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