java getGraphics() returns null, how to access paintComponent?

Question

I have a class with a JPanel, and paintComponent(). I also have a class that implements Runnable, and which I plan to draw images onto the paintComponent once the thread is started. My Constructor takes in the JPanel, and from there I call getGraphics(). However through testing, and searching this always seems to return null.

    System.err.println("Thread Started");
    isRunning = true;
    System.err.println(pap.getGraphics());
    Graphics g = pap.getGraphics();   //pap is the name of the JPanel
    while (isRunning)
    {
        while(xPos <= pap.getWidth() + 1)
        {
            xPos+=horizontalDirection;
            System.err.println(xPos);
            drawImage(upImgs[1], xPos, yPos, g);
            pap.repaint();
            pause();
            //g.drawOval(xPos, 10, 10, 10);
            if(xPos >= pap.getWidth())
                horizontalDirection = -horizontalDirection;
            if(xPos < 1)
                horizontalDirection = 1;
        }        
        //pap.repaint();
        //pause(); // let this thread sleep a bit
    }
    System.err.println("Thread Ended");

returns Thread Started 2 null Exception in thread "Thread-1" java.lang.nullPointerException

How can I properly get the paintComponent to draw on it from this separate class?

MTilsted · Accepted Answer · 2014-03-06 18:12:46Z

2

You can't do that. Awt/Swing are not thread safe, so drawing can only be done from the gui thread.

answered Mar 6, 2014 at 18:12

MTilsted

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3 Comments

David MacNeil Over a year ago

Could I have my thread "bump" a method in the GUI thread and it could paint itself from there?

Mark W Over a year ago

Use SwingUtilities.invoke or .invokelater to run the code on the gui thread.

MTilsted Over a year ago

What you should do, is make a class which extends JComponent and which then repaint itself. To force a repaint you then can call the repaint() method from your other thread.

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