java stream 把 list转成 Map<String,List>

文章目录

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class GroupTest {

    public static void main(String[] args){
        List<GroupTest> list = new ArrayList<>();
        list.add(new GroupTest(1,"AA"));
        list.add(new GroupTest(1,"BB"));
        list.add(new GroupTest(2,"CC"));
        list.add(new GroupTest(3,"33"));
        list.add(new GroupTest(3,"44"));
        list.add(new GroupTest(2,"55"));
        list.add(new GroupTest(2,"66"));

        Map<Integer,List<GroupTest>> m1 = list.stream().collect(
                Collectors.groupingBy(GroupTest::getAge)
        );
        Map<Integer,List<String>> m2 = list.stream().collect(
                Collectors.groupingBy(GroupTest::getAge,
                        Collectors.mapping(GroupTest::getTitle,Collectors.toList())
                )
        );
        System.out.println(m1);
        System.out.println(m2);
    }
    Integer age;
    String title;

    @Override
    public String toString() {
        return "GroupTest{" +
                "age=" + age +
                ", title='" + title + '\'' +
                '}';
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public GroupTest(Integer age, String title) {
        this.age = age;
        this.title = title;
    }

}

输出：

{1=[GroupTest{age=1, title='AA'}, GroupTest{age=1, title='BB'}], 2=[GroupTest{age=2, title='CC'}, GroupTest{age=2, title='55'}, GroupTest{age=2, title='66'}], 3=[GroupTest{age=3, title='33'}, GroupTest{age=3, title='44'}]}

{1=[AA, BB], 2=[CC, 55, 66], 3=[33, 44]}