在JFrame中的JPanel上绘图

Question

It's been a long time since I did any drawing programs in Java and I can't get this one to work. The window stays empty even though there should be a filled oval there. Before the code was throwing a NullPonterException in the draw() function.

public class Map extends JFrame {

    private static final long serialVersionUID = 1L;
    private JPanel contentPane;
    private Graphics graphics;


    public static void main(String[] args) {
        Map frame = new Map();
    }

    public Map() {
        setTitle("Bugs");
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 600, 600);
        contentPane = new JPanel();
        contentPane.setLayout(new BorderLayout(0, 0));
        setContentPane(contentPane);

        JPanel drawingPanel = new JPanel();

        contentPane.add(drawingPanel, BorderLayout.CENTER);
        this.setVisible(true);
        graphics = drawingPanel.getGraphics();

        draw(graphics);

    }

    public void draw(Graphics g){
        Bug b = new Bug();
        g.setColor(Color.RED);
        g.fillOval(b.getPosX(), b.getPosY(), b.getRadius(), b.getRadius());
    }

}

Answer 1

It is not enough to call draw(...) only one time. This way the drawn content is lost immediately after your component is redrawn.

The correct approach would be as follows:

public class BugPanel extends JPanel {

    private Bug bug;

    public BugPanel(Bug b) {
        bug = b;
    }

    @Override
    public void paint(Graphics g) {
        // draw your bug here
    }
}

Then add this panel to the center region of your frame.