Types of Exception

All exception types are subclasses of the built-in class Throwable. Thus, Throwable is at the
top of the exception class hierarchy. Immediately below Throwable are two subclasses that
partition exceptions into two distinct branches. One branch is headed by Exception. This class
is used for exceptional conditions that user programs should catch. This is also the class that
you will subclass to create your own custom exception types. There is an important subclass
of Exception, called RuntimeException. Exceptions of this type are automatically defined for
the programs that you write and include things such as division by zero and invalid array
indexing.
The other branch is topped by Error, which defines exceptions that are not expected to
be caught under normal circumstances by your program. Exceptions of type Error are used
by the Java run-time system to indicate errors having to do with the run-time environment,
itself. Stack overflow is an example of such an error. This chapter will not be dealing with
exceptions of type Error, because these are typically created in response to catastrophic failures
that cannot usually be handled by your program.