arraylist not work as expected

You were modifying the list and adding it one more time, so that's why [567] appeared twice. I suggest you change code as following:

ArrayList<List<Integer>> main = new ArrayList<>(); 
ArrayList<Integer> sub = new ArrayList<>();
sub.add(1);
sub.add(2);
main.add(sub);
sub = new ArrayList<>();
sub.add(5);
sub.add(6);
sub.add(7);
main.add(sub);

You are referring to same instance

sub.add(1);
sub.add(2);
main.add(sub);
sub.clear(); // you are clearing same instance you added to `main` list

you need to create new instance

You need

 ArrayList<Integer> sub1 = new ArrayList<>();
 sub1.add(1);
 sub1.add(2);
 main.add(sub1);
 ArrayList<Integer> sub2 = new ArrayList<>();
 sub2.add(10);
 sub2.add(20);
 main.add(sub2);

Adding to an ArrayList using the .add function, adds the reference of the same memory address onto the ArrayList.

So in your solution, a reference pointing to what sub was pointing to was added in the ArrayList. This reference internally pointed to the same location.

Now when you cleared 'sub', the actual data got cleared and as there was only one copy of the data(with two references pointing to it), it cleared.

Now when you added to sub back , it got added to the same memory location which was pointed by both the references. Hence you see its copy in main.

Hope you understood.

you can clone the same object like this:

public static void main(String[] args) {
 ArrayList<List<Integer>> main = new ArrayList<List<Integer>>(); 
 ArrayList<Integer> sub=new ArrayList<Integer>();
 sub.add(1);
 sub.add(2);
 main.add(sub);
 sub=(ArrayList<Integer>) sub.clone();// clone here
 sub.clear(); 
 System.out.println("sub = "+sub);
 System.out.println("main = "+main);
 sub.add(5);
 sub.add(6);
 sub.add(7);
 main.add(sub);
 System.out.println("sub = "+sub);
 System.out.println("main = "+main);
 }

output :

sub = []
main = [[1, 2]]
sub = [5, 6, 7]
main = [[1, 2], [5, 6, 7]]

Assuming your Integer's are already in a collection that is the basis of your "Big Loop":

ArrayList<List<Integer> main_list = new ArrayList<>();
Collection<Collection<Integer>> mySubs = xxxx;
for (Collection<Integer> group : mySubs){
 List<Integer> sub = new ArrayList<>(group);
 main_list.add(sub)
}

All of the Collection classes have constructors that can take other Collection's

If you don't have an existing collection to start with, then you'll just need to use sub.clone().clear() or sub = new ArrayList<>()

• java
• arraylist