Declaring New Exception Types

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 applications. The methods of those classes typically are declared to throw appropriate exceptions when problems occur. Programmers write code that processes these existing exceptions to make programs more robust.

If you are a programmer who builds classes that other programmers will use in their programs, you might find it useful to declare your own exception classes that are specific to the problems that can occur when another programmer uses your reusable classes.

Software Engineering Observation 13.13

If possible, indicate exceptions from your methods by using existing exception classes, rather than creating new exception classes. The Java API contains many exception classes that might be suitable for the type of problem your method needs to indicate.

A new exception class must extend an existing exception class to ensure that the class can be used with the exception-handling mechanism. Like any other class, an exception class can contains fields and methods. However, a typical new exception class contains only two constructorsone that takes no arguments and passes a default exception message to the superclass constructor, and one that receives a customized exception message as a string and passes it to the superclass constructor.

Good Programming Practice 13.3

Associating each type of serious execution-time malfunction with an appropriately named Exception class improves program clarity.

Software Engineering Observation 13.14

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 are 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. The new exception class should be a checked exception (i.e., extend Exception but not RuntimeException) if possible clients should be required to handle the exception. The client application should be able to reasonably recover from such an exception. The new exception class should extend RuntimeException if the client code should be able to ignore the exception (i.e., the exception is an unchecked exception).

In Chapter 17, Data Structures, we provide an example of a custom exception class. We declare a reusable class called List that is capable of storing a list of references 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 (i.e., no items can be removed, as the List does not currently contain any items). For this reason, some List methods throw exceptions of exception class EmptyListException.

Good Programming Practice 13.4

By convention, all exception-class names should end with the word Exception.