User Defined Exceptions:

• In some situation a programmer needs to create and throw his own created exception and such type of exceptions are known as User defined Exception or Custom Exceptions.
• To create own exceptions we have to Extend the Exception class then only we can create custom exceptions.

Example:

class CustomExc extends Exception{
    String string1;
    CustomExc(String string2) {
       string1=string2;
    }
    public String toString(){ 
       return ("Output String = "+string1) ;
    }
}

class CustomException{
    public static void main(String args[]){
       try{
          throw new CustomExc("CustomEXC");
          // It will throw user defined custom exception 
       }
       catch(CustomExc exp){
          System.out.println("This is my catch block") ;
          System.out.println(exp) ;
       }
    }
}

Output:

This is my catch block
Output String = CustomEXC

In the above program we have created our own exception "CustomExc" and we can use it using throw keyword.