Fastest prime number finder

Question

Hello,
I'm a high school student and am making a program which calculates the next prime number greater than the inputed number. The rules are that you cannot use any other classes other than those that are imported, but I think you can use any class in lang (you can definitely use the math class).
The program determines how fast it took to find the prime number. Entering the number 9*10^16 (9 with sixteen 0s), my PC calculated it in about 22534 milliseconds (22 seconds). I'm trying to improve this since I've seen people do at about 7 seconds. Any ideas on how to make this program faster, about 15 seconds faster?
Here is my code for the best i could come up with:
import javax.swing.JOptionPane;
import javax.swing.JApplet;
import javax.swing.JTextArea;
import java.awt.Container;
public class PrimeFinder extends JApplet
     public void init()
          String ms = JOptionPane.showInputDialog("Please enter a number:");
          long b = Long.parseLong(ms);
          long base = b;
          long num = 3;
          long start = System.currentTimeMillis();
          while(num<=Math.sqrt(base))
               if(base%2==0 || base%num==0)
                    base++;
                    num=3;
               else num+=2;
          long time = System.currentTimeMillis() - start;
          String result = "Prime Finder:\n\nBase Number:\t" + b + "\nNext Prime:\t" + base + "\nTime:\t" + time;
          JTextArea y = new JTextArea();
          y.setText(result);
          Container container = getContentPane();
          container.add(y);
}PS: please run my code on your PC to check how fast your PC does it before writing new code since a better PC will obviously do it faster with the same code.

Answer 1

This is what I am using:
public static void main(String[] args)
          String ms = JOptionPane.showInputDialog("Please enter a number:");
          long b = Long.parseLong(ms);
          long base = b;
          double sqrt = Math.sqrt(base);
          System.out.println("Best Before: 19469");
          long start = System.currentTimeMillis();
          for(long num = 3; num <= sqrt; num += 2)
               if(base%num==0)
                    base++;
                    num = 3;
                    sqrt = Math.sqrt(base);
               else
                    num += 2;
          long time = System.currentTimeMillis() - start;
          String result = "Prime Finder:\n\nBase Number:\t" + b + "\nNext Prime:\t" + base + "\nTime:\t" + time;
          System.out.println(result);
     }I see a very significant time change between the while loop and for loop.