How make a List of ArrayList in Java?

The reason for being the last list empty is that Java stores everything in the form of references i.e in the form of memory address and manipulates them in the same way.

Hence ,the ArrayList object list is also a reference. So

1) Everytime the inner loop is run a reference to list object is added to the ArrayList list_of_lists.

2) Hence each time the list is manipulated the list_of_lists stores the value of list as many times as the inner loop is run i.e. the list is added to it.

Hence Each time System.out.println(list_of_lists) is called

1) it prints list same no of times it is added to list_of_lists.

2) the list is cleared with list.clear() after that which makes all the references to list empty.

You can also check this code to get list_of_lists at each iteration to get a clear picture of what is happening.

ArrayList<ArrayList<Integer>> list_of_lists = new ArrayList<ArrayList<Integer>>();
List<Integer> list = new ArrayList<Integer>();          
for(int x = 0; x < 5; x++) {
    for(int y = 0; y < 2; y++) {                
        list.add(x);
        list.add(y);                
        list_of_lists.add((ArrayList<Integer>) list);
        System.out.println(list_of_lists);
        list.clear();
    }
    System.out.println(list_of_lists); //composes of empty lists 
                                        //the same of times list is added to it
}
System.out.println(list_of_lists);

**Solution:*

To create a List of Lists you have to allocate memory each time new list is added to list_of_lists:

  ArrayList<ArrayList<Integer>> list_of_lists = new 
    ArrayList<ArrayList<Integer>>();
        List<Integer> list;        
        for(int x = 0; x < 5; x++) {
            for(int y = 0; y < 2; y++) {  
                list = new ArrayList<>();              
                list.add(x);
                list.add(y);                
                list_of_lists.add((ArrayList<Integer>) list);
                System.out.println(list_of_lists);
                // list.clear();
            }
         }
          System.out.println(list_of_lists);
        }