Java Reference
In-Depth Information
Most Java programmers use
existing
classes from the Java API, third-party vendors and
freely available class libraries (usually downloadable from the Internet) to build Java appli-
cations. The methods of those classes typically are declared to throw appropriate excep-
tions when problems occur. You write code that processes these existing exceptions to
make your programs more robust.
If you build classes that other programmers will use, it's often appropriate to declare
your own exception classes that are specific to the problems that can occur when another
programmer uses your reusable classes.
A New Exception Type Must Extend an Existing One
A new exception class must extend an existing exception class to ensure that the class can
be used with the exception-handling mechanism. An exception class is like any other class;
however, a typical new exception class contains only four constructors:
•
one that takes no arguments and passes a default error message
String
to the su-
perclass constructor
•
one that receives a customized error message as a
String
and passes it to the su-
perclass constructor
•
one that receives a customized error message as a
String
and a
Throwable
(for
chaining exceptions) and passes both to the superclass constructor
•
one that receives a
Throwable
(for chaining exceptions) and passes it to the super-
class constructor.
Good Programming Practice 11.2
Associating each type of serious execution-time malfunction with an appropriately named
Exception
class improves program clarity.
Software Engineering Observation 11.12
When defining your own exception type, study the existing exception classes in the Java
API and try to extend a related exception class. For example, if you're creating a new class
to represent when a method attempts a division by zero, you might extend class
ArithmeticException
because division by zero occurs during arithmetic. If the existing
classes are not appropriate superclasses for your new exception class, decide whether your
new class should be a checked or an unchecked exception class. If clients should be
required
to handle the exception, the new exception class should be a checked exception (i.e., extend
Exception
but not
RuntimeException
). The client application should be able to
reasonably recover from such an exception. If the client code should be able to ignore the
exception (i.e., the exception is an unchecked one), the new exception class should extend
RuntimeException
.
Example of a Custom Exception Class
In Chapter 21, Custom Generic Data Structures, we provide an example of a
custom
ex-
ception class. We declare a reusable class called
List
that's capable of storing a list of ref-
erences to objects. Some operations typically performed on a
List
are not allowed if the
List
is empty, such as removing an item from the front or back of the list. For this reason,
some
List
methods throw exceptions of exception class
EmptyListException
.
