Hint: Using an exception

Throwing an exception is one way to get out of a bunch of recursive calls. There are two ways that you can throw an exception: you can reuse an already existing exception, or you can define your own.

Reusing an existing exception

If there is an existing exception that does the job well, you can throw one of those. For example, the Scanner.nextLine() method already throws a NoSuchElementException on end-of-file (which you cause by typing control-D). That doesn't need to be declared in a throws clause because it is a subtype of RuntimeException.

Remember that you will need to construct a new instance of the exception class when you throw it. The constructors for the pre-defined classes will accept a String parameter as a message.

Creating your own exception class

You can create your own class of Exception and throw it. If you were to dig out your notes from long ago, you'd find the following tips:

• Declare a new kind of exception as a class that extends the existing class Exception.
• Give your class a default (no-argument) constructor and one that takes a String argument.
• Make those constructors call super() to use the existing exception machinery.

I've included a file SampleException.java that shows you how all of this can be done.

Note that throwing your own exception will probably require you to add a throws clause to method declarations.