Is this the right way to use the Java 2D Graphics API?

Question

I am creating a GUI for modeling JBox2D. The simulation is progressive, and between updates it is assumed that the content is being modeled. Like a game, except without input.

I only need geometric primitives for modeling JBox2D. This API seemed the easiest choice, but its design is a bit confusing.

Currently, I have one class with the name Windowextending JFrame, which contains as a member another class called Renderer. The class Windowinitializes itself and provides a method updateDisplay()(which is called by the main loop) that calls the method updateDisplay(objects)on Renderer. I made these two methods on my own, and their only goal is to call repaint()on Renderer.

Is it supposed to JPanelbe used that way? Or should I use a more complex method for animation (one that includes events and / or time intervals in some kind of back-end stream)?

Answer 1

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, Swing, :

• paintComponent, .
• repaint , . (Swing , , , - Swing.)

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, paintComponent.

---

:

import javax.swing.*;
import java.awt.*;
import java.awt.image.*;
import java.awt.event.*;

/**
 * Holding left-click draws, and
 * right-clicking cycles the color.
 */
class PaintAnyTime {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new PaintAnyTime();
            }
        });
    }

    Color[]    colors = {Color.red, Color.blue, Color.black};
    int  currentColor = 0;
    BufferedImage img = new BufferedImage(256, 256, BufferedImage.TYPE_INT_ARGB);
    Graphics2D  imgG2 = img.createGraphics();

    JFrame frame = new JFrame("Paint Any Time");
    JPanel panel = new JPanel() {
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            // Creating a copy of the Graphics
            // so any reconfiguration we do on
            // it doesn't interfere with what
            // Swing is doing.
            Graphics2D g2 = (Graphics2D) g.create();
            // Drawing the image.
            int w = img.getWidth();
            int h = img.getHeight();
            g2.drawImage(img, 0, 0, w, h, null);
            // Drawing a swatch.
            Color color = colors[currentColor];
            g2.setColor(color);
            g2.fillRect(0, 0, 16, 16);
            g2.setColor(Color.black);
            g2.drawRect(-1, -1, 17, 17);
            // At the end, we dispose the
            // Graphics copy we've created
            g2.dispose();
        }
        @Override
        public Dimension getPreferredSize() {
            return new Dimension(img.getWidth(), img.getHeight());
        }
    };

    MouseAdapter drawer = new MouseAdapter() {
        boolean rButtonDown;
        Point prev;

        @Override
        public void mousePressed(MouseEvent e) {
            if (SwingUtilities.isLeftMouseButton(e)) {
                prev = e.getPoint();
            }
            if (SwingUtilities.isRightMouseButton(e) && !rButtonDown) {
                // (This just behaves a little better
                // than using the mouseClicked event.)
                rButtonDown  = true;
                currentColor = (currentColor + 1) % colors.length;
                panel.repaint();
            }
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (prev != null) {
                Point  next = e.getPoint();
                Color color = colors[currentColor];
                // We can safely paint to the
                // image any time we want to.
                imgG2.setColor(color);
                imgG2.drawLine(prev.x, prev.y, next.x, next.y);
                // We just need to repaint the
                // panel to make sure the
                // changes are visible
                // immediately.
                panel.repaint();
                prev = next;
            }
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            if (SwingUtilities.isLeftMouseButton(e)) {
                prev = null;
            }
            if (SwingUtilities.isRightMouseButton(e)) {
                rButtonDown = false;
            }
        }
    };

    PaintAnyTime() {
        // RenderingHints let you specify
        // options such as antialiasing.
        imgG2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        imgG2.setStroke(new BasicStroke(3));
        //
        panel.setBackground(Color.white);
        panel.addMouseListener(drawer);
        panel.addMouseMotionListener(drawer);
        Cursor cursor =
            Cursor.getPredefinedCursor(Cursor.CROSSHAIR_CURSOR);
        panel.setCursor(cursor);
        frame.setContentPane(panel);
        frame.pack();
        frame.setResizable(false);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

---

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, -:

import java.awt.*;
import javax.swing.*;
import java.awt.image.*;
import java.awt.event.*;
import java.util.*;

/**
 * Left-click to spawn a new background
 * painting task.
 */
class DoubleBuffer {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new DoubleBuffer();
            }
        });
    }

    final int  width = 640;
    final int height = 480;

