Exception Handling - Absolute Java - page 533

EXAMPLE: (continued)

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
includes 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 Exception 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.
The throw operator causes a change in the flow of control and delivers the Exception
object to a suitable place, as we are about to explain. When the throw operator is executed,
the try block ends immediately and control passes to the following catch block. (If it
helps, you can draw an analogy between the execution of the throw operator in a try block
and the execution of a break statement in a loop or switch statement.) When control is
transferred to the catch block, the Exception object that is thrown is plugged in for the
catch block parameter e . So, the expression e.getMessage() returns the string “Lesson
is canceled. No men.” The method getMessage() of the class Exception is an
accessor method that retrieves the String in the private instance variable of the Exception
object—that is, the String used as an argument to the Exception constructor.
To see if you get the basic idea of how this exception throwing mechanism works,
study the Sample Dialogues in Displays 9.2 and 9.3 . The next few sections explain
this mechanism in more detail.

Display 9.2 Handling a Special Case without Exception Handling (part 1 of 3)

1 import java.util.Scanner;

2 public class DanceLesson
3 {
4 public static void main(String[] args)
5 {
6 Scanner keyboard = new Scanner(System.in);
7
8 System.out.println("Enter number of male dancers:");
9 int men = keyboard.nextInt();

10 System.out.println("Enter number of female dancers: ");
11

int women = keyboard.nextInt();

(continued)