public Collection<PlanCoordinator> coordinators() {
    return Collections.unmodifiableCollection(mCoordinators.values());
  }
@Test
  public void testAddAndPurge() throws Exception {
    assertEquals("tracker should be empty", 0, mTracker.coordinators().size());
    fillJobTracker(CAPACITY);
    try {
      addJob(100);
      fail("Should have failed to add a job over capacity");
    } catch (ResourceExhaustedException e) {
      // Empty on purpose
    }
    finishAllJobs();
    try {
      addJob(100);
    } catch (ResourceExhaustedException e) {
      fail("Should not have failed to add a job over capacity when all are finished");
    }
  }