Exception Handling - Java - page 474

EXAMPLE: (continued)

The throw operator causes a change in the flow of control and it 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. The next few sections explain this mechanism in more detail.

try-throw-catch Mechanism
The basic way of handling exceptions in Java consists of the try-throw-catch trio.
The general setup consists of a try block followed by one or more catch blocks. First
let's describe what a try block is. A try block has the syntax
try
{
Some_Code
}

try block

This try block contains the code for the basic algorithm that tells what to do when
everything goes smoothly. It is called a try block because it “tries” to execute the case
where all goes smoothly.
Now, if something exceptional does happen, you want to throw an exception,
which is a way of indicating that something unusual happened. So the basic outline,
when we add a throw , is as follows:
try
{
Code_That_May_Throw_An_Exception
}

The following is an example of a try block with throw statements included (copied
from Display 9.2):
try
{
if (men == 0 && women == 0)
throw new Exception("Lesson is canceled. No students.");
else if (men == 0)
throw new Exception("Lesson is canceled. No men.");
else if (women == 0)
throw new Exception("Lesson is canceled. No women.");