### Question:
WalkGraphOp extends AbstractGeoGigOp<Void> { public WalkGraphOp setListener(final Listener listener) { this.listener = listener; return this; }  WalkGraphOp setListener(final Listener listener); WalkGraphOp setReference(final String reference);  }### Answer:
@Test public void testRefNotProvided() { Exception e = assertThrows(IllegalStateException.class, repo.command(WalkGraphOp.class).setListener(new AccumulatingListener())::call); assertThat(e.getMessage(), containsString("Reference not provided")); }