Java Reference
In-Depth Information
9.1
Exception Handling Basics
Well the program works for most cases. I didn't know it had to work for
that  case.
COMPUTER SCIENCE STUDENT, appealing a grade
Exception handling is meant to be used sparingly and in some situations that are more
involved than what is reasonable to include in an introductory example. So, in some cases,
we will teach you the exception handling details of Java by means of simple examples
that would not normally use exception handling. This makes a lot of sense for learning
about the exception handling details of Java, but do not forget that these examples are toy
examples and, in practice, you would not use exception handling for anything this simple.
try-catch Mechanism
The basic way of handling exceptions in Java consists of the try-throw-catch trio. At
this point, we will start with only try and catch . 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 following syntax:
try block
try
{
Some_Code_That_May_Throw_An_Exception
}
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 well.
Now, an exception can be “thrown” as a way of indicating that something unusual
happened. For example, if our code tries to divide by zero, then an ArithmeticException
object is thrown. In most of this chapter, our own code will throw the exception, but
initially we will have existing Java classes do the throwing.
As the name suggests, when something is “thrown,” something goes from one place
to another place. In Java, what goes from one place to another is the flow of control
as well as the exception object that is thrown. When an exception is thrown, the code
in the surrounding try block stops executing and (normally) another portion of code,
known as a catch block , begins execution. The catch block has a parameter, and the
exception object thrown is plugged in for this catch block parameter. This executing
of the catch block is called catching the exception or handling the exception . When
an exception is thrown, it should ultimately be handled by (caught by) some catch
block. The appropriate catch block immediately follows the try block; for example,
catch block
handling an
exception
catch (Exception e)
{
String message = e.getMessage();
System.out.println(message);
System.exit(0);
}
 
 
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