Exception Handling - Java - page 473

Display 9.2 Same Thing Using Exception Handling (part 3 of 3)

Sample Dialogue 3
Enter number of male dancers:
0
Enter number of female dancers:
5
Lesson is canceled. No men.

Sample Dialogue 4
Enter number of male dancers:
4
Enter number of female dancers:
0
Lesson is canceled. No women.

EXAMPLE:

(continued)

If the number of men or the number of women is zero (or both), that is an exceptional case in
this program and results in an exception being . To make things concrete, let's say that

the number of men is zero, but the number of women is not zero. In that case the following
statement is executed, which is how Java throws an exception:

thrown

throw new Exception("Lesson is canceled. No men.");

Let's analyze this statement. The following is the invocation of a constructor for the class
Exception , which is the standard Java package java.lang .

new Exception("Lesson is canceled. No men.");

The created Exception object is not assigned to a variable, but rather is used as an (anonymous)
argument to the throw operator. (Anonymous arguments were discussed in Chapter 5.) The key-
word throw is an operator with syntax similar to the unary + or unary − operators. To make it
look more like an operator, you can write it with parentheses around the argument, as follows:

throw ( new Exception("Lesson is canceled. No men."));

Although it is perfectly legal and sensible to include these extra parentheses, nobody does
include them.
To understand this process of throwing, you need to know two things: What is this
Exception class? And what does the throw operator do with the Exception object? The class
Exception is another class from the standard Java package java.lang . As you have already
seen, the class Exception has a constructor that takes a single String argument. The Excep-
tion object created stores this String argument (in a private instance variable). As you will
see, this String argument can later be retrieved from the Exception object.

(continued)