Use map set or list set to store list set

1. use list Collection storage list

Be careful ：

Every time List<List> Add list when , Should be re opened in memory list Space is convenient for storing new data to list in
List<List> Deposit list Type of data to use add(int i,list) Method, not add(list), In this way, every time new One list When the collection ,List<List> Only one data is stored , Previous list The data will be overwritten （ still list The problem of pointing in the opened memory ）, Only the data of the last cycle is output . So it is necessary to specify where the data should be stored each time , Use add(int i,list) Method .
To give list When storing data , Every time new One list, No new Words list Only one is opened in memory list object , Every time you put data in it ,list All point to this object . So to achieve each cycle has a different list when , Create a new one in memory under the loop list

public static void main(String[] args) {
        List<List<User>> list = new ArrayList<>();

        for (int i = 0; i < 2; i++) {
            List<User> list1 = new ArrayList<>();
            list1.add(new User("1","zhangsan"));
            list1.add(new User("2","lisi"));
            list1.add(new User("3","wangwu"));
            list.add(list1);
        }
        System.out.println("list: "+list);
}

Running results  

It can't be written like this

public static void main(String[] args) {
        List<List<User>> list = new ArrayList<>();
        List<User> list1 = new ArrayList<>();
        for (int i = 0; i < 2; i++) {
            
            list1.add(new User("1","zhangsan"));
            list1.add(new User("2","lisi"));
            list1.add(new User("3","wangwu"));
            list.add(list1);
        }
        System.out.println("list: "+list);
}

Otherwise, the result is ：

The above problems will occur , Each time according to the defined cycle add Add after （ Will cause duplicate data ） 

2. use map Collective storage list

// When adding multiple values ：
 public static void main(String[] args) {
        List<List<User>> list = new ArrayList<>();
        Map<String,List<User>> map = new HashMap<>();
        for (int i = 0; i < 2; i++) {
            List<User> list2 = new ArrayList<>();
            list2.add(new User("1","zhangsan"));
            list2.add(new User("2","lisi"));
            list2.add(new User("3","wangwu"));
            map.put("list"+i,list2);
        }
}

// When you add a list When the collection ：
public static void main(String[] args) {
        List<List<User>> list = new ArrayList<>();
        Map<String,List<User>> map = new HashMap<>();
        List<User> list2 = new ArrayList<>();
        list2.add(new User("1","zhangsan"));
        list2.add(new User("2","lisi"));
        list2.add(new User("3","wangwu"));
        map.put("list"+i,list2);
        
}

  Multiple list The result of the value

  One list The result of the value