how to refresh JFrame after after adding java 2d components

Question

in this code i added two components to the jframe and also used revalidate and repaint but only one of the components is being viewed. i need a way to refresh the jframe

class A extends JPanel{
    int i ,j;
    A(int i,int j){
        this.i = i;
        this.j = j;
    }
    @Override
    protected void paintComponent(Graphics g){
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.drawOval(i,j,10,10);
    }
}

public class T2d extends JPanel
{

    public static void main(String[] args) {

        JFrame jFrame = new JFrame("hi");
        jFrame.setExtendedState(JFrame.MAXIMIZED_BOTH);

        jFrame.add(new A(50,50));
        jFrame.revalidate();//revalidate and repaint doesn't refresh the frame
        jFrame.repaint();//i have read all the past question about this
                        //but none of them solved my problem.

        jFrame.add(new A(1000,100));
        jFrame.revalidate();
        jFrame.repaint();

        jFrame.setVisible(true);

    }
}

Answer 1

JFrame uses a BorderLayout by default, so only the last component you added is been updated by the layout manager (BorderLayout)

You could try changing the layout manager, maybe FlowLayout or GridLayout, but since A doesn't override the getPrefferedSize method, it will be sized to 0x0 by the layout managers anyway.

See Laying Out Components Within a Container for more details

It's also kind of pointless calling revalidate and repaint on a window that hasn't been made visible