Java stream 分组操作

根据单/多字段分组

单字段分组可以直接使用指定字段，多字段分组则采用拼接Key的形式

单字段:

//单字段分组Map> singleAttr = list.stream().collect(                Collectors.groupingBy(t -> t.getNativePlace()));//还可以写成简化版//Map> collect = list.stream().collect(// Collectors.groupingBy(User::getNativePlace));

多字段：

//多字段分组Map> multiAttr = list.stream().collect(                Collectors.groupingBy(t -> t.getNativePlace()+"_"+t.getAge()));

多层级：

//多级分组Map>> multiLevel = list.stream().collect(        Collectors.groupingBy(                t -> t.getNativePlace(),                Collectors.groupingBy(t -> t.getAge())        ));

测试

package test;import java.util.ArrayList;import java.util.List;import java.util.Map;import java.util.Optional;import java.util.stream.Collectors;public class Test {    public static void main(String[] args) {        ArrayList list=new ArrayList();        list.add(new User(11,"a"));        list.add(new User(12,"a"));        list.add(new User(14,"c"));        list.add(new User(14,"c"));        //单字段分组        Map> singleAttr = list.stream().collect(Collectors.groupingBy(t -> t.getNativePlace()));        //还可以写成简化版        //Map> collect = list.stream().collect(Collectors.groupingBy(User::getNativePlace));        //多字段分组        Map> multiAttr = list.stream().collect(Collectors.groupingBy(t -> t.getNativePlace()+"_"+t.getAge()));        //多级分组        Map>> multiLevel = list.stream().collect(                Collectors.groupingBy(                        t -> t.getNativePlace(),                        Collectors.groupingBy(t -> t.getAge())                )        );    }}class User{    private int age;    private String nativePlace;    public String getNativePlace() {        return nativePlace;    }    public void setNativePlace(String nativePlace) {        this.nativePlace = nativePlace;    }    public int getAge() {        return age;    }    public void setAge(int age) {        this.age = age;    }    public User(int age,String nativePlace) {        this.age = age;        this.nativePlace=nativePlace;    }}

来源地址：https://blog.csdn.net/zcy_wxy/article/details/130192370