public class Tester {
    public static void main(String[] args) {
        Frame frame  = new Frame();
        frame.start();
    }
}
class Frame {
    private JFrame frame;
    private Paint oval;
    private int x,y;
    Frame(){
        x = y = 120;
        frame =  new JFrame();
        oval = new Paint();
    }
    public void start() {
        frame.getContentPane().add(oval);
        frame.setSize(700, 700);
        frame.setVisible(true);

        for(int count = 0;count <400;count++) {
            x++;
            y++;
            oval.repaint();

            try {
                Thread.sleep(25);
            } catch (Exception ex) {

            }
        }
    }
    class Paint extends JPanel {
        public void paintComponent(Graphics g) {
            g.setColor(Color.WHITE);
            g.fillRect(0, 0, this.getWidth(), this.getHeight());
            g.setColor(Color.ORANGE);
            g.fillOval(x,y, 100, 100);
        }
    }
}

g.setColor(Color.WHITE);
g.fillRect(0, 0, this.getWidth(), this.getHeight());

public class Tester {
    public static void main(String[] args) {
        Frame frame  = new Frame();
        frame.start();
    }
}
class Frame {
    private JFrame frame;
    private Paint oval;
    private int x,y;
    Frame() {
        x = y = 120;
        frame =  new JFrame();
        oval = new Paint();
    }
    public void start() {
        frame.getContentPane().add(oval);
        frame.setSize(700, 700);
        frame.setVisible(true);

        for (int count = 0;count <400;count++) {
            x++;
            y++;
                **frame.repaint();**

            try {
                Thread.sleep(25);
            } catch (Exception ex) {

            }
        }
    }
    class Paint extends JPanel {
        public void paintComponent(Graphics g) {
            g.setColor(Color.ORANGE);
            g.fillOval(x,y, 100, 100);
        }
    }
}

public void paintComponent(Graphics g){
    super.paintComponent(g); // <<<
    g.setColor(Color.ORANGE);
    g.fillOval(x,y, 100, 100);
}