Java making changes to an already added object

Question

I'm currently working on a little project for school and I'm still at the very beginning. I've just started reading into JFrame and all that stuff, so don't wonder why I may not be so familiar with everything you'll show me.

The goal for now is to have a program that gives out an image and to be able to change every single pixel of that image manually. Thus, I've written the following code:

public class JavaGraphicsTest {
    private static Pixel pixel;

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
        frame.setVisible(true);
        pixel = new Pixel(1600, 900);
        frame.getContentPane().add(pixel);
        //pixel.testChange();
    }
}

and:

public class Pixel extends Component {
    private BufferedImage img;
    private int width;
    private int height;
    private Graphics graphics;

    public Pixel(int w, int h) {
        width = w;
        height = h;
    }

    public void create() {
        img = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

        //Set any color for now
        for(int wc = 0; wc < width; wc++) {
            for(int hc = 0; hc < height; hc++) {
                img.setRGB(wc, hc, new Color(0xAAFFBB).getRGB());
            }
        }
    }      

    @Override
    public void paint(Graphics g) {
        super.paint(g);
        graphics = g;
        create();
        update();
    }

    public void update() {
        graphics.drawImage(img, 0, 0, null);
    }

    public void testChange() {
        for(int i = 50; i < 80; i++) {
            for(int  j = 80; j < 120; j++) {
                img.setRGB(i, j, new Color(0xFF8876).getRGB());
            }
            for(int  j = 460; j < 493; j++) {
                img.setRGB(i, j, new Color(0xFF8876).getRGB());
            }
        }
    }
}

Well, the code works so far (after many hours of nasty error spotting xD), but what I want to do now seems not to work so far: I want to call the method "pixel.testChange()" in the main method (it's currently commented). But as far as I have understood how JFrame works, I can't do anything with an object once I've added it to the JFrame. But who should it work then? How can I modify any active object without deleting and re-adding it?

PS: If you don't understand what the testChange method is supposed to do: It should change two blocks of the image to another color, it's basically a test to see if I successfully changed the image.

If you need further information on the project, please ask me :)

Thanks in advance, Julian

Answer 1

Don't store the Graphics object. Ever.

public class Pixel extends Component {
    private Graphics graphics;  // <<-- DO NOT DO THIS

    @Override
    public void paint(Graphics g) {
        // ...
        graphics = g;    // <<-- DO NOT DO THIS
        // ...
    }
}

The graphics object may be recreated each time #paint(Graphics g) is called, and can be invalidated, destroyed or corrupted when #paint(Graphics g) exits.

Ditto, do not CREATE images during the #paint(Graphics g) call. This should be done only once, when your Pixel is created.

public class Pixel extends Component {

    @Override
    public void paint(Graphics g) {
        // ...
        create();    // <<-- DO NOT DO THIS, EITHER.
        // ...
    }
}

But you may safely pass the Graphics object to other methods called from paint(Graphics g).

public class Pixel extends Component {
    // ... 

    public Pixel(int w, int h) {
        width = w;
        height = h;
        create();
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);
        update(g);
    }

    private void update(Graphics g) {
        graphics.drawImage(img, 0, 0, null);
    }
}

Ok. To the business at hand -- your #testChange() method. After changing the image, you simply ask Swing to draw your component again, using the #repaint() call.

public class Pixel extends Component {

    public void testChange() {
        int rgb = new Color(0xFF8876).getRGB());  // Cached, for efficiency.

        for(int i = 50; i < 80; i++) {
            for(int  j = 80; j < 120; j++) {
                img.setRGB(i, j, rgb);
            }
            for(int  j = 460; j < 493; j++) {
                img.setRGB(i, j, rgb);
            }
        }

        repaint(); // <<-- Ask Swing to repaint the component.
    }
}

---

One final note: you really shouldn't change Swing objects except in the Event Dispatching Thread (EDT). An exception is usually allowed when creating the first window, but before it is set visible.

public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
    frame.setVisible(true);             // <<-- Window becomes visible here
    pixel = new Pixel(1600, 900);
    frame.getContentPane().add(pixel);  // <<-- DANGEROUS!!
}

Instead, you can restructure the code like this:

public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
    pixel = new Pixel(1600, 900);
    frame.getContentPane().add(pixel);  // <<-- Safe - window not visible yet.
    frame.setVisible(true);             // <<-- Window becomes visible here
}

Better, is to actually switch to the EDT, using SwingUtilities.invokeLater(...):

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
            JFrame frame = new JFrame();
            frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
            frame.setVisible(true);             // <<-- Window becomes visible here
            pixel = new Pixel(1600, 900);
            frame.getContentPane().add(pixel);  // <<-- Safe - running on EDT.
            pixel.testChange();  // <<-- Also safe - running on EDT
        }
    });
}

Oh, and with Java8, you can get rid of much of the Runnable boiler plate code:

public static void main(String[] args) {
    SwingUtilities.invokeLater( () -> {
        JFrame frame = new JFrame();
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
        frame.setVisible(true);             // <<-- Window becomes visible here
        pixel = new Pixel(1600, 900);
        frame.getContentPane().add(pixel);  // <<-- Safe - running on EDT.
        pixel.testChange();  // <<-- Also safe - running on EDT
    });
}

Answer 2

At the end of your testChange method (or after calling it) you should call invalidate on the Pixel object, that is a signal to notify the component to be redrawn as it has been changed.

Check also this SO question.