抽象方法的疑惑解决思路

public class GamePanel extends JPanel {    MainFrame mainFrame;        private Image background = ImageUtil.getImage("images/background.jpg");        public GamePanel(MainFrame mainFrame) {        this.mainFrame = mainFrame;//        this.setPreferredSize(new Dimension())    }        public void paint(Graphics g) {        g.drawImage(this.background, 0, 0, this.getWidth(),                 this.getHeight() , null);        Piece currentPiece = this.mainFrame.getCurrentPiece();        ImageUtil.paintPiece(g, currentPiece);        Square[][] squares = this.mainFrame.getSquares();        if (squares == null) return;        for (int i = 0; i < squares.length; i++) {            for (int j = 0; j < squares[i].length; j++) {                Square s = squares[i][j];                if (s != null) {                    g.drawImage(s.getImage(), s.getBeginX(), s.getBeginY(), this);                }            }        }    }}