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 }); }}