Exception Handling - Java - page 485

catch block. The catch block outputs the message of the exception and then calls the
method secondChance . The method secondChance gives the user a second chance to
enter the input correctly and then carries out the calculation. If the user tries a second
time to divide by zero, the method ends the program. The method secondChance is
there only for this exceptional case. So, we have separated the code for the exceptional
case of a division by zero into a separate method, where it will not clutter the code for
the normal case.

TIP: Preserve getMessage

For all predefined exception classes, getMessage will return the string that is passed as
an argument to the constructor (or will return a default string if no argument is used
with the constructor). For example, if the exception is thrown as follows:

throw new Exception("Wow, this is exceptional!");

then "Wow, this is exceptional!" is used as the value of the String instance variable
of the object created. If the object is called e , the method invocation e.getMessage()
returns "Wow, this is exceptional!" You want to preserve this behavior in the excep-
tion classes you define.
For example, suppose you are defining an exception class named NegativeNumber-
Exception . Be sure to include a constructor with a string parameter that begins with
a call to super , as illustrated by the following constructor:

public NegativeNumberException(String message)
{
super (message);
}

The call to super is a call to a constructor of the base class. If the base class constructor
handles the message correctly, then so will a class defined in this way.
You should also include a no-argument constructor in each exception class. This no-
argument constructor should set a default value to be retrieved by getMessage . The con-
structor should begin with a call to super , as illustrated by the following constructor:

public NegativeNumberException()
{
super ("Negative Number Exception!");
}

If getMessage works as we described for the base class, then this sort of no-
argument constructor will work correctly for the new exception class being defined. A
full definition of the class NegativeNumberException is given in Display 9.8. ■