Why explicitly throw a NullPointerException rather than letting it happen naturally?

Question

When reading JDK source code, I find it common that the author will check the parameters if they are null and then throw new NullPointerException() manually. Why do they do

Answer 1

It's because it's possible for it not to happen naturally. Let's see piece of code like this:

bool isUserAMoron(User user) {
    Connection c = UnstableDatabase.getConnection();
    if (user.name == "Moron") { 
      // In this case we don't need to connect to DB
      return true;
    } else {
      return c.makeMoronishCheck(user.id);
    }
}

(of course there is numerous problems in this sample about code quality. Sorry to lazy to imagine perfect sample)

Situation when c will not be actually used and NullPointerException will not be thrown even if c == null is possible.

In more complicated situations it's becomes very non-easy to hunt down such cases. This is why general check like if (c == null) throw new NullPointerException() is better.