Completed code:

public class Game
{
	public int score()
	{
		return scoreForFrame(itsCurrentFrame);
	}

	public void add(int pins)
	{
		itsScorer.addThrow(pins);
		adjustCurrentFrame(pins);
	}

	private void adjustCurrentFrame(int pins)
	{
    if (lastBallInFrame(pins))
      advanceFrame();
    else
      firstThrowInFrame = false;
	}

  private boolean lastBallInFrame(int pins)
  {
    return strike(pins) || !firstThrowInFrame;
  }

  private boolean strike(int pins)
  {
    return (firstThrowInFrame && pins == 10);
  }

  private void advanceFrame()
	{
		itsCurrentFrame = Math.min(10, itsCurrentFrame + 1);
	}

	public int scoreForFrame(int theFrame)
	{
		return itsScorer.scoreForFrame(theFrame);
	}

	private int itsCurrentFrame = 0;
	private boolean firstThrowInFrame = true;
	private Scorer itsScorer = new Scorer();
}

public class Scorer
{
	public void addThrow(int pins)
	{
		itsThrows[itsCurrentThrow++] = pins;
	}

	public int scoreForFrame(int theFrame)
	{
		ball = 0;
		int score=0;
		for (int currentFrame = 0;
		     currentFrame < theFrame;
			   currentFrame++)
		{
			if (strike())
			{
				score += 10 + nextTwoBallsForStrike();
        ball++;
			}
			else if ( spare() )
			{
				score += 10 + nextBallForSpare();
				ball+=2;
			}
			else
			{
				score += twoBallsInFrame();
				ball+=2;
			}
		}

		return score;
	}

	private boolean strike()
	{
		return itsThrows[ball] == 10;
	}

	private boolean spare()
	{
		return (itsThrows[ball] + itsThrows[ball+1]) == 10;
	}

	private int nextTwoBallsForStrike()
	{
		return itsThrows[ball+1] + itsThrows[ball+2];
	}

	private int nextBallForSpare()
	{
		return itsThrows[ball+2];
	}

	private int twoBallsInFrame()
	{
		return itsThrows[ball] + itsThrows[ball+1];
	}

	private int ball;
	private int[] itsThrows = new int[21];
	private int itsCurrentThrow = 0;
}

import junit.framework.*;

public class TestGame extends TestCase
{
	public TestGame(String name)
	{
		super(name);
	}

	private Game g;

	public void setUp()
	{
		g = new Game();
	}

	public void testTwoThrowsNoMark()
	{
		g.add(5);
		g.add(4);
		assertEquals(9, g.score());
	}

	public void testFourThrowsNoMark()
	{
		g.add(5);
		g.add(4);
		g.add(7);
		g.add(2);
		assertEquals(18, g.score());
		assertEquals(9,  g.scoreForFrame(1));
		assertEquals(18, g.scoreForFrame(2));
	}

	public void testSimpleSpare()
	{
		g.add(3);
		g.add(7);
		g.add(3);
		assertEquals(13, g.scoreForFrame(1));
	}

	public void testSimpleFrameAfterSpare()
	{
		g.add(3);
		g.add(7);
		g.add(3);
		g.add(2);
		assertEquals(13, g.scoreForFrame(1));
		assertEquals(18, g.scoreForFrame(2));
		assertEquals(18, g.score());
	}

	public void testSimpleStrike()
	{
		g.add(10);
		g.add(3);
		g.add(6);
		assertEquals(19, g.scoreForFrame(1));
		assertEquals(28, g.score());
	}

	public void testPerfectGame()
	{
		for (int i=0; i<12; i++)
		{
			g.add(10);
		}
		assertEquals(300, g.score());
	}

	public void testEndOfArray()
	{
		for (int i=0; i<9; i++)
		{
			g.add(0);
			g.add(0);
		}
		g.add(2);
		g.add(8); // 10th frame spare
		g.add(10); // Strike in last position of array.
		assertEquals(20, g.score());
	}

	public void testSampleGame()
	{
		g.add(1);
		g.add(4);
		g.add(4);
		g.add(5);
		g.add(6);
		g.add(4);
		g.add(5);
		g.add(5);
		g.add(10);
		g.add(0);
		g.add(1);
		g.add(7);
		g.add(3);
		g.add(6);
		g.add(4);
		g.add(10);
		g.add(2);
		g.add(8);
		g.add(6);
		assertEquals(133, g.score());
	}

	public void testHeartBreak()
	{
		for (int i=0; i<11; i++)
			g.add(10);
		g.add(9);
		assertEquals(299, g.score());
	}

	public void testTenthFrameSpare()
	{
		for (int i=0; i<9; i++)
			g.add(10);
		g.add(9);
		g.add(1);
		g.add(1);
		assertEquals(270, g.score());
	}
}