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//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

//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

import javax.swing.*;
//Descripcion: Dibuja graficos en una ventana.
//Autor: alvk4r
import java.awt.*;
 
public class GraphicsDrawWindow extends JComponent {
  public void paint(Graphics g){
    g.setColor(Color.red);
    g.drawString("Esto es una linea:",10,20);
    g.drawLine(10,30,200,30);
    g.drawString("Esto es un rectangulo:",10,60);
    g.drawRect(10,70,190,100);
    g.drawString("Esto es una curva",10,200);
    g.drawArc(10,210,190,100,0,180);
  }
  public static void main(String args[]) {
    JFrame DrawWindow = new JFrame("Sample Window");
    DrawWindow.getContentPane().add(new GraphicsDrawWindow());
    DrawWindow.setSize(220,310);
    DrawWindow.setLocation(100, 150);
    DrawWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    DrawWindow.setVisible(true);
 
  }
}

import java.applet.Applet;
import java.awt.Graphics;
 
public class GraphicsDrawApplet extends Applet{
  public void paint(Graphics g){
    g.drawString("Esto es una linea:",10,20);
    g.drawLine(10,30,200,30);
    g.drawString("Esto es un rectangulo:",10,60);
    g.drawRect(10,70,190,100);
    g.drawString("Esto es una curva",10,200);
    g.drawArc(10,210,190,100,0,180);
  }
}

<applet code="GraphicsDrawApplet.class" width="400" height="400" />

//Descripcion: Suma las filas de una matrix de 3x3, devolviendo el valor de cada fila.
//Autor: alvk4r
public class sumArray {
  public static void main(String[] args) {
    int[][] myMatrix = new int[][] {{5,4,2}, {3,4,8}, {3,7,3}};
    int fila1, fila2, fila3;
 
    fila1 = myMatrix [0][0] + myMatrix [0][1] + myMatrix [0][2];
    fila2 = myMatrix [1][0] + myMatrix [1][1] + myMatrix [1][2];
    fila3 = myMatrix [2][0] + myMatrix [2][1] + myMatrix [2][2];
 
    System.out.println("La fila 1 suma: " + fila1);
    System.out.println("La fila 2 suma: " + fila2);
    System.out.println("La fila 3 suma: " + fila3);
  }
}
 

public class SumRowArrayI {
  public static void main (String[] args) {
    int[][] myArray = {{2,4,5}, {2,3,4,}, {9,7,3}};
    int[] row = {
      myArray[0][0] + myArray[0][1] + myArray[0][2],
      myArray[1][0] + myArray[1][1] + myArray[1][2],
      myArray[2][0] + myArray[2][1] + myArray[2][2],
    };
    for (int i = 0; i < row.length; i++)
      System.out.println("El valor de la fila " + i + " es: " + row[i]);
  }
}
 

public class Primes 
  { 
    public static void main(String[] args) 
      { int N = Integer.parseInt(args[0]); 
        boolean[] a = new boolean[N]; 
        for (int i = 2; i < N; i++) a[i] = true; 
        for (int i = 2; i < N; i++) 
          if (a[i] != false) 
            for (int j = i; j*i < N; j++) 
              a[i*j] = false; 
        for (int i = 2; i < N; i++) 
          if (i > N - 2000) 
            if (a[i]) System.out.print(" " + i); 
        System.out.println(); 
      } 
  } 
 

public class OrdBurbuja { 
 
   public static void main (String [] args) { 
      int i, b; 
      int [] myArray = { 113, 23, 54, -10, -8, 72, 46 }; 
 
       for (b = 0; b <= myArray.length - 2; b++) 
           for (i = 0; i <= myArray.length - b - 2; i++) 
                if (myArray [i] >  myArray [i + 1]) { 
                    int temp = myArray [i]; 
                    myArray [i] = x [i + 1]; 
                    myArray [i + 1] = temp; 
                } 
 
      for (i = 0; i <  myArray.length; i++) 
           System.out.println (myArray [i]); 
   } 
}
 

//Fuente: Java Cryptography
//Uso: 
      /* cifrar:
            java SecretWriting -e [cadena_texto_plano]
          descifrar:
            java SecretWriting -e [cadena_texto_cifrada]
      */
import java.io.*;
import java.security.*;
import javax.crypto.*;
import sun.misc.*;
public class SecretWriting {
  public static void main(String[] args) throws Exception {
// Check arguments.
    if (args.length < 2) {
      System.out.println("Uso: SecretWriting -e || -d [text]");
      return;
    }
// Get or create key.
    Key key;
    try {
      ObjectInputStream in = new ObjectInputStream(
      new FileInputStream("SecretKey.ser"));
      key = (Key)in.readObject();
      in.close();
      }
    catch (FileNotFoundException fnfe) {
      KeyGenerator generator = KeyGenerator.getInstance("DES");
      generator.init(new SecureRandom());
      key = generator.generateKey();
      ObjectOutputStream out = new ObjectOutputStream(
      new FileOutputStream("SecretKey.ser"));
      out.writeObject(key);
      out.close();
    }
// Get a cipher object.
    Cipher cipher = Cipher.getInstance("DES/ECB/PKCS5Padding");
// Encrypt or decrypt the input string.
    if (args[0].indexOf("e") != -1) {
      cipher.init(Cipher.ENCRYPT_MODE, key);
      String amalgam = args[1];
      for (int i = 2; i < args.length; i++)
        amalgam += " " + args[i];
      byte[] stringBytes = amalgam.getBytes("UTF8");
      byte[] raw = cipher.doFinal(stringBytes);
      BASE64Encoder encoder = new BASE64Encoder();
      String base64 = encoder.encode(raw);
      System.out.println(base64);
    }
    else if (args[0].indexOf("d") != -1) {
      cipher.init(Cipher.DECRYPT_MODE, key);
      BASE64Decoder decoder = new BASE64Decoder();
      byte[] raw = decoder.decodeBuffer(args[1]);
      byte[] stringBytes = cipher.doFinal(raw);
      String result = new String(stringBytes, "UTF8");
      System.out.println(result);
    }
  }
}