Exception Handling - Absolute Java - page 545

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For example, suppose you are defi ning 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 defi ned 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
constructor 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 defi ned. A full defi ni-
tion of the class NegativeNumberException is given in Display 9.9 . ■

Exception Object Characteristics
The two most important things about an exception object are its type (the exception class) and
a message that it carries in an instance variable of type String . This string can be recovered
with the accessor method getMessage . This string allows your code to send a message
along with an exception object, so that the catch block can use the message.

Programmer-Defined Exception Classes
You may define your own exception classes, but every such class must be a derived class
of an already existing exception class (either from one of the standard Java libraries or
programmer defined).
GUIDELINES
• If you have no compelling reason to use any other class as the base class, use the
class Exception as the base class.
• You should define two (or more) constructors, as described later in this list.

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