李四", 21, "123");
        UserEntity user6 = new UserEntity("2001", "王五",23, "123");
        List list = new ArrayList<>();
        list.add(user1);
        list.add(user2);
        list.add(user3);
        list.add(user4);
        list.add(user5);
        list.add(user6);
        Map map = list.stream().collect(Collectors.groupingBy(UserEntity::getId, Collectors.counting()));
        map.forEach((key, value)->{
            System.out.println(key+":"+value);
            
        });
    }

    private static void groupByIdGetAgeAve() {

        UserEntity user1 = new UserEntity("1001", "zhangsan", 18, "123");
        UserEntity user2 = new UserEntity("1001", "lisi", 20, "123");
        UserEntity user3 = new UserEntity("1001", "wangwu", 15, "123");
        UserEntity user4 = new UserEntity("2001", "张三", 22, "123");
        UserEntity user5 = new UserEntity("2001", "李四", 21, "123");
        UserEntity user6 = new UserEntity("2001", "王五",23, "123");
        List list = new ArrayList<>();
        list.add(user1);
        list.add(user2);
        list.add(user3);
        list.add(user4);
        list.add(user5);
        list.add(user6);
        Map map = list.stream().collect(Collectors.groupingBy(UserEntity::getId,
                Collectors.averagingInt(UserEntity::getAge)));
        map.forEach((key, value)->{
            BigDecimal bigDecimal = new BigDecimal(value);
            bigDecimal.setScale(2, BigDecimal.ROUND_HALF_UP);
            System.out.println(key+":"+value); // 要想保留两位小数 使用bigDecimal， 四舍五入
            
        });
    }

    private static void groupByIdGetAgeSum() {
        UserEntity user1 = new UserEntity("1001", "zhangsan", 18, "123");
        UserEntity user2 = new UserEntity("1001", "lisi", 20, "123");
        UserEntity user3 = new UserEntity("1001", "wangwu", 15, "123");
        UserEntity user4 = new UserEntity("2001", "张三", 22, "123");
        UserEntity user5 = new UserEntity("2001", "李四", 21, "123");
        UserEntity user6 = new UserEntity("2001", "王五",23, "123");
        List list = new ArrayList<>();
        list.add(user1);
        list.add(user2);
        list.add(user3);
        list.add(user4);
        list.add(user5);
        list.add(user6);
        Map map = list.stream().collect(Collectors.groupingBy(UserEntity::getId,
                Collectors.summingInt(UserEntity::getAge)));
        map.forEach((key, value)->{
            System.out.println(key+":"+value); // 要想保留两位小数 使用bigDecimal， 四舍五入
            
        });
    }
}