Question

When I press the ‘draw’ button, the clock doesn’t show up. I know the button works since calling the setText method on a textField object inside it comes through just fine. I tried various things and cannot find anything that works. I even approached my professor and he also had a tough time. But then I resized the frame and it suddenly showed up… I don’t understand why and I’m trying to figure out how to make the clock show up simply from pressing the ‘draw’ button. Any help is appreciated and thank you so much in advance!

Here are my classes in their entirety just in case you wish to run them yourself but for those preferring brevity, focus on inner class within ClockFrame class:

            import java.awt.Graphics;
            import java.awt.Graphics2D;
            import java.awt.geom.Ellipse2D;
            import java.awt.geom.Line2D;

            import javax.swing.JComponent;

            public class ClockComponent extends JComponent
            {
                final int HOURS_ON_CLOCK = 12;
                final double MINUTES_IN_HOUR = 60;
                final double HOUR_LENGTH = .4;
                final double MINUTE_LENGTH = .9;

                private double radius;
                private double posXY;
                private double hourHandLength;
                private double minuteHandLength;
                private double center;

                private Line2D.Double hourHand;
                private Line2D.Double minuteHand;

                public ClockComponent(double rad, double xy)
                {
                    radius = rad;
                    posXY = xy;

                    center = posXY + radius;

                    hourHandLength = radius * HOUR_LENGTH;
                    minuteHandLength = radius * MINUTE_LENGTH;

                    hourHand = new Line2D.Double(center, center, 
                            center, center - hourHandLength);
                    minuteHand = new Line2D.Double(center, center, 
                            center, center - minuteHandLength);
                }

                public void setClock(int hour, int minute)
                {
                    final double HOURS_TO_RADIANS = 2 * Math.PI / HOURS_ON_CLOCK;
                    final double MINUTES_TO_RADIANS = 2 * Math.PI / MINUTES_IN_HOUR;

                    hourHand.setLine(center, center, center + hourHandLength * 
                            Math.sin((hour + minute / MINUTES_IN_HOUR) * HOURS_TO_RADIANS), 
                            center - hourHandLength * 
                            Math.cos((hour + minute / MINUTES_IN_HOUR) * HOURS_TO_RADIANS));
                    minuteHand.setLine(center, center,
                            center + minuteHandLength * Math.sin(minute * MINUTES_TO_RADIANS),
                            center - minuteHandLength * Math.cos(minute * MINUTES_TO_RADIANS));
                }

                public void paintComponent(Graphics g)
                {
                    Graphics2D g2 = (Graphics2D) g;

                    double diameter = 2 * radius;

                    Ellipse2D.Double clockFace = 
                            new Ellipse2D.Double(posXY, posXY, diameter, diameter);

                    g2.draw(clockFace);
                    g2.draw(hourHand);
                    g2.draw(minuteHand);
                }
            }

ClockFrame class with the ActionListener inner class. I feel that the ActionListener inner class is where I am missing something but I can’t figure out what exactly…

            import java.awt.BorderLayout;
            import java.awt.GridLayout;
            import java.awt.event.ActionEvent;
            import java.awt.event.ActionListener;

            import javax.swing.JButton;
            import javax.swing.JFrame;
            import javax.swing.JLabel;
            import javax.swing.JPanel;
            import javax.swing.JTextField;
            import javax.swing.border.EtchedBorder;

            public class ClockFrame extends JFrame
            {
                final int FIELD_SIZE = 10;
                final int FRAME_WIDTH = 450;
                final int FRAME_HEIGHT = 450;
                final int FIELD_WIDTH = 5;

                private JPanel frameFace;
                private JPanel fieldPanel;
                private JPanel drawingPanel;

                private JLabel hourLabel;
                private JLabel minuteLabel;

                private JTextField hourField;
                private JTextField minuteField;

                private JButton drawButton;

                private ClockComponent clock;

                public ClockFrame(double rad, double xy)
                {
                    setTitle("Clock viewer");
                    setSize(FRAME_WIDTH, FRAME_HEIGHT);

                    frameFace = new JPanel(new BorderLayout());
                    fieldPanel = new JPanel();
                    drawingPanel = new JPanel();
                    drawingPanel.setBorder(new EtchedBorder());

                    hourLabel = new JLabel();
                    hourLabel.setText("Hour ");

                    minuteLabel = new JLabel();
                    minuteLabel.setText("Minute ");

                    hourField = new JTextField(FIELD_SIZE);
                    minuteField = new JTextField(FIELD_SIZE);

                    drawButton = new JButton("Draw");
                    drawingPanel.setLayout(new GridLayout(1, 1));

                    clock = new ClockComponent(rad, xy);

                    fieldPanel.add(hourLabel);
                    fieldPanel.add(hourField);
                    fieldPanel.add(minuteLabel);
                    fieldPanel.add(minuteField);
                    fieldPanel.add(drawButton);

                    ActionListener clockListener = new DrawListener();
                    drawButton.addActionListener(clockListener);

                    frameFace.add(fieldPanel, BorderLayout.NORTH);
                    frameFace.add(drawingPanel, BorderLayout.CENTER);

                    add(frameFace);
                }

                class DrawListener implements ActionListener
                {
                    public void actionPerformed(ActionEvent event)
                    {
                        int hour = Integer.parseInt(hourField.getText());
                        int minute = Integer.parseInt(minuteField.getText());

                        clock.setClock(hour, minute);
                        drawingPanel.add(clock);
                    }
                }
            }

And a main method

            import javax.swing.JFrame;

            public class ClockViewer
            {
                public static void main(String[] args)
                {
                    JFrame frame = new ClockFrame(75, 100);
                    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                    frame.setVisible(true);
                }
            }

Answer 1

Whenever you add a component to a container or remove one from the container after the GUI has been realized, you need to tell that container to re-lay out its components and re-draw them. This is done by calling revalidate() and then repaint() on the container, here the drawingPanel:

clock.setClock(hour, minute);
drawingPanel.add(clock);
drawingPanel.revalidate();
drawingPanel.repaint();