User defined exceptions in java

User defined exception in Java

User defined exceptions in java are also known as Custom exceptions. Most of the times when we are developing an application in java, we often feel a need to create and throw our own exceptions. These exceptions are known as User defined or Custom exceptions. In this tutorial we will see how to create and throw such  exceptions in a java program.

Example of User defined exception in Java

class MyException extends Exception{
    String str1;
    MyException(String str2) {
       str1=str2;
    }
    public String toString(){ 
       return ("Output String = "+str1) ;
    }
}

class CustomException{
    public static void main(String args[]){
       try{
          throw new MyException("Custom");
          // I'm throwing user defined custom exception above
       }
       catch(MyException exp){
          System.out.println("Hi this is my catch block") ;
          System.out.println(exp) ;
       }
    }
}

Output:

Hi this is my catch block
Output String = Custom

Key-points from above example:

You can see that while throwing my custom exception I gave a string in parenthesis ( throw new MyException("Custom");). That’s the reason we have a parametric constructor (with a String parameter) in my custom exception class.

Notes :

• User defined exception needs to inherit (extends) Exception class in order to act as an exception.
• throw keyword is used to throw such exceptions.

Another example where in we will modify the error message of Exception Class

Part 1: 
I have created my own exception class- MyException by inheriting the parent class Exception then I have defined a parametric constructor of my class with a String parameter. In the constructor I called super(), super refers to the super class { My class has inherited Exception class so Exception class is my superclass }. In this way I have modified the system generated message by my own message.

public class MyException extends Exception
{
    public MyException(String mymsg)
    {
        super(mymsg);
    }
}

Part 2 :

public class ExceptionSample
{
    public static void main(String args[]) throws Exception
    {
        ExceptionSample es = new ExceptionSample();
        es.displayMymsg();
    }
    public void displayMymsg() throws MyException
    { 
        for(int j=8;j>0;j--)
        {
          System.out.println("j= "+j);
          if(j==7)
          {
              throw new MyException("This is my own Custom Message");
          }
        }
    }
}

Output:

j = 8
j = 7
Exception in thread "main" MyException: This is my own Custom Message
at ExceptionSample.displayMymsg( ExceptionSample.java.19)
...