Use BigInteger : BigInteger « Data Type « Java Tutorial

1. Java Tutorial
2. Data Type
3. BigInteger

import java.math.BigInteger;

public class MainClass {
  public final static int pValue = 47;

  public final static int gValue = 71;

  public final static int XaValue = 9;

  public final static int XbValue = 14;

  public static void main(String[] args) throws Exception {
    BigInteger p = new BigInteger(Integer.toString(pValue));
    BigInteger g = new BigInteger(Integer.toString(gValue));
    System.out.println("p = " + p);
    System.out.println("g = " + g);

    BigInteger Xa = new BigInteger(Integer.toString(XaValue));
    BigInteger Xb = new BigInteger(Integer.toString(XbValue));
    System.out.println("Xa = " + Xa);
    System.out.println("Xb = " + Xb);

    BigInteger Ya = g.modPow(Xa, p);
    System.out.println("Ya = " + Ya);

    BigInteger Yb = g.modPow(Xb, p);
    System.out.println("Yb = " + Yb);

    BigInteger Ka = Ya.modPow(Xa, p);
    System.out.println("Users A, K = " + Ka);

    BigInteger Kb = Yb.modPow(Xb, p);
    System.out.println("Users B, K = " + Kb);

  }

}