【Java】list根據某一條件進行分組

集合中為對象，根據對象中的某一屬性值分組

public static void main(String[] args){
        List<User> list = new ArrayList<>();
        User user1 = new User("1", "張三1", "10");
        User user2 = new User("2", "張三2", "10");
        User user3 = new User("2", "張三3", "10");
        User user4 = new User("4", "張三4", "11");
        User user5 = new User("5", "張三5", "11");
        User user6 = new User("6", "張三6", "11");
        User user7 = new User("7", "張三7", "12");
        User user8 = new User("8", "張三8", "12");
        User user9 = new User("9", "張三9", "12");
        list.add(user1);
        list.add(user2);
        list.add(user3);
        list.add(user4);
        list.add(user5);
        list.add(user6);
        list.add(user7);
        list.add(user8);
        list.add(user9);
        List<List<User>> groupList = new ArrayList<>();
        list.stream().collect(Collectors.groupingBy(User::getAge,Collectors.toList()))
                    .forEach((age,fooListByAge)->{
                        groupList.add(fooListByAge);
                    });
        
        groupList.forEach((userList)->{
            System.out.println(userList.size());
            userList.forEach((user)->System.out.println(user));
        });
    }

注意：
list.stream().collect(Collectors.groupingBy(User::getAge,Collectors.toList()))
                    .forEach((age,fooListByName)->{
                        groupList.add(fooListByName);
                    });

此句中的User::getAge 

getAge 為對象中某屬性的get方法