Check out example codes for "how to create a circle in java". It will help you in understanding the concepts better.

Code Example 1

public class scratch {
    public static void main(String[] args) {
        JFrame window = new JFrame();
        window.setTitle("Christmas Tree");
        window.setSize(700, 600);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setVisible(true);
        DrawingComponent DC = new DrawingComponent();
        window.add(DC);
    }
}
class DrawingComponent extends JComponent{
      public void paint(Graphics graph0){
        Graphics2D graph = (Graphics2D) graph0;
        Ellipse2D.Double circle = new Ellipse.Double(5, 5, 25, 25);
        graph.fill(circle);
      }
}

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