When traverse a HashMap, I get NullPointerException [closed]

Answer 1

Where is map initialized? If you don't actually assign anything to it, of course it will be null.

(It's not clear if it's a class member or a local variable.)

Answer 2

If the map does contain elements, then map.entrySet() won't throw NullPointerException (because the map is not null), so the exception must be coming from accessing one of the elements (or its subobjects) in the loop.

Answer 3

If map is a local variable (and not an instance field), it is theoretically impossible to get an NPE on map.entrySet() after map.entrySet().size() already succeeded, even if the map itself was accessible to another thread and that thread changed it in every way imaginable. If the first entry in your stacktrace is a line of your code, as opposed to some code that was called from yours, then the only way for that to happen is for the map variable itself to be null. Therefore I think you need to revisit your diagnostic clues.

Answer 4

I just tried this: and it worked fine. Your map must be assigned to null somewhere.

HashMap<String, HashSet<String>> map;
map = new HashMap<String, HashSet<String>>();

for(Map.Entry<String, HashSet<String>> entry : map.entrySet()) {

}