Go4Expert - Why is it throwing "number format exception" ?

Code:

//WAP That convert integer value into binary,octal and hexdecimal.



import java.io.*;



class numconv

{

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

   { 

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



     char ch='y'; 

     int opt,num;

     String bin,oct,hex;





     do

      {



       System.out.println("*******  1: Convert number to Binary format ********");

       System.out.println("*******  2: Convert number to Octal format *********");

       System.out.println("*******  3: Convert number to Hexadecimal format ***");

       

       System.out.print("\n\n Enter Option :");

       opt=Integer.parseInt(br.readLine());



       switch(opt)

         {

            case 1:

                    System.out.print("\n\nEnter Number :");

                    num=Integer.parseInt(br.readLine());

                    bin=Integer.toBinaryString(num);

                    System.out.println("\nBinary equivalent of "+num+" is "+bin);

                    break; 



            case 2:

                    System.out.print("\n\nEnter Number :");

                    num=Integer.parseInt(br.readLine());

                    oct=Integer.toOctalString(num);

                    System.out.println("\nOctal equivalent of "+num+" is "+oct);

                    break;  



            case 3:

                    System.out.print("\n\nEnter Number :");

                    num=Integer.parseInt(br.readLine());

                    hex=Integer.toHexString(num);

                    System.out.println("\nHexadecimal equivalent of "+num+" is "+hex);

                    break;



           

         }



       System.out.print("\n\n Enter your choice(y/n) :");

       ch=(char)br.read();

       

      }while(ch!='n');



   }

}

Here is the output

******* 1: Convert number to Binary format ********
******* 2: Convert number to Octal format *********
******* 3: Convert number to Hexadecimal format ***

Enter Option :1

Enter Number :5643

Binary equivalent of 5643 is 1011000001011

Enter your choice(y/n) :y

Here, when i am entering 'y' or value other than 'n' it is throwing exception.

******* 1: Convert number to Binary format ********
******* 2: Convert number to Octal format *********
******* 3: Convert number to Hexadecimal format ***

Enter Option :Exception in thread "main" java.lang.NumberFormatException: For i
nput string: ""
at java.lang.NumberFormatException.forInputString(Num berFormatException.
java:48)
at java.lang.Integer.parseInt(Integer.java:447)
at java.lang.Integer.parseInt(Integer.java:476)
at numconv.main(numconv.java:24)