Using JOptionPAne Converter for Binary To Hexadecimal

import javax.swing.JOptionPane;
 import java.io.*;
 import java.lang.*;
public class BinaryToHexadecimal {
    public static void main(String[] args){
    String hex;
  hex = JOptionPane.showInputDialog(null, "Enter the Binary number:");
  long num = Long.parseLong(hex);
  long rem;
  while(num > 0){
  rem = num % 10;
  num = num / 10;
  if(rem != 0 && rem != 1){
  JOptionPane.showMessageDialog(null, "This is not a binary number.","a", JOptionPane.WARNING_MESSAGE);
  JOptionPane.showMessageDialog(null, "Please try once again.","w", JOptionPane.WARNING_MESSAGE);
  System.exit(0);
  }
  }
  int i= Integer.parseInt(hex,2);
  String hexString = Integer.toHexString(i);
  JOptionPane.showMessageDialog(null, "Hexa decimal: " + hexString,"result", JOptionPane.INFORMATION_MESSAGE);
  }
}