The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
When faced with choosing the type of exception to throw, you can either use one written by someone else the Java platform provides a lot of exception classes you can use or you can write one of your own. You should write your own exception classes if you answer yes to any of the following questions; otherwise, you can probably use someone else's.
Suppose you are writing a linked list class. The class supports the following methods, among others:
objectAt(int n)
Returns the object in the n
th position in the list. Throws an exception if the argument is less than 0 or more than the number of objects currently in the list.firstObject()
Returns the first object in the list. Throws an exception if the list contains no objects.indexOf(Object o)
Searches the list for the specified Object
and returns its position in the list. Throws an exception if the object passed into the method is not in the list.The linked list class can throw multiple exceptions, and it would be convenient to be able to catch all exceptions thrown by the linked list with one exception handler. Also, if you plan to distribute your linked list in a package, all related code should be packaged together. Thus, the linked list should provide its own set of exception classes.
The next figure illustrates one possible class hierarchy for the exceptions thrown by the linked list.
Example exception class hierarchy.
Any Exception
subclass can be used as the parent class of LinkedListException
. However, a quick perusal of those subclasses shows that they are inappropriate because they are either too specialized or completely unrelated to LinkedListException
. Therefore, the parent class of LinkedListException
should be Exception
.
Most applets and applications you write will throw objects that are Exception
s. Error
s are normally used for serious, hard errors in the system, such as those that prevent the JVM from running.
Exception
to the names of all classes that inherit (directly or indirectly) from the Exception
class.