public interface StockExchange {
    Money currentPrice(String symbol);
}

public class PortfolioTest {
    private FixedStockExchangeStub exchange;
    private Portfolio portfolio;
    @Before
    protected void setUp() throws Exception {
        exchange = new FixedStockExchangeStub();
        exchange.fix("MSFT", 100);
        portfolio = new Portfolio(exchange);
    }
    @Test
    public void GivenFiveMSFTTotalShouldBe500() throws Exception {
        portfolio.add(5, "MSFT");
        Assert.assertEquals(500, portfolio.value());
    }
}