Question

I have a problem with canvas i wanted to show some moving balls on canvas (animation) but there is nothing I can see except black background.

can somebody tell me the mistake in this code and how it will work please.

public CopyOfCleanBallPanel2() throws IOException, InterruptedException {

        frame = new JFrame("simple gaming loop in java");
        frame.setSize(BOX_WIDTH, BOX_WIDTH);
        frame.setResizable(false);

        displayCanvas = new CustomCanvas();
        displayCanvas.setLocation(0, 0);
        displayCanvas.setSize(CANVAS_WIDTH, CANVAS_HEIGHT);
        displayCanvas.setBackground(Color.BLACK);
        displayCanvas.setFont(new Font("Arial", Font.BOLD, 14));
        displayCanvas.setPreferredSize(new Dimension(CANVAS_WIDTH,CANVAS_HEIGHT));

        frame.add(displayCanvas);
        displayCanvas.requestFocus();

        frame.setLocationRelativeTo(null);

        try {
            this.aBall = (BallServer) Naming
                    .lookup("rmi://localhost/BouncingBalls");

        } catch (Exception e) {
            System.out.println("Exception: " + e);
        }


        frame.pack();
        frame.setVisible(true);

        aBall.start();
        startFrameTimer();  
    }

    /*
     * Initializes the frame (also game update) timer.
     */
    private void startFrameTimer() {
        frameTimer.schedule(new FrameTimerTask(), 1, GAME_TIMER_COOLDOWN);
    }

    public void updateSimulation() throws RemoteException {
        repaintCanvas();
    }
    /*
     * This method gets called by the timer. It updates the game simulation and
     * redraws it.
     */
    private void onFrameTimer() throws RemoteException {
        updateSimulation();
    }

    /*
     * Causes the whole canvas to get repainted.
     */
    private final void repaintCanvas() throws RemoteException  {
        Graphics g =  displayCanvas.getGraphics();
        drawworld(g);
    }

    private class FrameTimerTask extends TimerTask {
        public void run() {
            try {
                onFrameTimer();
            } catch (RemoteException e) {
                e.printStackTrace();
            }
        }
    }
    /*
     * This custom canvas overrides the paint method thus allowing for a custom
     * painting on the component.
     */
    private class CustomCanvas extends Canvas {
        @Override
        public void paint(Graphics g) {
            // Currently the game message gets drawn over the inner border
            // of the canvas so we have to repaint the whole thing.
            try {
                repaintCanvas();
            } catch (RemoteException e) {
                e.printStackTrace();
            }
        }
    }   
    public void drawworld(Graphics g) throws RemoteException {
            g.setColor(Color.BLACK);
             g.fillRect(0, 0, CANVAS_WIDTH, CANVAS_HEIGHT);

            System.out.println("i m in drawworld ");

            serBall = aBall.getState1(); ***// here it is remote call and there is thread going on suspension*** 

            for (int i = 0; i < currentNumBalls; i++) {
                g.setColor(serBall[i].getBallColor(velocity.getLength()));
                g
                        .fillOval((int) (serBall[i].position.getX() - serBall[i]
                                .getRadius()),
                                (int) (serBall[i].position.getY() - serBall[i]
                                        .getRadius()), (int) (2 * serBall[i]
                                        .getRadius()), (int) (2 * serBall[i]
                                        .getRadius()));

                // Draw our framerate and ball count
                g.setColor(Color.WHITE);
                g.drawString("FPS: " + currentFrameRate + " Balls: "
                        + currentNumBalls, 15, 15);
            }   
        }

P.S: I thought there is some thread problem while I m calling the remote method and rendering the drawworld, either of thread is going on suspension or blocked

Please Help.

jibby lala

Answer 1

When using Swing custom painting is done by overriding the paintComponent() method of a JPanel (or JComponent), not a Canvas. Canvas is an AWT component and should not be used with Swing. See the Swing tutorial on Custom Painting for more information and examples.

Animation should be done by using a Swing Timer so that code is executed on the EDT. The Swing tutorial also has section on “How to Use Swing Timers” and “Concurrency” which helps explain these concepts.

The repaintCanvas() method is unnecessary. To repaint a component you simply invoke repaint() on the component. You should never use the getGraphics() method. All painting methods already receive the Graphics class as a parameter. That is the Grapphics object you should use for painting.