Updating a java Canvas that is a component of a JFrame

Question

How do I update the graphics on my screen from any class for a canvas that is a component of a JFrame? I believe that it is JFrame.update(Graphics g). But I'm not sure. If it is JFrame.update(Graphics g), What is the correct way to call it, and where do I get the Graphics object for the parameters?

Answer 1

Just call repaint() .

Any custom painting you want to do should be done in paintComponent(Graphics) , and be sure to call super.paintComponent(g);

In general, leave the handling of the Graphics instances to Swing. It will provide its own in compliance with its own standard.

This Oracle tutorial shows just that

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Here's an example that uses a Swing Timer to request repaint updates from the JFrame every 15 milliseconds.

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;
import javax.swing.WindowConstants;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

/**
 * @author Obicere
 */
public class ShapeDrawer {

    public ShapeDrawer() {
        final JFrame frame = new JFrame("Shape Drawer");
        final MyPanel panel = new MyPanel();

        frame.add(panel);

        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);

        createRepaintTimer(frame);
    }

    // Just makes calls for the frame to paint every 15 milliseconds
    private void createRepaintTimer(final JFrame frame) {
        final Timer timer = new Timer(15, null);

        timer.addActionListener(e -> {
            if (!frame.isVisible()) {
                timer.stop();
            } else {
                frame.repaint();
            }
        });

        timer.start();
    }

    public static void main(final String[] args) {
        SwingUtilities.invokeLater(ShapeDrawer::new);
    }

    public class MyPanel extends JPanel {

        private long start;

        public MyPanel() {
            start = System.currentTimeMillis();
        }

        @Override
        protected void paintComponent(final Graphics g) {
            // Calling to clear the artifacts!
            super.paintComponent(g);

            g.setColor(Color.RED);

            // Difference between when the current time and when we started
            final long difference = System.currentTimeMillis() - start;

            // Difference in seconds
            final double seconds = difference / 1000d;

            // 1 rotation per second
            final double rotation = seconds * 1440 / 1000;

            final int x = (int) (Math.cos(rotation) * 100);
            final int y = (int) (Math.sin(rotation) * 100);

            g.drawOval(200 + x - 5, 200 + y - 5, 10, 10);
        }

        // Just setting the default size of the panel
        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 400);
        }

    }

}

As you can see, the createRepaintTimer(JFrame) method makes the calls to request the graphics to be updated. We don't have to provide a Graphics instance, Swing handles this for us. We then render to the Graphics swing has provided us in the MyPanel#paintComponent(Graphics) method after calling super.paintComponent(Graphics) .