User defined Exception

User defined Exception

• We can throw our own exceptions using the keyword throw.

• The syntax for throwing out own exception is

throw new Throwable's subclass

• Here the Throwable's subclass is actually a subclass derived from the Exception class.

For example -

throw new ArithmeticException();

Throwable's subclass

• Let us see a simple Java program which illustrates this concept.

Java Program[MyExceptDemo.java]

import java.lang.Exception;

class MyOwnException extends Exception

{

MyOwnException(String msg)

{

super(msg);

}

}

class MyExceptDemo

{

public static void main (String args[])

{

int age;

age=15;

try

{

if(age<21)

throw new MyOwnException("Your age is very less than the condition");

}

catch (MyOwnException e)

{

System.out.println ("This is My Exception block");

System.out.println (e.getMessage());

}

finally

{

System.out.println ("Finally block:End of the program");

}

}

}

Output

This is My Exception block

Your age is very less than the condition

Finally block:End of the program

Program Explanation

• In above code, the age value is 15 and in the try block exception if the value is less than 21. As soon as the exception is thrown the catch block gets executed. Hence as an output we get the first message "This is My Exception block". Then the control is transferred to the class MyOwnException(defined at the top of the program). The message is set and it is "Your age is very less than the condition". This message can then printed by the catch block using the System.out.println statement by means of e.message.

• At the end the finally block gets executed.

Ex. 3.9.1: Write an exception class for a time of day that can accept only 24 hour representation of clock hours. Write a java program to input various formats of timings and throw suitable error messages.

Sol.:

import java.lang.Exception;

import java.io.*;

import java.util.*;

class MyException extends Exception

{

MyException(String msg)

{

super(msg);

}

}

class Clock

{

private int hour;

private int minute;

public void input() throws IOException

{

 BufferedReader br=new BufferedReader (new InputStreamReader(System.in));

System.out.println("\n Enter the time in hh:mm format");

String str=br.readLine();

StringTokenizer tokn=new StringTokenizer(str,":");

String h=tokn.nextToken();

String m=tokn.nextToken();

hour=Integer.parseInt(h);

minute= Integer.parseInt(m);

try

{

System.out.println("Hour: "+hour);

if((hour < 0) || (hour>24))

throw new MyException("Fatal error: invalid hour");

}

catch(MyException e)

{

System.out.println(e.getMessage());

}

try

{

System.out.println("Minute: "+minute);

if((minute <0) || (minute > 59))

throw new MyException("Fatal error: invalid minute");

}

catch(MyException e)

{

System.out.println(e.getMessage());

}

}

}

class ClockDemo

{

public static void main(String[] args) throws IOException

{

Clock c=new Clock();

c.input();

}

}

Output(Run1)

Enter the time in hh:mm format

25:80

Hour: 25

Fatal error: invalid hour

Minute: 80

Fatal error: invalid minute

Output(Run2)

Enter the time in hh:mm format

10:70

Hour: 10

Minute: 70

Fatal error: invalid minute

Output(Run3)

Enter the time in hh:mm format

30:40

Hour: 30

Fatal error: invalid hour

Minute: 40

Review Question

1. With suitable Java program, explain user defined exception handling.