# Java: Example - Calc GUI

Example - Calc Main, Example - Calc GUI, Example - Calc Extensions

This is the source for the graphical user interface of a simple calculator. It shows how a listener can be shared by many buttons, which then get the "action command" from the button and decide what to do with it. All the numeric buttons share one listener and all operators share another.

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    /** 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