为什么画布设置不了边框

class DiceCanvas extends JPanel{
Dice dice=new Dice();

    public Dice getDice() {
        return dice;
    }

    public void setDice(Dice dice) {
        this.dice = dice;
    }

    @Override
    public void paint(Graphics g) {
        int type=dice.getDiceType();
        try {
            switch (type)
            {
                case 1:
                    g.drawImage(ImageIO.read(new FileInputStream("src/images/t1.png")),40,40,80,80,null);
                    break;
                case 2:
                    g.drawImage(ImageIO.read(new FileInputStream("src/images/t1.png")),40,40,80,80,null);
                    break;
                case 3:
                    g.drawImage(ImageIO.read(new FileInputStream("src/images/t1.png")),40,40,80,80,null);
                    break;
                case 4:
                    g.drawImage(ImageIO.read(new FileInputStream("src/images/t1.png")),40,40,80,80,null);
                    break;
                case 5:
                    g.drawImage(ImageIO.read(new FileInputStream("src/images/t1.png")),40,40,80,80,null);
                    break;
                case 6:
                    g.drawImage(ImageIO.read(new FileInputStream("src/images/t1.png")),40,40,80,80,null);
                    break;
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }