Adding element in 2d arraylist in java

Question

ArrayList<Integer> a =new ArrayList<Integer>(); ArrayList<ArrayList<Integer>> j =new ArrayList<ArrayList<Integer>>(); a.add(1); a.add(2); a.add(3); for(int c=0; c<10; c++){ j.add(a); } j.get(3).add(1); System.out.println(j); 

Does anyone know why this code adds 1 to each element of j, and not just to the third element, and what can I do to fix this?

Answer 1

This is what happens when you add array list a to array list j 10 times.

This is what happens when you add 1 to the list of array a .

Thus, basically all 10 ArrayList j indices point to one ArrayList a . Therefore, printing a value from any index j will always give you the same result.

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So that each pointer points to a different list of arrays:

Answer 2

You use the same instance of ArrayList a in each j element. You must create a new ArrayList instance for each j element if you want them to be different.

Answer 3

  for(int c=0; c<10; c++) { j.add(new ArrayList<>(a)); } 

In your code a, there is a pointer to the cell in which the ArrayList is located.

Answer 4

In fact, j.get (3) refers to an array of List a like every j.get (c), so every cell j pointing to a changes. The following image shows how this works.