Java: Example - Calc GUI

/** CalcGUI.java - Subclass of JPanel for Calc GUI
    @author Fred Swartz
    @version 2004-04-20 Rodenbach
*/
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

////////////////////////////////////////////////////////// class CalcGUI
class CalcGUI extends JPanel {
    //=============================================== instance variables
    //--- Component referenced during execution
    private JTextField _displayField;        // display result / input.
    //--- Variables representing state of the calculator
    private boolean    _startNumber = true;  // true: num key next
    private int        _resultValue = 0;     // result so far
    private String     _previousOp  = "=";   // previous operation

    //========================================= static (class) variables
    private static final Font BIGGER_FONT = 
                                  new Font("monspaced", Font.PLAIN, 20);

    //====================================================== constructor
    public CalcGUI() {
        //--- Display field
        _displayField = new JTextField("0", 12);
        _displayField.setHorizontalAlignment(JTextField.RIGHT);
        _displayField.setFont(BIGGER_FONT);

        //--- Clear button
        JButton clearButton = new JButton("CLEAR");
        clearButton.setFont(BIGGER_FONT);
        clearButton.addActionListener(new ClearListener());

        //--- One listener for all numeric keys.
        ActionListener numListener = new NumListener();
        
        //--- Layout numeric keys in a grid.  Generate the buttons
        //    in a loop from the chars in a string.
        String buttonOrder = "789456123 0 ";
        JPanel buttonPanel = new JPanel(new GridLayout(5, 3));
        for (int i = 0; i < buttonOrder.length(); i++) {
            String keyTop = buttonOrder.substring(i, i+1);
            if (keyTop.equals(" ")) {
                buttonPanel.add(new JLabel(""));
            } else {
                JButton b = new JButton(keyTop);
                b.addActionListener(numListener);
                b.setFont(BIGGER_FONT);
                buttonPanel.add(b);
            }
        }
        
        //--- One ActionListener to use for all operator buttons.
        ActionListener opListener = new OpListener();
        
        //--- Create panel with gridlayout to hold operator buttons.
        //    Use array of button names to create buttons in a loop.
        JPanel opPanel = new JPanel(new GridLayout(5, 1));
        String[] opOrder = {"+", "-", "*", "/", "="};
        for (int i = 0; i < opOrder.length; i++) {
            JButton b = new JButton(opOrder[i]);
            b.addActionListener(opListener);
            b.setFont(BIGGER_FONT);
            opPanel.add(b);
        }

        //--- Layout the top-level panel.
        this.setLayout(new BorderLayout());
        this.add(_displayField, BorderLayout.NORTH );
        this.add(buttonPanel  , BorderLayout.CENTER);
        this.add(opPanel      , BorderLayout.EAST  );
        this.add(clearButton  , BorderLayout.SOUTH );
    }//end constructor

    //====================================================== action_clear
    /** Called by Clear btn action listener and elsewhere.*/
    private void action_clear() {
        _startNumber = true;
        _displayField.setText("0");
        _resultValue = 0;
        _previousOp  = "=";
    }

    /////////////////////////////////////// inner listener class OpListener
    /** Listener for all op buttons. */
    class OpListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            // The calculator is always in one of two states.
            // 1. A number must be entered -- this operator is wrong.
            // 2. An operator must be entered -- we're ok.
            if (_startNumber) { // Error: needed number, not operator
                action_clear();
                _displayField.setText("ERROR");
            } else {
                _startNumber = true;  // Next thing must be a number
                try {
                    // Get value from display field, convert, do prev op
                    // If this is the first op, _previousOp will be =.
                    String displayText = _displayField.getText();
                    int currentValue = Integer.parseInt(displayText);
    
                    if (_previousOp.equals("=")) {
                        _resultValue = currentValue;
                    } else if (_previousOp.equals("+")) {
                        _resultValue += currentValue;
                    } else if (_previousOp.equals("-")) {
                        _resultValue -= currentValue;
                    } else if (_previousOp.equals("*")) {
                        _resultValue *= currentValue;
                    } else if (_previousOp.equals("/")) {
                        _resultValue /= currentValue;
                    }
    
                    _displayField.setText(""+_resultValue);
    
                } catch (NumberFormatException ex) {
                    action_clear();
                    _displayField.setText("Error");
                }
    
                //--- set _previousOp for the next operator.
                _previousOp = e.getActionCommand();
            }//endif _startNumber
        }//endmethod
    }//end class

    //////////////////////////////////// inner listener class ClearListener
    // Action listener for numeric keys
    class NumListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            String digit = e.getActionCommand(); // Get text from button
            if (_startNumber) {
                // This is the first digit, clear field and set
                _displayField.setText(digit);
                _startNumber = false;
            } else {
                // Add this digit to the end of the display field
                _displayField.setText(_displayField.getText() + digit);
            }
        }
    }//end class
    
    //////////////////////////////////// inner listener class ClearListener
    class ClearListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            action_clear();
        }
    }
}//endclass CalcGUI