Implementing User Defined Exceptions
In addition to the built-in exceptions, you can create customized exception as per the application requirements. Each application that you create might have specific constraints.
For example, in case of a train reservation application, a passenger must specify the destination. Also, a ticket has to be bought for a passenger who is above three years of age.
Similarly, in the case of a banking application, if a customer is below 18 years of age, he is allowed to open a joint account only. Error handling becomes a necessity while developing applications that account for such constraints. The Exception class or its subclasses in Java do not address these application specific constraints. You can create your own exception classes to address these constraints and ensure the integrity of application data.
Creating and Handling User-defined Exceptions
The throw, throws, try, catch, and finally keywords are used while implementing userdefined exceptions. The Exception class inherits all the methods provided by the Throwable class. The user-defined Exception class also inherits the methods defined in the Throwable class. The following table lists the methods defined by the Throwable class:
| Methods | Description |
| String getMessage() |
Returns a description of the |
| String toString() |
Returns a string object containing a |
| Throwable fillInStackTrace() |
Returns a Throwable object that |
| void printStackTrace() | Prints the stack trace |
|
void printStackTrace(PrintStream |
Propels the stack trace to a specific |
|
void printStackTrace(PrintWriter |
Propels the stack trace to a specific |
| String getLocalizedMessage |
Provides a localized description of |
You can use the following code to create a user-defined exception to catch the exception raised when the sum of two numbers exceeds 20:
class UserException extends Exception
{
int num1,num2,sum;
UserException(int a,int b)
{
num1 = a; num2 = b; sum = a + b;}
public String toString()
{
return "UserException Caught: The sum of the numbers Exceeds
20.."; }
}
class UserExceptionDemo
{
static void calculate(int a,int b) throws UserException
{
int sum;
System.out.println("Calculate Method(" + a + "," + b +
")");
sum=a+b;
if(sum>20)
throw new UserException(a,b);
System.out.println("The Value of the sum of two numbers
is:" + sum);
}
public static void main(String args[])
{
try
{
calculate(12,1);
calculate(15,7);
}
catch (UserException Obja)
1B.42 Programming in Java
{
System.out.println("Caught:" + Obja);
}
}
}
In the preceding code, the UserException subclass of the Exception class is created. The UserException class has one constructor, UserException() and the toString() method that returns a string containing the description of the exception. The UserExceptionDemo class defines the calculate() method that throws an object of the user-defined exception, UserException. The exception is thrown when the sum of two numbers passed as parameters to the calculate() method exceeds 20.
The output of the preceding code is:
