Exception Handling - Java - page 510

and in the catch block, give code for some alternative action, such as asking the user to
reenter the input. InputMismatchException is a descendent class of RuntimeException ,
so you are not required to account for an InputMismatchException by catching it in a
catch block or declaring it in a throws clause. However, you are allowed to catch an
InputMismatchException in a catch block, which can sometimes be useful.
InputMismatchException is in the standard Java package java.util and so if your
program mentions InputMismatchException , then it needs an import statement, such
as the following:

import java.util.InputMismatchException;

Display 9.11 contains a sample of how you might usefully catch an Input-
MismatchException . This program gets an input int value from the keyboard and
then does nothing with it other than echo the input value. However, you can use code
like this to give robust input to any program that uses keyboard input. The following
Tip explains the general technique we used for the loop in Display 9.11.

TIP: Exception Controlled Loops

Sometimes when an exception is thrown, such as an InputMismatchException for an
ill-formed input, you want your code to simply repeat some code so that the user (or
whatever) can get things right on a second or subsequent try. One way to set up your
code to repeat a loop every time a particular exception is thrown is as follows:

boolean done = false ;

while (! done)
{
try
{
< Code that may throw an exception in the class Exception_Class . >
done = true ; //Will end the loop.
< Possibly more code. >
}
catch ( Exception_Class e)
{
< Some code. >
}
}

Note that if an exception is thrown in the first piece of code in the try block, then the
try block ends before the line that sets done to true is executed and so the loop body
is repeated. If no exception is thrown, then done is set to true and the loop body is not
repeated.
Display 9.11 contains an example of such a loop. Minor variations on this outline can
accommodate a range of different situations for which you want to repeat code on
throwing an exception.

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