ScoreBox.java

class ScoreBox
{
    /**
     * 倒したピンの数(-1は未入力)
    */
    private int pins;
    private boolean isFirstBox;

    public ScoreBox(boolean isFirstBox)
    {
        this.pins = -1;
        this.isFirstBox = isFirstBox;
    }

    public int getPins()
    {
        return this.pins;
    }
    public void setPins(int pins)
    {
        this.pins = pins;
    }
}

ScoreFrame.java

class ScoreFrame
{
    private boolean isFinalFrame;
    private ScoreFrame previousFrame;
    private ScoreBox[] boxes = new ScoreBox[3];
    private int currentBox = 0;

    private int nextPins = 0;
    private int nextnextPins = 0;

    public int getScore()
    {
        int total = 0;
        for(int i=0; i<2; i++){
            int pins = boxes[i].getPins();
            if(pins != -1)
                total += pins;
        }
        // スペア
        if(total == 10){
            if(nextPins == -1){
                return -1;
            }
            else{
                total += nextPins;
            }
        }
        // ストライク
        if(boxes[0].getPins() == 10){
            if(nextnextPins == -1){
                return -1;
            }
            else{
                total += nextnextPins;
            }
        }
        return total;
    }

    public void setPins(int pins)
    {
        boxes[currentBox].setPins(pins);
        currentBox++;

        // 前のフレームに得点の変化を通知
        previousFrame.scoreChanged(this);
    }

    public void scoreChanged(ScoreFrame nextFrame)
    {
        this.nextPins = next;
        this.nextnextPins = nextnext;
        //自身の表示を更新する
    }
}