Here is a java program that throws an user-defined exception.

import java.util.*;
class MyException extends Exception{
  String str;
   MyException(String st) {
     str=st;
   }
   public String toString(){
     return ("Invalid Marks entered! "+ str) ;
  }
}

class UserDefinedException{
   public static void main(String args[]){
  try{
      Scanner input=new Scanner(System.in);
    System.out.print("Enter marks: ");
    int marks=input.nextInt();
        if(marks < 15){
       throw new MyException("Error! Marks are less than 15.");
        }
  }
 catch(MyException e){
    System.out.println(e) ;
 }
}
}