Java8實現以對象的某屬性分組並對分組后的結果list

轉自：https://www.cnblogs.com/wwjj4811/p/13246031.html

1、對象類

@Data
@AllArgsConstructor
public class User {
    private Integer age;
    private String name;
}

2、普通寫法

//構建的list數據
private List<User> users =Arrays.asList(new User(20,"張三"),new User(18,"李四"),new User(20,"王五"));
 

@Test
public void testGroup(){
    Map<Integer,List<User>>  userMap =  new HashMap<>();
    users.forEach(user->{
        List<User> users = userMap.get(user.getAge());
        if(users==null){
            users = new ArrayList<>();
            users.add(user);
            userMap.put(user.getAge(),users);
        }else{
            users.add(user);
        }
    });
    userMap.forEach((x,y)->{
        System.out.println(x+":"+y);
    });
}

3、Java8 寫法

@Test
public void testGroup2(){
    Map<Integer,List<User>>  userMap =  new HashMap<>();
    users.stream().collect(Collectors.groupingBy(User::getAge,Collectors.toList()))
            .forEach(userMap::put);
    userMap.forEach((x,y)->{
        System.out.println(x+":"+y);
    });
}

4、運行結果