question: public final TestSubscriber < T > assertError ( ) { assertNotComplete ( ) ; int s = errors . size ( ) ; if ( s == 0 ) { throw new AssertionError ( "No error" , null ) ; } if ( s > 1 ) { throw new AssertionError ( "Multiple errors: " + s , null ) ; } return this ; }
target:  Assert an error signal has been received.