another question on calculator applet

I am almost done with my applet. I have one last thing to do; I have to program it so that if the user hits the equal button, the calculator will return the last computed result. As it is, it is not doing that, it is repeating the operator function over and over again. I am not sure how to get it to display the result when the user hits the equal sign. Any advice would be helpful.

import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;
public class CalcApplet extends Applet
              implements MouseListener {
   boolean first_num_flag = true;
   boolean clearscreen = false;
   String display = "0";
   int num1, num2 = 0;
   char Op = 0;
   int result = 0;
//Declare the text field to be used for input and output:
   private TextField textInput = new TextField("");
//Declare the buttons of the calculator:
   private Button btn1 = new Button("1");
   private Button btn2 = new Button("2");
   private Button btn3 = new Button("3");
   private Button btnDiv = new Button("/");
   private Button btn4 = new Button("4");
   private Button btn5 = new Button("5");
   private Button btn6 = new Button("6");
   private Button btnMult = new Button("*");
   private Button btn7 = new Button("7");
   private Button btn8 = new Button("8");
   private Button btn9 = new Button("9");
   private Button btnSub = new Button("-");
   private Button btn0 = new Button("0");
   private Button btnClear = new Button("C");
   private Button btnEqual = new Button("=");
   private Button btnAdd = new Button("+");
   private String sSavedValue;
   private char charPreviousCalc;

public void init() {
      
//Declare a panel to group all the calculator buttons together:
      Panel calcPanel = new Panel();
      calcPanel.setLayout(new GridLayout(4,4));
      calcPanel.add(btn1);
      calcPanel.add(btn2);
      calcPanel.add(btn3);
      calcPanel.add(btnDiv);
      calcPanel.add(btn4);
      calcPanel.add(btn5);
      calcPanel.add(btn6);
      calcPanel.add(btnMult);
      calcPanel.add(btn7);
      calcPanel.add(btn8);
      calcPanel.add(btn9);
      calcPanel.add(btnSub);
      calcPanel.add(btn0);
      calcPanel.add(btnClear);
      calcPanel.add(btnEqual);
      calcPanel.add(btnAdd);
      btn1.addMouseListener(this);
      btn2.addMouseListener(this);
      btn3.addMouseListener(this);
      btnDiv.addMouseListener(this);
      btn4.addMouseListener(this);
      btn5.addMouseListener(this);
      btn6.addMouseListener(this);
      btnMult.addMouseListener(this);
      btn7.addMouseListener(this);
      btn8.addMouseListener(this);
      btn9.addMouseListener(this);
      btnSub.addMouseListener(this);
      btn0.addMouseListener(this);
      btnClear.addMouseListener(this);
      btnEqual.addMouseListener(this);
      btnAdd.addMouseListener(this);
//Add the panel & text field to the applet in a border layout:
      setLayout(new BorderLayout());
      add(textInput,BorderLayout.NORTH);
      add(calcPanel,BorderLayout.CENTER);
  }
   public void mousePressed(MouseEvent event) {
      Object source = event.getSource();
   
      if (source == btn0)
         if (first_num_flag == true) {
            display = "0";
            first_num_flag = false;
         }
         else 
            display += "0";
         
      textInput.setText(display);
      
      if (source == btn1)
         if (first_num_flag == true) {
            display = "1";
            first_num_flag = false;
         }
         else
            display += "1";             // concatenate "1" to display
      textInput.setText(display);
      
      if (source == btn2)
         if (first_num_flag == true) {
            display = "2";
            first_num_flag = false;
         }
         else
            display += "2";            // concatenate "2" to display
      textInput.setText(display);
      if (source == btn3)
         if (first_num_flag == true) {
            display = "3";
            first_num_flag = false;
         }
         else
            display += "3";            // concatenate "3" to display
      textInput.setText(display);
      if (source == btn4)
         if (first_num_flag == true) {
            display = "4";
            first_num_flag = false;
         }
         else
           display += "4";            // concatenate "4" to display
      textInput.setText(display);
      if (source == btn5)
         if (first_num_flag == true) {
            display = "5";
            first_num_flag = false;
         }
         else
            display += "5";           // concatenate "5" to display
      textInput.setText(display);
      if (source == btn6)
         if (first_num_flag == true) {
            display = "6";
            first_num_flag = false;
         }
         else
            display += "6";           // concatenate "6" to display
      textInput.setText(display);
      if (source == btn7)
         if (first_num_flag == true) {
            display = "7";
            first_num_flag = false;
         }
         else
            display += "7";           // concatenate "7" to display
      textInput.setText(display);
      if (source == btn8)
         if (first_num_flag == true) {
            display = "8";
            first_num_flag = false;
         }
         else
            display += "8";           // concatenate "8" to display
      textInput.setText(display);
      if (source == btn9)
         if (first_num_flag == true) {
            display = "9";
            first_num_flag = false;
         }
         else
            display += "9";           // concatenate "9" to display
      textInput.setText(display);
      
