How to add an ArrayList<Integer> to an ArrayList<ArrayList<Integer>> again and again with different elements in ArrayList<Integer>

Question

I have created an ArrayList of ArrayList in java. And iam storing values in them as follows :

import java.util.ArrayList;


    class Test {
        public static void main(String args[]) {
                ArrayList<ArrayList<Integer>> connections = new ArrayList<ArrayList<Integer>>();
                ArrayList<Integer> currentConnects = new ArrayList<>();
                currentConnects.add(7);
                currentConnects.add(4);
                connections.add(currentConnects);
                currentConnects.clear();
                currentConnects.add(10);
                currentConnects.add(15);
                connections.add(currentConnects);
                System.out.println(connections);
        }
    }

I am expecting the output to be :

[[7, 4], [10, 15]]

but the output is :

[[10, 15], [10, 15]]

How can i achieve my expected output without using any other extra variables?

Answer 1

You're actually referencing the same object twice in your connections object, hence the result you have.

Instead of currentConnects.clear(); try currentConnects = new ArrayList<>();

Answer 2

You need to remove currentConnects.clear(); and create a new ArrayList Object.Your ArrayList objects are pointing to the same Object in heap.The ArrayList<ArrayList<Integer>> contains same reference of ArrayList twice

import java.util.ArrayList;

    class Test {
        public static void main(String args[]) {
                ArrayList<ArrayList<Integer>> connections = new ArrayList<ArrayList<Integer>>();
                ArrayList<Integer> currentConnects = new ArrayList<>();
                currentConnects.add(7);
                currentConnects.add(4);
                connections.add(currentConnects);

                currentConnects = new ArrayList<>();
                currentConnects.add(10);
                currentConnects.add(15);
                connections.add(currentConnects);
                System.out.println(connections);
        }
    }

Answer 3

You have to create new array lists everytime.

When you do:

  connections.add(currentConnects);
  currentConnects.clear();

You first add a reference of currentConnects, and then clear the content of currentConnects, you are not creating a new arraylist. Instead do:

  ArrayList<Integer> currentConnects = new ArrayList<>();
  currentConnects.add(7);
  currentConnects.add(4);
  connections.add(currentConnects);
  currentConnects = new ArrayList<>();
  currentConnects.add(10);
  currentConnects.add(15);
  connections.add(currentConnects);

Answer 4

When you call connections.add(currentConnects); , since connections is of type ArrayList<ArrayList<Integer>> , you're actually adding a reference to entire ArrayList connections, NOT just the integers in the ArrayList itself.

So because you call connections.add(currentConnects); twice, there are two references to connections ArrayList, which is why the integers held in connections are printed out twice.