//Descripcion: Dibuja una ventana cuadriculada, que pienso usar mas adelante para desarrollar un Sistema de Coordenadas.
//Autor: alvk4r
 
import javax.swing.*;
import java.awt.*;
 
public class SquaredPaper extends JFrame {
  public void paint(Graphics g) {
    Dimension d = getSize();
    int x = d.width;
    int y = d.height;
 
    g.setColor(Color.yellow);
    g.fillRect(0,0,x,y);
 
    g.setColor(Color.green);
    for (int i = 0; i < y; i+=25) g.drawLine(0,i,x,i);
    for (int i = 0; i < x; i+=25) g.drawLine(i,0,i,y);
 
    g.setColor(Color.red);
    g.drawLine(x/2,0,x/2,y);
    g.drawLine(0,y/2,x,y/2);
  }
 
  public static void main(String args[]) {
    SquaredPaper DrawWindow = new SquaredPaper();
 
    DrawWindow.setSize(500,500);
    DrawWindow.setResizable(false);
    DrawWindow.setLocation(200, 50);
    DrawWindow.setTitle("Sistema de Coordenadas Cartesianas.");
    DrawWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    DrawWindow.setVisible(true);
 
  }
}
 

> jar cvfe SquaredPaper.jar SquaredPaper SquaredPaper.class

//Descripcion: Programa  que permite comprobar si un numero dado es primo.            
//Autor: alvk4r                                                                                                                    
//Uso: java PrimeCheck [numero]
import java.lang.*;
 
public class PrimeCheck {
  public static void main(String[] args) {
    int N = Integer.parseInt(args[0]);
    int mod, i = 0;
    int[] primes = { 2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,
      73,79,83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167,
      173,179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263,269,
      271,277,281,283,293,307,311,313,317,331,337,347,349,353,359,367,373,379,
      383,389,397,401,409,419,421,431,433,439,443,449,457,461,463,467,479,487,
      491,499,503,509,521,523,541,547,557,563,569,571,577,587,593,599,601,607,
      613,617,619,631,641,643,647,653,659,661,673,677,683,691,701,709,719,727,
      733,739,743,751,757,761,769,773,787,797,809,811,821,823,827,829,839,853,
      857,859,863,877,881,883,887,907,911,919,929,937,941,947,953,967,971,977,
      983,991,997,1009,1013,1019,1021,1031,1033,1039,1049,1051,1061,1063,1069,
      1087,1091,1093,1097,1103,1109,1117,1123,1129,1151,1153,1163,1171,1181,
      1187,1193,1201,1213,1217,1223,1229,1231,1237,1249,1259,1277,1279,1283,
      1289,1291,1297,1301,1303,1307,1319,1321,1327,1361,1367,1373,1381,1399,
      1409,1423,1427,1429,1433,1439,1447,1451,1453,1459,1471,1481,1483,1487,
      1489,1493,1499,1511,1523,1531,1543,1549,1553,1559,1567,1571,1579,1583,
      1597,1601,1607,1609,1613,1619,1621,1627,1637,1657,1663,1667,1669,1693,
      1697,1699,1709,1721,1723,1733,1741,1747,1753,1759,1777,1783,1787,1789,
      1801,1811,1823,1831,1847,1861,1867,1871,1873,1877,1879,1889,1901,1907,
      1913,1931,1933,1949,1951,1973,1979,1987,1993,1997,1999};
    boolean ver = true;
 
    while ((i <= 303) && (primes[i] < N)) {
      mod = N % primes[i];
      if (mod == 0) {
        ver = false;
        break;
      }
      i++;
    }
 
    if (ver == false){
    System.out.println("El numero " + N + " es divisible por " + primes[i]);
    System.out.println("El numero " + N + " no es primo.");
    }
    else
      System.out.println("El numero " + N + " probablemente sea primo.");
  }
}

java PrimeCheck 23543