[Java] [Exer2] Máy tính đơn giản

/*
* cusomized by: mathhoang
* Y!M: vietnam_hoangminhnguyen@yahoo.com
* addr: http://mathhoang.blogspot.com
*
*/

import java.awt.Container;
import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;


public class test extends JPanel implements ActionListener {
private JTextField display = new JTextField("0");
private String buttonText = "789/456*123-0.=+";

private double result = 0;

private String operator = "=";

private boolean calculating = true;

public test() {
 setLayout(new BorderLayout());

 display.setEditable(false);
 add(display, "North");

 JPanel p = new JPanel();
 p.setLayout(new GridLayout(4, 4));

 for (int i = 0; i < buttonText.length();i++){
  JButton b = new JButton(buttonText.substring(i,i+1));
  p.add(b);
  b.addActionListener(this);
 }
 add(p, "Center");;
}

public void actionPerformed(ActionEvent e){
    String cmd = e.getActionCommand();
    if ('0' <= cmd.charAt(0) && cmd.charAt(0) <= '9' || cmd.equals(".")) {
      if (calculating)
        display.setText(cmd);
      else
        display.setText(display.getText() + cmd);
      calculating = false;
    } else {
      if (calculating) {
        if (cmd.equals("-")) {
          display.setText(cmd);
          calculating = false;
        } else
          operator = cmd;
      } else {
        double x = Double.parseDouble(display.getText());
        calculate(x);
        operator = cmd;
        calculating = true;
      }
    }


}

public void calculate(double n){
    if (operator.equals("+"))
        result += n;
      else if (operator.equals("-"))
        result -= n;
      else if (operator.equals("*"))
        result *= n;
      else if (operator.equals("/"))
        result /= n;
      else if (operator.equals("="))
        result = n;
      display.setText("" + result);
}

  public static void main(String[] args) {
     JFrame frame = new JFrame();
     frame.setTitle("Calculator");
     frame.setSize(200, 200);
     frame.addWindowListener(new WindowAdapter() {
       public void windowClosing(WindowEvent e) {
         System.exit(0);
       }
     });

     Container contentPane = frame.getContentPane();
     contentPane.add(new test());
     frame.show();
   }
}

---
name: nguyen minh hoang
Y!M : vietnam_hoangminhnguyen@yahoo.com
net name: mathhoang