"Warning: [unchecked] unchecked cast" when casting Object to ArrayList<String[]>

Question

Strange situation - below is the code:

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

String str = (String) obj[0];//OK
ArrayList<String[]> list = (ArrayList<String[]>) obj[1];//warning: [unchecked] unchecked cast

When project is built (with compiler option -Xlint:unchecked in project properties), I get one warning:

warning: [unchecked] unchecked cast
ArrayList list = (ArrayList) obj[1];
required: ArrayList
found: Object

But casting String in the same way is OK. What is the problem here?

Answer 1

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];