Google Data

Michael Feathers defines the qualities of a good unit test as: “they run fast, they help us localize problems.” This can be hard to accomplish when your code accesses a database, hits another server, is time-dependent, etc.

By substituting custom objects for some of your module's dependencies, you can thoroughly test your code, increase your coverage, and still run in less than a second. You can even simulate rare scenarios like database failures and test your error handling code.

A variety of different terms are used to refer to these “custom objects”. In an effort to clarify the vocabulary, Gerard Meszaros provides the following definitions:

• Test Double is a generic term for any test object that replaces a production object.
• Dummy objects are passed around but not actually used. They are usually fillers for parameter lists.
• Fakes have working implementations, but take some shortcut (e.g., InMemoryDatabase).
• Stubs provide canned answers to calls made during a test.
• Mocks have expectations which form a specification of the calls they do and do not receive.

For example, to test a simple method like getIdPrefix() in the IdGetter class:

public class IdGetter {  // Constructor omitted.
  public String getIdPrefix() {
    try {
      String s = db.selectString("select id from foo");
      return s.substring(0, 5);
    } catch (SQLException e) { return ""; }
  }
}

You could write:

  db.execute("create table foo (id varchar(40))");  // db created in setUp().
  db.execute("insert into foo (id) values ('hello world!')");
  IdGetter getter = new IdGetter(db);
  assertEquals("hello", getter.getIdPrefix());

The test above works but takes a relatively long time to run (network access), can be unreliable (db machine might be down), and makes it hard to test for errors. You can avoid these pitfalls by using stubs:

  public class StubDbThatReturnsId extends Database {
    public String selectString(String query) { return "hello world"; }
  }
  public class StubDbThatFails extends Database {
    public String selectString(String query) throws SQLException {
      throw new SQLException("Fake DB failure");
    }
  }
  public void testReturnsFirstFiveCharsOfId() throws Exception {
    IdGetter getter = new IdGetter(new StubDbThatReturnsId());
    assertEquals("hello", getter.getIdPrefix());
  }
  public void testReturnsEmptyStringIfIdNotFound() throws Exception {
    IdGetter getter = new IdGetter(new StubDbThatFails());
    assertEquals("", getter.getIdPrefix());
  }

Remember to download this episode of Testing on the Toilet and post it in your office.