About Question enthuware.ocpjp.v21.2.4040 :

[giginar]

the default one (option 3) is false because: "all the building subclasses(hotel, hospital) are covered, except for "null" case, so we need null, not default" is that true?

[admin]

No, did you read the explanation?

This is syntactically valid but does not implement the logic given in the problem statement. If b is null, this code will throw a NullPointerException while the code in the problem statement will not.

So default will work but not exactly as the logic given in the problem statement.