Code:
//By GT3X for www.bifrostworld.org

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;


public class CTaschenRechner extends JFrame {
  
  private JTextField jTextField1 = new JTextField();
  private JTextField jTextField2 = new JTextField();
  private JButton jButton1 = new JButton();
  private JButton jButton2 = new JButton();
  private JButton jButton3 = new JButton();
  private JButton jButton4 = new JButton();
  private JLabel jLabel1 = new JLabel("Ergebnis..");
  int zahl1,zahl2;
  int ergebnis;
 

  public CTaschenRechner (String title) {

    super (title);
    setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    int frameWidth = 217;
    int frameHeight = 300;
    setSize(frameWidth, frameHeight);
    Dimension d = Toolkit.getDefaultToolkit().getScreenSize();
    int x = (d.width - getSize().width) / 2;
    int y = (d.height - getSize().height) / 2;
    setLocation(x, y);
    Container cp = getContentPane();
    cp.setLayout(null);


    jTextField1.setBounds(56, 8, 105, 24);
    jTextField1.setText("");
    cp.add(jTextField1);
    jTextField2.setBounds(56, 40, 105, 24);
    jTextField2.setText("");
    cp.add(jTextField2);
    jButton1.setBounds(8, 80, 89, 41);
    jButton1.setText("+");
    jButton1.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
        jButton1_ActionPerformed(evt);
      }
    });
    cp.add(jButton1);
    jButton2.setBounds(104, 80, 89, 41);
    jButton2.setText("-");
    jButton2.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
        jButton2_ActionPerformed(evt);
      }
    });
    cp.add(jButton2);
    jButton3.setBounds(8, 128, 89, 41);
    jButton3.setText("*");
    jButton3.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
        jButton3_ActionPerformed(evt);
      }
    });
    cp.add(jButton3);
    jButton4.setBounds(104, 128, 89, 41);
    jButton4.setText("/");
    jButton4.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
        jButton4_ActionPerformed(evt);
      }
    });
    cp.add(jButton4);
    jLabel1.setBounds(0, 200, 205, 37);
    jLabel1.setText("Ergebnis");
    jLabel1.setFont(new Font("MS Sans Serif", Font.PLAIN, 27));
    cp.add(jLabel1);


    setResizable(false);
    setVisible(true);
  }


  public void jButton1_ActionPerformed(ActionEvent evt) {
  zahl1=Integer.valueOf(jTextField1.getText());
  zahl2=Integer.valueOf(jTextField2.getText());
  ergebnis=zahl1+zahl2;
  jLabel1.setText(String.valueOf(ergebnis));
  }

  public void jButton2_ActionPerformed(ActionEvent evt) {
  zahl1=Integer.valueOf(jTextField1.getText());
  zahl2=Integer.valueOf(jTextField2.getText());
  ergebnis=zahl1-zahl2;
  jLabel1.setText(String.valueOf(ergebnis));
  }

  public void jButton3_ActionPerformed(ActionEvent evt) {
  zahl1=Integer.valueOf(jTextField1.getText());
  zahl2=Integer.valueOf(jTextField2.getText());
  ergebnis=zahl1*zahl2;
  jLabel1.setText(String.valueOf(ergebnis));
  }

  public void jButton4_ActionPerformed(ActionEvent evt) {
  zahl1=Integer.valueOf(jTextField1.getText());
  zahl2=Integer.valueOf(jTextField2.getText());
  ergebnis=zahl1/zahl2;
  jLabel1.setText(String.valueOf(ergebnis));
  }



  public static void main(String[] args) {
    new CTaschenRechner("TaschenRechner");
  }
}