Question

This code works. it prints out the values in the list

ArrayList<String> menuItemsUrl=null;

for (int i = 0; i < modules.size(); i++) {
                String tmp = modules.get(i).attr("href");
                Log.d("", tmp);

            }

but this one generates an null pointer. why?

for (int i = 0; i < modules.size(); i++) {
                String tmp = modules.get(i).attr("href");
                menuItemsUrl.add(tmp);

            }

Answer 1

In your code you havent initialized menuItemsUrl, hence NPE.

You have declared your list here ArrayList<String> menuItemsUrl=null; and when you try to access it menuItemsUrl.add(tmp);, menuItemsUrl isn’t refering to anything it is pointing to null which means nothing hence NPE, to get rid of NPE, initialize it before you use.

menuItemsUrl = new ArrayList<String>();