import java.awt.*;
import java.applet.*;
import java.awt.event.*;
/**
* This class demonstrates an arrays of objects (Buttons and Strings)
* It's a very simple cash register class.
*
* It also demonstrates use of a panel to create a grid of buttons
*/
public class CashRegister extends Applet implements ActionListener 
{
  TextArea display = new TextArea(8, 20); // register display area
  String current = new String("");        // current string in the display
  double sumSoFar = 0d;                   // sum of numbers entered so far

  String [] btnLabels = // Labels for the register buttons
                       {
                          "1", "2", "3", "Add",
                          "4", "5", "6", "Clear", 
                          "7", "8", "9", "Sum",
                          "0", "."
                                                 }; 
  // Notice that arrays can be initialised wuith values when they are declared
  // rather than be given a size; in this case the size is implied.

  /**
   * Method to set up the display
   */
  public void init()
  {
    int nButtons = btnLabels.length;          // number of buttons in the display (= label array size)
    add(display);                             // the text area
    Panel keypad = new Panel();               // put the buttons in a panel
    keypad.setLayout( new GridLayout(4, 3) ); // lay them out in a grid
    Button [] keys = new Button[nButtons];    // Array of buttons

    for (int i = 0; i < nButtons; i++ )       // for each label
    {
      keys[i] = new Button(btnLabels[i]);     // create a button
      keypad.add( keys[i] );                  // add it to the panel
      keys[i].addActionListener(this);        // set listener
      keys[i].setActionCommand(btnLabels[i]); // and associated command
    }
    add(keypad);                              // add the pad
  }
  /**
   * actionPerformed, as ever, carries out the main processing
   * based on the Button's which are pressed.
   * 
   * @param e carries information about the event that generated the call
   */
  public void actionPerformed(ActionEvent e) 
  {
    // a numeric key (0-9) adds the number to the current string
    // the . key adds a decimal point
    // the Add key sums in the last entry and starts a new line
    // the Sum key prints the sum so far and clears the sum to zero
    // the Clr key clears the current entry

    String command = e.getActionCommand();    // action string - set in init()
    char com = command.charAt(0);             // get first character

    // see if the character is part of a number:
    if ( (com >= '0') && (com <= '9') || (com == '.') )
    {
      current = current + com;                // if it is append to the current String
      display.append("" + com);               // and to the display
    }
    else
    {
      // check here for function buttons
      if ( com == 'A')                       // Add this entry into the sum
      {
        display.append(" +\n");              // start a new display line
        addThis(current);                    // helper method to add to sum
        current = "";                        // re-set current String to empty
      }
      else if (com == 'C')
      {
        current = "";                        // clear the current String
        display.append(" - err\n");          // indicate in the display
      }
      else if (com == 'S')
      {
        display.append("\n-------------\n");     // draw a line
        display.append("" + sumSoFar + "\n\n");  // show the current sum
        current = "";                            // clear the current String
        sumSoFar = 0.0;                          // reset the sum
      }
      // more functions could be added!
    }
  }

  /**
   *  This method converts the current string and adds to sum so far
   *  it gives an error if the number is incorrectly formatted
   *  
   *  @param The String to be converted and added to the sum
   */
  private void addThis(String s)
  {
    if (s == "")
    {
      display.append("empty string!\n");
    }
    else
    {
      try
      {
        sumSoFar = sumSoFar + Double.parseDouble(s);
      }
      catch ( NumberFormatException n )
      {
        display.append("Error, re-enter\n");
        current = "";
      }
    }
  }
}