如何在JPanel上绘制矩形？

Question

I want to draw a rectange on top of JPanel . Next I want to set proper location for this rectangle. This is how I do this. I tried to change number in g.fillRect(margin, margin, 30, 30) , but my rectangle is always located in the center of JPanel . Why?

JPanel chartPanel = new JPanel();

// here we add components to chartPanel
// ...

chartPanel.add(new LegoBox());

class LegoBox extends JPanel {

    public void paintComponent(Graphics g) {
            super.paintComponent(g);
            int margin = 0;          
            g.setColor(Color.red);
            g.fillRect(margin, margin, 30, 30);
    }
}

Answer 1

Perhaps it is the LayoutManager from chartPanel ?
For reproducing your problem you have to post a working example.
Try filling the chartPanel with you custom panel:

chartPanel.setLayoutManager(new BorderLayout());
chartPanel.add(new LegoBox(), BorderLayout.CENTER); 

---

paintComponent:

super.paintComponent(g);
g.setColor(Color.red);
int rectWidth = 30;
int rectHeight = 30; 
int marginLeft = 12; // a pixel value
int marginTop = 17;      
g.fillRect(marginLeft, marginTop, rectWidth, rectHeight);