code: public TestRunFixture getFixture(String runId) {
          if (runId.isEmpty() && this.fixtures.size() == 1) {
              runId = this.fixtures.keySet().iterator().next();
          }
          return fixtures.get(runId);
      }
docs: Gets the fixture for the specified test run. If runId is an empty String
  and only one fixture exists this is returned.
  
  @param runId
  The test run identifier (may be an empty String).
  @return A TestRunFixture, or {@code null } if a matching one cannot be
  found.