Thread: Diffie Hellman Key exchange implemetation

when running the following code it runs one time correctly and after that when I try to run it again, it fails.
Any Help. I'm using jGRASP editor
Thanks

import java.util.*;
import java.math.*;
 
 
public class DiffieHellmanBigInt
{  
 
   public static void main(String args[]) 
   {
 
      int p;
 
      System.out.println("Enter the number of bits to compute the value of the Prime Number:");
      Scanner userInput = new Scanner(System.in);
      int ans = Integer.parseInt(userInput.next());
 
      p = getPrime(ans);
 
      System.out.println("The value of p selected:"+ p);
 
 
       //Random numbers Generation
      Random rand=new Random();
      int g = rand.nextInt((p - 2) + 1) + 2; //between 2 and (p-1)
      System.out.println("The value of g selected: "+ g);
 
      int a = rand.nextInt(99); //generating value of a
      System.out.println("The value of a selected by Alice: "+ a);
 
 
      int b = rand.nextInt(99);//generating value of b
      System.out.println("The value of b selected by Bob: "+ b);
 
       // A's calculation.
      int comput_A=(int)Math.pow(g, a)%p;      
      System.out.println("The value of A sent to Bob by Alice: "+ comput_A);          
 
       // B's calculation.
      int comput_B = (int)Math.pow(g, b)%p;   
      System.out.println("The value of B sent to Alice by Bob: "+ comput_B);
 
 
      int KeyA = (int) Math.pow(comput_B, a)%p;
      int KeyB = (int)Math.pow(comput_A, b)%p;
 
      if(KeyA==KeyB)
      {
      System.out.println("The value of key shared between Alice and Bob: "+ KeyA);
      }
      else
      {
       System.out.println("There is an Error"); 
 
     // System.out.println("The Key A calculates is "+KeyA);
      //System.out.println("The Key B calculates is "+KeyB);
      }
   }
// evaluate if a number is prime or not  
 
   public static boolean isPrime(int n) 
   {
      for(int i=2;i<n;i++) 
      {
         if(n%i==0)
         {
            return false;
         }
      }
      return true;
   }
 
   // find a prime number
 
   public static int getPrime(int ans) 
   {
      boolean foundPrime= false;
      Random rand_pr = new Random();
 
      if(!foundPrime)
      {
         int num1 = (int)Math.pow(2, ans-1);
         int num2 = (int)Math.pow(2, ans);           
         int  pr = rand_pr.nextInt(num2 - num1 + 1) + num1;
 
         if (isPrime(pr))         
         { 
           return pr; 
         }
      }        
           return 0;        
 
    }     
}

Output 1:
Enter the number of bits to compute the value of the Prime Number:
5
The value of p selected:19
The value of g selected: 3
The value of a selected by Alice: 57
The value of b selected by Bob: 88
The value of A sent to Bob by Alice: 2
The value of B sent to Alice by Bob: 2
The value of key shared between Alice and Bob: 2



Output 2:

Enter the number of bits to compute the value of the Prime Number:
5
The value of p selected:0
Exception in thread "main" java.lang.IllegalArgumentException: bound must be positive
at java.util.Random.nextInt(Random.java:388)
at DiffieHellmanBigInt.main(DiffieHellmanBigInt.java: 24)

----jGRASP wedge2: exit code for process is 1.

it fails

Please explain.
Post any output that shows what you are talking about.