    BufferedImage createCompatibleImage() {
        GraphicsConfiguration gc =
            GraphicsEnvironment
                .getLocalGraphicsEnvironment()
                .getDefaultScreenDevice()
                .getDefaultConfiguration();
        // createCompatibleImage creates an image that is
        // optimized for the display device.
        // See http://docs.oracle.com/javase/8/docs/api/java/awt/GraphicsConfiguration.html#createCompatibleImage-int-int-int-
        return gc.createCompatibleImage(width, height, Transparency.TRANSLUCENT);
    }

    // The front image is the one which is
    // displayed in the panel.
    BufferedImage front = createCompatibleImage();
    // The back image is the one that gets
    // painted to.
    BufferedImage  back = createCompatibleImage();
    boolean  isPainting = false;

    final JFrame frame = new JFrame("Double Buffer");
    final JPanel panel = new JPanel() {
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            // Scaling the image to fit the panel.
            Dimension actualSize = getSize();
            int w = actualSize.width;
            int h = actualSize.height;
            g.drawImage(front, 0, 0, w, h, null);
        }
    };

    final MouseAdapter onClick = new MouseAdapter() {
        @Override
        public void mousePressed(MouseEvent e) {
            if (!isPainting) {
                isPainting = true;
                new PaintTask(e.getPoint()).execute();
            }
        }
    };

    DoubleBuffer() {
        panel.setPreferredSize(new Dimension(width, height));
        panel.setBackground(Color.WHITE);
        panel.addMouseListener(onClick);
        frame.setContentPane(panel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    void swap() {
        BufferedImage temp = front;
        front = back;
        back = temp;
    }

    class PaintTask extends SwingWorker<Void, Void> {
        final Point pt;

        PaintTask(Point pt) {
            this.pt = pt;
        }

        @Override
        public Void doInBackground() {
            Random rand = new Random();

            synchronized(DoubleBuffer.this) {
                Graphics2D g2 = back.createGraphics();
                g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                                    RenderingHints.VALUE_ANTIALIAS_ON);
                g2.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL,
                                    RenderingHints.VALUE_STROKE_PURE);
                g2.setBackground(new Color(0, true));
                g2.clearRect(0, 0, width, height);
                // (This computes pow(2, rand.nextInt(3) + 7).)
                int  depth = 1 << ( rand.nextInt(3) + 7 );
                float  hue = rand.nextInt(depth);
                int radius = 1;
                int c;
                // This loop just draws concentric circles,
                // starting from the inside and extending
                // outwards until it hits the outside of
                // the image.
                do {
                    int rgb = Color.HSBtoRGB(hue / depth, 1, 1);
                    g2.setColor(new Color(rgb));

                    int x = pt.x - radius;
                    int y = pt.y - radius;
                    int d = radius * 2;

                    g2.drawOval(x, y, d, d);

                    ++radius;
                    ++hue;
                    c = (int) (radius * Math.cos(Math.PI / 4));
                } while (
                       (0 <= pt.x - c) || (pt.x + c < width)
                    || (0 <= pt.y - c) || (pt.y + c < height)
                );

                g2.dispose();
                back.flush();

                return (Void) null;
            }
        }

        @Override
        public void done() {
            // done() is completed on the EDT,
            // so for this small program, this
            // is the only place where synchronization
            // is necessary.
            // paintComponent will see the swap
            // happen the next time it is called.
            synchronized(DoubleBuffer.this) {
                swap();
            }

            isPainting = false;
            panel.repaint();
        }
    }
}

- , :

Answer 2

javax.swing.Timer - . paintComponent(), , .

SwingWorker, . publish() .

.

Answer 3

? . JPanel, . 2D- 2D.

. JPanel, : https://github.com/dmurph/jbox2d/blob/master/jbox2d-testbed/src/main/java/org/jbox2d/testbed/framework/j2d/TestPanelJ2D.java

: https://github.com/dmurph/jbox2d/blob/master/jbox2d-testbed/src/main/java/org/jbox2d/testbed/framework/j2d/DebugDrawJ2D.java

TestbedMain.java, , , , :)

: : jbox2d

Here is the package for the testbed framework: https://github.com/dmurph/jbox2d/tree/master/jbox2d-testbed/src/main/java/org/jbox2d/testbed/framework

TestbedMain.java is located in the j2d folder, here: https://github.com/dmurph/jbox2d/tree/master/jbox2d-testbed/src/main/java/org/jbox2d/testbed/framework/j2d