import java.awt.Button;
import java.awt.Color;
import java.awt.Frame;
import java.awt.GridLayout;
import java.awt.Panel;
import java.awt.TextField;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

class cl {
 int mn, fir, sec;
 double last;
 Frame f = new Frame("계산기");
 String num[];
 ActionListener al;
 TextField tf=new TextField();
 Panel p;
 

 public cl() {

  p = new Panel();
  Button[] numButtons = null;
  p.setLayout(new GridLayout(4, 4));

  String numStr[] = { "7", "8", "9", "*", "4", "5", "6", "/", "1",
    "2", "3", "+", "C", "0", "=", "-" };
  
  numButtons = new Button[numStr.length];
  for (int i = 0; i < numStr.length; i++) {
   numButtons[i] = new Button(numStr[i]);
   numButtons[i].setForeground(Color.blue);
   p.add(numButtons[i]);
  }
  al = new ActionListener() {

   @Override
   public void actionPerformed(ActionEvent e) {
    // TODO Auto-generated method stub
    if (e.getActionCommand() == "0") {
     tf.setText("0");
    } else if (e.getActionCommand() == "1") {
     tf.setText("1");
    } else if (e.getActionCommand() == "2") {
     tf.setText("2");
    } else if (e.getActionCommand() == "3") {
     tf.setText("3");
    } else if (e.getActionCommand() == "4") {
     tf.setText("4");
    } else if (e.getActionCommand() == "5") {
     tf.setText("5");
    } else if (e.getActionCommand() == "6") {
     tf.setText("6");
    } else if (e.getActionCommand() == "7") {
     tf.setText("7");
    } else if (e.getActionCommand() == "8") {
     tf.setText("8");
    } else if (e.getActionCommand() == "9") {
     tf.setText("9");
    } else if (e.getActionCommand() == "+") {
     last = Integer.parseInt(tf.getText());
     mn = 1;
    } else if (e.getActionCommand() == "-") {
     last = Integer.parseInt(tf.getText());
     mn = 2;
    } else if (e.getActionCommand() == "*") {
     last = Integer.parseInt(tf.getText());
     mn = 3;
    } else if (e.getActionCommand() == "/") {
     last = Integer.parseInt(tf.getText());
     mn = 4;
    } else if (e.getActionCommand() == "C") {
     fir = 0;
     sec = 0;
     last = 0;
     tf.setText("0");

    } else if (e.getActionCommand() == "=") {
     last = Integer.parseInt(tf.getText());
     if (mn == 1) {
      last = fir += sec;
      tf.setText(String.valueOf(last));
     } else if (mn == 2) {
      last = fir -= sec;
      tf.setText(String.valueOf(last));
     } else if (mn == 3) {
      last = fir *= sec;
      tf.setText(String.valueOf(last));
     } else if (mn == 4) {
      if (sec == 0) {
       last = 0;
       tf.setText(String.valueOf(last));
      } else if (sec != 0) {
       last = fir /= sec;
       tf.setText(String.valueOf(last));
      }

     }
    }
   }
  };
  for (int i = 0; i < 16; i++) {
   numButtons[i].addActionListener(al);
  }
  f.add("North", tf);
  f.add("Center", p);
  f.addWindowListener(new WindowAdapter() {

   @Override
   public void windowClosing(WindowEvent arg0) {
    // TODO Auto-generated method stub
    System.exit(0);
   }

  });
  f.setSize(200, 150);
  f.setVisible(true);
  
 }
}

public class cal {

 /**
  * @param args
  */
 public static void main(String[] args) {
  // TODO Auto-generated method stub
  cl aa = new cl();
 }

}

여기까진 내가 짯는데....

연산결과가 미친듯이 0만튀어나와요...왜그런지 좀 알려줘요 제발 ㅠㅠ