Pragmatic Unit Testing in Java with JUnit, 3rd Edition

Increment 2: Generalizing the Implementation

Madhu’s second batch of requirements for you is a batch of one: track the number of shares owned for a given symbol. A happy path test case:

utj3-tdd/16/src/test/java/app/APortfolio.java

@Test
void returnsSharesGivenSymbol() {
    portfolio.purchase("AAPL", 42);

    assertEquals(42, portfolio.sharesOf("AAPL"));
}

The tests to any point in time represent the set of assumptions you make. Currently, you are assuming there will only ever be a single purchase. As such, you can track the shares purchased using a single discrete field:

utj3-tdd/17/src/main/java/app/Portfolio.java

public class Portfolio {
    private Set symbols = new HashSet<String>();
    private int shares;

    // ...