Download Basic Exception Handling

Basic Exception
Handling
Exceptions in Java
 Either the Java language itself or your
code provides a mechanism that signals
when something unusual happens.
 This is called throwing an exception.
 Another part of your code contains code
for handling the exception.
Example of Exception
Handling
import java.util.*;
public class GotMilk
{
public static void main(String[] args)
{
int donutCount, milkCount;
double donutsPerGlass;
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter number of donuts:");
donutCount = keyboard.nextInt( );
System.out.println("Enter number of glasses of milk:");
milkCount = keyboard.nextInt( );
}
}
if (milkCount < 1)
{
System.out.println("No Milk!");
System.out.println("Go buy some milk.");
}
else
{
donutsPerGlass = donutCount/(double)milkCount;
System.out.println(donutCount + " donuts.");
System.out.println(milkCount + " glasses of milk.");
System.out.println("You have " + donutsPerGlass
+ " donuts for each glass of milk.");
}
System.out.println("End of program.");
Example of Exception Handling
(cont’d)
import java.util.*;
public class ExceptionDemo
{
public static void main(String[] args)
{
int donutCount, milkCount;
double donutsPerGlass;
Scanner keyboard = new Scanner(System.in);
try
{
}
}
System.out.println("Enter number of donuts:");
donutCount = keyboard.nextInt( );
System.out.println("Enter number of glasses of milk:");
milkCount = keyboard.nextInt( );
if (milkCount < 1)
throw new Exception("Exception: No Milk!");
donutsPerGlass = donutCount/(double)milkCount;
System.out.println(donutCount + " donuts.");
System.out.println(milkCount + " glasses of milk.");
System.out.println("You have " + donutsPerGlass
+ " donuts for each glass of milk.");
}
catch(Exception e)
{
System.out.println(e.getMessage( ));
System.out.println("Go buy some milk.");
}
System.out.println("End of program.");
How Exception Handling
Works
 Exception is a predefined class.
 The throw statement creates an object of
the class Exception and throws it.
 When an exception is thrown, the code
in the surrounding try block stops
execution, and another portion of code,
know as a catch block, begins
execution.
 Execution of the catch block is called
catching the exception.
The try Block
 The basic way of handling exceptions in
Java consists of the try-throw-catch
threesome.
 A try block contains the code for the
basic algorithm that tells the computer
what to do when everything goes
smoothly.
 It is called try because you are not 100
percent sure everything will go smoothly.
The throw Statement
 If something goes wrong, you want to
throw an exception.
 The throw statement creates a new
object of the class Exception and
throws it.
 When an exception is thrown, the code
in the try block stops execution, and
another portion of code, the catch
block, begins execution.
The throw Statement (cont’d)
 In the example,
new Exception(“Exception: No Milk!”)
the string “Exception: No Milk!” is an
argument for the constructor for the class
Exception.
 The Exception object, created with new, stores
this string in an instance variable of the object, so
that it can be recovered in the catch block.
The catch block
 Although it is not a method definition it
looks a little like one.
 It is a separate piece of code that is
executed when a program throws an
exception.
 The throw statement is similar to a
method call, but instead of calling a
method, it calls the catch block.
The catch Block Parameter
 The following the word catch is the catch-block
parameter.
 The class name preceding the catch-block
parameter specifies what kind of exception the
catch-block can catch.
 The catch-block parameter gives you a name for
the exception that is caught, so that you can write
code in the catch block in order to manipulate
that exception object.
The getMessage Method
 Every exception has a String instance
variable that contains some message,
which typically identifies the reason for
the exception.
 If the object is called e, then the method
call e.getMessage() returns this
string.