I don't understand this NullPointerException

Question

I have the following code:

RoomCell r = b.GetRoomCellAt(1, 2);
assertTrue(r.isDoorway());

The second line is failing with a null pointer exception, as if r hadn't been instantiated. But I'm setting it equal to a cell that that method retrieves, so it is obviously not null.

Why would I possibly be getting this error?

b.GetRoomCellAt(1, 2) must return null. Check the method docs o source... — Thihara, Oct 03 '13 at 02:44

score 3 · Answer 1 · answered Oct 03 '13 at 02:45

3

What's your full stacktrace? It is needed in order to determine which of the following is the cause:

It's possible that GetRoomCellAt doesn't find a value and returns null

It's possible that the implementation of GetRoomCellAt accesses a null pointer

It's possible that the implementation of isDoorway accesses a null pointer

FYI, it is good practice to use the @Nullable annotation to annotate parameters and return types

answered Oct 03 '13 at 02:45

Uri

88,451
51
221
321

+1 for making me read about the `@Nullable` annotation. A quick Google search led me to http://stackoverflow.com/questions/14076296/nullable-annotation-usage :) – icedwater Oct 03 '13 at 04:03
If you use Eclipse, you now get built in Nullability analysis (though you may have to turn it on). That being said, I often find Guava's Optional to be more effective. – Uri Oct 03 '13 at 05:05

score 2 · Answer 2 · answered Oct 03 '13 at 02:44

2

The function GetRoomCellAt() can probably still return NULL.

answered Oct 03 '13 at 02:44

Jesus Ramos

22,940
10
58
88

newuser · Answer 3 · 2013-10-03T02:59:31.677

0

if(r != null)
{
  assertTrue(r.isDoorway());
}

instead of

assertTrue(r.isDoorway());


b.GetRoomCellAt(1, 2);

This cell may be null. So only you are getting nullPointerException

edited Oct 03 '13 at 02:59

answered Oct 03 '13 at 02:45

newuser

8,338
2
25
33

score 0 · Answer 4 · answered Oct 03 '13 at 03:05

The only way you'd be getting a NullPointerException is if r was indeed null. You can put this in your test conditions to ensure that, if r actually is null, then the test will fail:

RoomCell r = b.GetRoomCellAt(1, 2);
assertNotNull(r);
assertTrue(r.isDoorway());

If r is null, then the test fails early, and you avoid the NPE.

I don't understand this NullPointerException

4 Answers4