Java8:按照對象的屬性，對List集合分組

一.對象類

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

 二.普通寫法

    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);
        });
    }

　三.java8 stream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);
        });
    }

運行結果：

二.List<對象>分組成List<List<對象>>,按照某一個屬性

List<List<User>>users.stream().collect(   
 Collectors.collectingAndThen(Collectors.groupingBy(User::getAge,Collectors.toList(),map->new ArrayList<>(map.values())))   
);