Stream流的groupingBy
简单使用
业务场景：现在有100个人，这些人都年龄分部在18-30岁之间。现要求把他们按照年龄进行分组
key：年龄
value：数据列表
java
public void listToMapGroup() {
//这里假设通过listStreamService.list();方法查询了这100人
List<Perple> list = perpleService.list();
//按照age进行分组
Map<Integer, List<Perple>> result = list.stream()
.collect(Collectors.groupingBy(Perple::getAge));
//打印结果
result.forEach((k, v) -> {
System.out.println(k);
System.out.println(v);
System.out.println("--------------------");
});
}效果相当于是，把list这个集合里面存放的100个人每个人都调用Perple的getAge方法，按照getAge方法的返回值进行分组。每个组是一个Map<Integer, List>类型的对象。Map<Integer, List>的键是getAge的返回值，即，分组的依据。Map<Integer, List>的值是这个年龄对应组中的成员。
如果要进行排序
分组之后，是不能对每个分组进行比较的（也就无法排序），因为groupingBy是一个终结流。
Collectors.groupingBy(Function<? super T, ? extends K> classifier, Collector<? super T, A, D> downstream)
这里有两个思路：
1:提前排序，再进行分组
2:分组后，对Map进行处理，对其每个value排序
做法一：
提前排序，再进行分组.
这里还展示了一些groupingBy方法的扩展用法。
java
import com.alibaba.fastjson.JSONObject;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Test {
public static void main(String[] args) {
Student student1 = new Student(1, 1);
Student student2 = new Student(1, 1);
Student student3 = new Student(2, 2);
Student student4 = new Student(2, 3);
Student student5 = new Student(3, 3);
Student student6 = new Student(3, 4);
Student student7 = new Student(4, 1);
Student student8 = new Student(4, 1);
Student student9 = new Student(4, 2);
Student student10 = new Student(4, 1);
List<Student> list = Arrays.asList(student1, student2, student3, student4, student5, student6, student7, student8, student9, student10);
System.out.println("--------- 根据字段分组，求每个分组的sum ----------");
Map<Integer, Integer> collect = list.stream().collect(Collectors.groupingBy(Student::getId, Collectors.summingInt(Student::getScore)));
System.out.println(collect.toString());
System.out.println("--------- 根据字段分组，求每个分组的count ----------");
Map<Integer, Long> countMap = list.stream().collect(Collectors.groupingBy(Student::getId, Collectors.counting()));
System.out.println(countMap.toString());
System.out.println("--------- 根据字段分组，每个分组为：对象的指定字段 ----------");
Map<Integer, List<Integer>> groupMap = list.stream().collect(Collectors.groupingBy(Student::getId, Collectors.mapping(Student::getScore, Collectors.toCollection(ArrayList::new))));
System.out.println(groupMap.toString());
System.out.println("--------- 根据字段分组,默认分组 ----------");
Map<Integer, List<Student>> defaultGroupMap = list.stream().collect(Collectors.groupingBy(Student::getId));
System.out.println(JSONObject.toJSONString(defaultGroupMap));
System.out.println("--------- 根据字段分组，每个分组按照指定字段进行生序排序 ----------");
Map<Integer, List<Student>> sortGroupMap = list.stream().sorted(Comparator.comparing(Student::getScore))
.collect(Collectors.groupingBy(Student::getId));
System.out.println(JSONObject.toJSONString(sortGroupMap));
System.out.println("--------- 先排序，再分组 ----------");
Map<Integer, List<Student>> reversedSortGroupMap = list.stream().sorted(Comparator.comparing(Student::getScore).reversed())
.collect(Collectors.groupingBy(Student::getId));
System.out.println(JSONObject.toJSONString(reversedSortGroupMap));
}
}
class Student {
private Integer id;
private Integer score;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public Integer getScore() {
return score;
}
public void setScore(Integer score) {
this.score = score;
}
public Student(Integer id, Integer score) {
this.id = id;
this.score = score;
}
}执行结果：
java
--------- 根据字段分组，求每个分组的sum ----------
{1=2, 2=5, 3=7, 4=5}
--------- 根据字段分组，求每个分组的count ----------
{1=2, 2=2, 3=2, 4=4}
--------- 根据字段分组，每个分组为：对象的指定字段 ----------
{1=[1, 1], 2=[2, 3], 3=[3, 4], 4=[1, 1, 2, 1]}
--------- 根据字段分组,默认分组 ----------
{1:[{"id":1,"score":1},{"id":1,"score":1}],2:[{"id":2,"score":2},{"id":2,"score":3}],3:[{"id":3,"score":3},{"id":3,"score":4}],4:[{"id":4,"score":1},{"id":4,"score":1},{"id":4,"score":2},{"id":4,"score":1}]}
--------- 根据字段分组，每个分组按照指定字段进行生序排序 ----------
{1:[{"id":1,"score":1},{"id":1,"score":1}],2:[{"id":2,"score":2},{"id":2,"score":3}],3:[{"id":3,"score":3},{"id":3,"score":4}],4:[{"id":4,"score":1},{"id":4,"score":1},{"id":4,"score":1},{"id":4,"score":2}]}
--------- 先排序，再分组 ----------
{1:[{"id":1,"score":1},{"id":1,"score":1}],2:[{"id":2,"score":3},{"id":2,"score":2}],3:[{"id":3,"score":4},{"id":3,"score":3}],4:[{"id":4,"score":2},{"id":4,"score":1},{"id":4,"score":1},{"id":4,"score":1}]}做法二：
先分组，再排序。
先将数据进行groupingBy，得到Map<Long, List>
在通过forEach对每个List进行排序
java
Map<Long, List<StreamMapEntity>> map = getMap();
System.out.println("------ flatMap 处理Map<Long,List<StreamMapEntity>> ------");
System.out.println();
System.out.println("初始数据：" + JSONObject.toJSONString(map));
System.out.println();
map.keySet().forEach(key -> map.computeIfPresent(key, (k, v) -> v.stream().sorted(Comparator.comparing(StreamMapEntity::getId).reversed()).collect(Collectors.toList())));
System.out.println("处理map：对map内的每个list进行排序：");
System.out.println(JSONObject.toJSONString(map));执行结果：
java
------ flatMap 处理Map<Long,List<StreamMapEntity>> ------
初始数据：{1111:[{"id":1,"name":"aa"},{"id":2,"name":"bb"},{"id":3,"name":"cc"},{"id":4,"name":"dd"}],2222:[{"id":5,"name":"ee"},{"id":6,"name":"ff"},{"id":7,"name":"gg"},{"id":8,"name":"hh"}]}
处理map：对map内的每个list进行排序：
{1111:[{"id":4,"name":"dd"},{"id":3,"name":"cc"},{"id":2,"name":"bb"},{"id":1,"name":"aa"}],2222:[{"id":8,"name":"hh"},{"id":7,"name":"gg"},{"id":6,"name":"ff"},{"id":5,"name":"ee"}]}