Calculater.java


import java.awt.*;
import java.awt.event.*;
import java.awt.Toolkit;
import java.applet.*;
import java.lang.Object;
import  java.lang.*;

/**
 *  Define our applet class.  It extends the Applet object
 */
public class Calculator extends Applet{
//
// define and create the objects that make up the interface
//
 BorderLayout borderLayout = new BorderLayout(10,10); // 10 pixels of padding
 private TextField accumulator = new TextField("",25);
 private KeyPad keyPad = new KeyPad(this);
 private float answer = 0;    // holds intermediate answers
 private float accumulatorValue = 0; // holds value of accumulator after conversion
 private char lastOperation = ' '; // remembers last operation key
 private boolean startNewNumber = false; // tells the addDigit routine to start a new number

 /**
  * Constructor the applet
  */
  public Calculator() {
  }
/**
 *  The following methods initialize the applet and are
 * invoked automatically.
 */
   public void init(){

//
// first set the applets layout manager
//
   setLayout(borderLayout);
//
// add items to the layout
//
    add("North",accumulator);  // text box at top to enter operands and display answer
    add("Center",keyPad); // keypad, this is a grid panel

  }
/**
 * Add a digit to the window
 */
public void addDigit(String c)
{
  String text;

  if(startNewNumber)              // this is true after a operation button is pressed.
    text = "";                    // it allows the answer to be displayed until the
  else                            // first digit of the next operand is entered
    text = accumulator.getText();
  text += c;                      // add the character pressed
  accumulator.setText(text);      // change value of accumulator
  startNewNumber = false;         // clear this to allow building a number
}
/**
 * convert input text to a number
 */
public boolean convertInput()
{
 String text;

 text = accumulator.getText();    // get what is in the text box
 try { accumulatorValue = (Float.valueOf(text).floatValue());} // convert string to a float
    catch(NumberFormatException e)  // if its a bad number do something here
      {
       accumulatorValue = 0;
       answer = 0;
       accumulator.setText("Error");
       return false;              // let the caller know a bad value was entered
      }
 return true;
}
/**
 * process display a result
 */

public void equal()
{
 if(convertInput()){              // convert accumulator to a float
   switch (lastOperation){        // modify answer variable based on last operation
      case '+': answer+= accumulatorValue; break;
      case '-': answer-= accumulatorValue; break;
      case '*': answer = answer*accumulatorValue; break;
      case '/': answer  = answer /accumulatorValue; break;
      default:  answer = accumulatorValue; break;
    }
    accumulator.setText(String.valueOf(answer)); // set accumulator to show answer
    lastOperation = ' ';
  }
  startNewNumber = true; // tell addDigit to clear on the next digit
}
/**
 *  proces a + - * / operation
 */
public void operation(char c)
{
 equal(); // display the result
 lastOperation = c;  // store the operation for later
}
/**
 * process a clear
 */
public void clear()
{
 accumulator.setText("");
 answer = 0;
}
/**
 * process a negate
 */

public void negate()
{
 String text;
 text = accumulator.getText();
 if(text.charAt(0) == '-')
 {
   text = text.substring(1);
   accumulator.setText(text);
 }
 else
 {
  text = "-" + accumulator.getText();
  accumulator.setText(text);
 }

}
/**
 * process a backspace
 */
public void backSpace()
{
  String text;
  int length;

  if(startNewNumber)
  {
   accumulator.setText("");
   startNewNumber = false;
   return;
  }
  text = accumulator.getText(); // get contents
  length = text.length();       // determine the number of characters
  if(length != 0)               // do nothing if its empty
    accumulator.setText(text.substring(0,length-1)); // remove last character
}


}

------------------------------------------------------------------------

KeyPad.java

import java.awt.*;
import java.awt.event.*;
//
// keypad is a panel object that has a grid of buttons
//
public class KeyPad extends Panel
 implements ActionListener {

  GridLayout gridLayout = new GridLayout(4,5); // create the gridlayout with 20 buttons
  Calculator calculator;  // place to store the calculator object
/**
 * constructor fo rthe keypad
 */
  public KeyPad(Calculator theCalculator) {
    calculator = theCalculator; // remember calculator object so we can send it messages

    int i, nComponents;
//
// add all the keypad buttons
//
    add(new Button("7"));
    add(new Button("8"));
    add(new Button("9"));
    add(new Button("/"));
    add(new Button("*"));

    add(new Button("4"));
    add(new Button("5"));
    add(new Button("6"));
    add(new Button("+"));
    add(new Button("-"));

    add(new Button("1"));
    add(new Button("2"));
    add(new Button("3"));
    add(new Button("."));
    add(new Button("="));

    add(new Button("0"));
    add(new Button("+/-"));
    add(new Button("."));
    add(new Button("C"));
    add(new Button("<-"));
    this.setLayout(gridLayout);
//
// add an actionListener for each button
//
    nComponents = getComponentCount();
    for(i=0; i < nComponents; i++)
      ((Button)getComponent(i)).addActionListener(this);
  }

 /**
  * this method is called whenever a button is
  * pressed. Figure out what was pressed then do it
  */
  public void actionPerformed(ActionEvent e){
  //
  // first make sure that it was a button that sent this
  // event
  //
  if(e.getSource() instanceof Button){
  //
  // Now determine which button. Ask it for its label
  //
    String label = ((Button)e.getSource()).getLabel();
    switch(label.charAt(0))
    {
     case '0' :
     case '1' :
     case '2' :
     case '3' :
     case '4' :
     case '5' :
     case '6' :
     case '7' :
     case '8' :
     case '9' :
     case '.' :
          calculator.addDigit(label); // tell calculator a digit was pressed
          break;
     case '+' :
          if(label.compareTo("+/-") == 0)
            calculator.negate();
          else
            calculator.operation('+');
          break;

     case '*' :
     case '/' :
     case '-' :
          calculator.operation(label.charAt(0));
          break;

     case '=' :
          calculator.equal();
          break;
     case 'C' :
          calculator.clear();
          break;
     case '<' :
          calculator.backSpace();
          break;
    }
  }// end if (e.getSource
 }

}