Question

Is the use of NullPointerException recommended in this case:

/**
 * Drop the database referenced by the singleton.
 * 
 * @throws NullPointerException
 */
public static void dropDatabase() throws NullPointerException {
    if (store != null) {
        store.dropDatabase(DATABASE);
    } else {
        throw new NullPointerException("No valid database connection.");
    }
}

Answer 1

Well it’s pretty pointless code in that you’d still get a NullPointerException if it unconditionally called store.dropDatabase. You wouldn’t get the same message, but the stack trace would make it pretty clear.

In this case I would say it should be an IllegalStateException though:

Signals that a method has been invoked at an illegal or inappropriate time. In other words, the Java environment or Java application is not in an appropriate state for the requested operation.

Sounds exactly like the situation to me. I’d also do the check first, like this:

if (store == null)
{
    throw new IllegalStateException("No valid database connection.");
}
store.dropDatabase(DATABASE);

That way you can get all the preconditions out of the way at the start of the method, and then focus on the main body.

With Guava, I’d just change this to:

Preconditions.checkState(store != null, "No valid database connection.");
store.dropDatabase(DATABASE);