You could use:
Bitmap bitmap = cache.get(key, null);
But understand that this is the same as get(key):
Bitmap bitmap = cache.get(key);
The best way to use get(key, default) is to provide a generic default case, something to is a valid substitute when the key is not found.
But there is no good reason not to use if(get(key) != null) as a quick replacement for contains().