      // check if user entered addition operator
      if (source == btnAdd) {
         first_num_flag = true;
         clearscreen = false;
         num1 = Integer.parseInt(display);
         Op = '+';                         // Op is set to addition
      }
      else
      // check if user entered subtraction operator
      if (source == btnSub) {
         first_num_flag = true;
         clearscreen = false;
         num1 = Integer.parseInt(display);
         Op = '-';                         // Op is set to subtraction
      }
      else
      // check if user entered multiplication operator
      if (source == btnMult) {
         first_num_flag = true;
         clearscreen = false;
         num1 = Integer.parseInt(display);
         Op = '*';                         // Op is set to multiplication
      }
      else 
      // check if user entered division operator
      if (source == btnDiv) {
         first_num_flag = true;
         clearscreen = false;
         num1 = Integer.parseInt(display);
         Op = '/';                         // Op is set to division
      }
      else
      // check if user entered clear button
      if (source == btnClear) {
         first_num_flag = true;
         clearscreen = true;
         display = "0";
         num1 = 0;                         // Clears the screen
         num2 = 0;
         result = 0;
         Op = 0;
         textInput.setText(display);
      }
      else 
      // check if user entered equal operator
      if (source == btnEqual) {
         num2 = Integer.parseInt(display);
         
         switch(Op) 
         {
            case '+':
               result = num1 + num2;
               display = result + "";
               textInput.setText(display);
               break;
            case '-':
               result = num1 - num2;
               display = result + "";
               textInput.setText(display);
               break;
            case '*':
               result = num1 * num2;
               display = result + "";
               textInput.setText(display);
               break;
            case '/':
               if (num2 == 0) {
                 textInput.setText("Cannot divide by zero"); 
               }
               else {
                 result = num1 / num2;
                 display = result + "";
                 textInput.setText(display);
               }
               break;
            }
            
         }
         
      }
   
   public void mouseClicked(MouseEvent event) {
   }
   public void mouseReleased(MouseEvent event) {
   }
   public void mouseEntered(MouseEvent event) {
   }
   public void mouseExited(MouseEvent event) {
   }
}

[This message has been edited by Cindy Glass (edited December 20, 2001).]

Hi Jade,
Your code works great for me! I'm running Win2000 and JDK1.2. When I hit the equals button the result appears in the textfield just as it should.
Good Work,
Manfred.

Yes, it is working, but our teacher wanted it to display the last result when we hit the equals button. I thought that meant the previous result? Maybe I misunderstood.
Anyway, thanks.

'Previous' to me would be 'next to last,' as in "Show me the last step. Now show me the previous one."
If you change your point of reference at the right moment, they appear to be synonymous. This is where a broad liberal education, including lots of Looney Tunes re-runs, can be a big benefit:
Daffy Duck: Last week you said I could do my tholo this week!
Bugs Bunny: Right, Daffy. But this week last week was next week, not this week.
You gotta keep everything viewed from one place at one time, or you'll get dizzy.
------------------
Michael Ernest, co-author of: The Complete Java 2 Certification Study Guide

I think my education prior to this was too broad and liberal. I studied literary theory.

Would like some nitpicking of your code? There's a few ways it could be made shorter and better.
Junilu

Sure.

OK, you asked for it...

1. Something really nitpicky: Calculators normally have the number buttons arranged this way:
789
456
123
0
2. You can get rid of all the
if (source = btnXXX) { ... }
statements by factoring out the code in the if block to a separate object (hint: inner classes and MouseAdapter) that gets the label from the source and displays it etc.
3. You can do the same thing for the operator buttons as you do for the number buttons.
In general, try not to repeat yourself. If you find yourself writing the same code (with slight variations) more than twice, it's time to refactor.
Junilu

How do I refactor the if code out to a separate object? I don't know how to use inner classes.
I am very new to Java (like this is not obvious.)

OK, inner classes may be a little advanced for you at this point but it should be the next topic you'll have in class.
I'll give an example later...
Junilu

OK.

Hi Jade,
Since you are taking comments about the applet. I will just say that unless the exercise was to use a mouseListener that you should always try and make use of the objects that you are using. In your case an ActionListener would be more appropriate because all your objects are buttons.
Also you could make use of the buttons methods such as setActionCommand method and use getActionCommand method once inside your callback. It would get rid of much of your if/else blocks.
Regards,
Manfred.