Java ArrayList of ArrayList

Question

The following code outputs

[[100, 200, 300], [100, 200, 300]].  

However, what I expect is

[[100, 200, 300], [100, 200]],  

Where am I wrong?

public static void main(String[] args) {     ArrayList<ArrayList<Integer>> outer = new ArrayList<ArrayList<Integer>>();     ArrayList<Integer> inner = new ArrayList<Integer>();              inner.add(100);          inner.add(200);      outer.add(inner);     outer.add(inner);      outer.get(0).add(300);      System.out.println(outer);  } 

Answer 1

You are adding a reference to the same inner ArrayList twice to the outer list. Therefore, when you are changing the inner list (by adding 300), you see it in "both" inner lists (when actually there's just one inner list for which two references are stored in the outer list).

To get your desired result, you should create a new inner list :

public static void main(String[] args) {     ArrayList<ArrayList<Integer>> outer = new ArrayList<ArrayList<Integer>>();     ArrayList<Integer> inner = new ArrayList<Integer>();              inner.add(100);          inner.add(200);     outer.add(inner); // add first list     inner = new ArrayList<Integer>(inner); // create a new inner list that has the same content as                                              // the original inner list     outer.add(inner); // add second list      outer.get(0).add(300); // changes only the first inner list      System.out.println(outer); }