“Warning: [unchecked] unchecked cast” when casting Object to ArrayList

Question

Strange situation - below is the code:

ArrayList listArr = new ArrayList<>();
Object[] obj = new Object[]{\"str\", listArr};

String st         


        

Answer 1

This is because if you try to cast Integer to String you will get ClassCastException at runtime. But there will be no ClassCastException here:

    ArrayList<Integer[]> listArr = new ArrayList<>();
    ArrayList<String[]> list = (ArrayList<String[]>) obj[1];

Answer 2

The compiler complains

ArrayList<String[]> list = (ArrayList<String[]>) obj[1]

because a cast is a runtime check. So at runtime your ArrayList<String[]> could be a ArrayList<Whatever[]>, because the type of obj is unknown.

Answer 3

This is because the compiler can not verify the internal types at the list level, so you need to first verify for list. And the internal types individually.

Instead of ArrayList<String[]> list = (ArrayList<String[]>) obj[1];

It should be ArrayList<?> list = (ArrayList<?>) obj[1];