Thread: [JAVA]Keygen example

EDIT: Mods, sorry for posting in the wrong section, my intention was to post in the Android section, but it seems that I have mistakenly clicked the wrong sub-section... Please move the thread where it is appropriate

I wondered a long time how the hell do keygens work, but now it's all clear.
The source I am going to post in this thread is just a modified version of Dr0ax's (@LeetCoders), with a little addition.
There is 1 more function, which enables the user to check if the key he enters, is valid and generated by the algorithm in the keygen.
This is just a basic example, with a weak algorithm, but can be improved much much more, and easily translated to C# (for the .NET lovers)
The code is commented a lot, so nobody should have trouble understanding the commands.
Anyways, here is a pastebin link http://pastebin.com/ZiYH83rk
And attached to the board... All comments are welcomed =)

Code:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.*;
import javax.swing.*;

//Credits to Dr0ax@leetcoders.org, for coding the original keygen example
@SuppressWarnings("serial")
public class Keygen extends JPanel implements ActionListener{
	JTextField key_field = new JTextField(19);
	JButton generate = new JButton("Generate");
	JButton chk = new JButton("Check key");
	Random r = new Random();

	public Keygen() {
		this.add(key_field);
		this.add(chk);
		this.add(generate);
		generate.addActionListener(this);
		
		chk.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				String chkStr = key_field.getText();
				String[] parts = chkStr.split("-");
				boolean valid = false;
				//first check: if there are 4 parts total
				if(parts.length != 4)
					valid = false;
				else{
					
					//loop through the four parts of the string
					outloop: for (int i = 0; i < parts.length; i++)
					{
						//second check: break if the part size is not 4 characters total
						if(parts[i].length() != 4){
							valid = false;
							break;
						}
						
						else{
							//loop through each character of the string
							for(int x = 0; x < parts[i].length(); x++) {
								//get the integer value of the char
								int cv = (int)parts[i].charAt(x);
								
								//substract 48 from the number
								int rndVal = cv - 48;
								
								//check the number against the algorithm
								if (((rndVal >= 0 && rndVal <= 9) || (rndVal >= 17 && rndVal <= 42)) /*|| (rndVal >= 49 && rndVal <= 74)*/  && isPrime(rndVal)) {
									valid = true;
								}
								else{
									//if not valid, set the flag and exit the loop
									//you have to use a label (in my case "outloop") because you can't break a nested loop with just the break command
									valid = false;
									break outloop;
								}
							}
						}
					}
				}
				
				if(valid == true)
				key_field.setText("Valid Key!");
				else{key_field.setText("Not a valid key");}
			}
		});
		}
	
	
	public static void main(String[] args){
		JFrame f = new JFrame("Keygen Example");
		f.setLocation(100,100);
		f.setDefaultCloseOperation(3);
		f.setSize(250,100);
		f.add(new Keygen());
		f.setVisible(true);
	}
	
	boolean isPrime(int num){
		int i;
		boolean xD = false;
		for (i=2; i < num ;i++ ){
		  	int n = num%i;
		  	if (n==0){
		  		xD = false;
		  		break;
		  	}
		  }
		  if(i == num)
			  xD = true;
		  return xD;
	}
		  
	@Override
	public void actionPerformed(ActionEvent arg0) {
		int x = 0;
		int math = 48;
		String nStr = "";
		boolean valid;
		for (int i = 0; i < 19; i++) {
			if(i == 4 || i == 9 || i ==14) {
				nStr += "-";
			}
			else{
				valid = false;
				while(!valid){
					x = r.nextInt(74) +1;
					if (/*this makes x only available to hold the ASCII values of the numbers, and uppercase characters ... after you add 48 to it, of course =)*/ 
						((x >= 0 && x <= 9) || (x >= 17 && x <= 42)) && isPrime(x)) {
						valid = true;
						math += x;
						nStr += (char)math;
						System.out.println(x);
						break;
					}
				}
				math = 48;
			}
		}
		key_field.setText(nStr);
	}
}

// Thread Moved but comments are disabled in the right section ~ Mind