问如果只绘制特定的区域，为什么需要调用两个重绘？

EN

class MyPanel extends JPanel {

    private int squareX = 50;
    private int squareY = 50;
    private int squareW = 20;
    private int squareH = 20;

    public MyPanel() {

        setBorder(BorderFactory.createLineBorder(Color.black));

        addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                moveSquare(e.getX(),e.getY());
            }
        });

        addMouseMotionListener(new MouseAdapter() {
            public void mouseDragged(MouseEvent e) {
                moveSquare(e.getX(),e.getY());
            }
        });

    }

    private void moveSquare(int x, int y) {
        int OFFSET = 0;
        if ((squareX!=x) || (squareY!=y)) {
            //repaint(squareX,squareY,squareW+OFFSET,squareH+OFFSET);
            repaint(squareX,squareY,squareW,squareH);
            squareX=x;
            squareY=y;
            repaint(squareX,squareY,squareW+OFFSET,squareH+OFFSET);
            //repaint(squareX,squareY,squareW,squareH);
        }
    }


    public Dimension getPreferredSize() {
        return new Dimension(250,200);
    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawString("This is my custom Panel!",10,20);
        g.setColor(Color.RED);
        g.fillRect(squareX,squareY,squareW,squareH);
        g.setColor(Color.BLUE);
        g.drawRect(squareX,squareY,squareW,squareH);



  }
}