Bowling DDDified

Game.java

package de.munich.softwerkskammer;

import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;

public class Game {
    private Deque<Frame> frames = new LinkedList<Frame>();
    private List<Roll> rolls = new ArrayList<Roll>();

    public Game() {
        frames.push(new Frame(0));
    }

    public void roll(int pins) {

        Roll roll = new Roll(pins);

        if (rolls.size() >= 21) throw new RuntimeException("game is over, stop rolling");
        rolls.add(roll);

        if (frames.peek().isComplete() && frames.size() < 10) {
            frames.push(new Frame(rolls.size()-1));
        }

        frames.peek().addRoll(roll);
    }

    public int score() {
        int score = 0;
        for (Frame f : frames) {
            if (f.isStrike()) score += 10 + strikeBonus(f);
            else if (f.isSpare()) score += 10 + spareBonus(f);
            else score += f.sumOfBallsInFrame();
        }
        return score;
    }


    private int strikeBonus(Frame frame) {
        return scoreOf(frame.rollIndex+1) + scoreOf(frame.rollIndex+1);
    }

    private int scoreOf(int roll) {
        if (rolls.size() <= roll) return 0;
        else return rolls.get(roll).pins;
    }

    private int spareBonus(Frame frame) {
        return scoreOf(frame.rollIndex + 2);
    }

    private int sumOfBallsInFrame(Frame frame) {
        return scoreOf(frame.rollIndex) + scoreOf(frame.rollIndex + 1);
    }

    static class Frame {
        private List<Roll> rolls = new ArrayList<Roll>();
        public final int rollIndex;

        Frame(int rollIndex) {
            this.rollIndex = rollIndex;
        }

        void addRoll(Roll roll) {
            rolls.add(roll);
        }

        boolean isComplete() {
            return rolls.size() == 2 || isStrike();
        }

        public boolean isStrike() { return rolls.size() == 1 && rolls.get(0).isStrike(); }
        public boolean isSpare() { return rolls.size() == 2 && sumOfBallsInFrame() == 10; }
        public int sumOfBallsInFrame() {
            int sum = 0;
            for (Roll r : rolls)
                sum += r.pins;
            return sum;
        }
    }

    static class Roll {
        public final int pins;

        Roll(int pins) {
            this.pins = pins;
        }

        public boolean isStrike() {
            return pins == 10;
        }
    }

}