Java, A Beginner's Guide, 5th Edition, 5th Edition

Catching Subclass Exceptions

There is one important point about multiple catch statements that relates to subclasses. A catch clause for a superclass will also match any of its subclasses. For example, since the superclass of all exceptions is Throwable, to catch all possible exceptions, catch Throwable. If you want to catch exceptions of both a superclass type and a subclass type, put the subclass first in the catch sequence. If you don’t, then the superclass catch will also catch all derived classes. This rule is self-enforcing because putting the superclass first causes unreachable code to be created, since the subclass catch clause can never execute. In Java, unreachable code is an error.

For example, consider the following program: