question: @ Override public void removeHandshakeCompletedListener ( HandshakeCompletedListener listener ) { if ( listener == null ) { throw new IllegalArgumentException ( "Provided listener is null" ) ; } if ( listeners == null ) { throw new IllegalArgumentException ( "Provided listener is not registered" ) ; } if ( ! listeners . remove ( listener ) ) { throw new IllegalArgumentException ( "Provided listener is not registered" ) ; } }
target:  This method works according to the specification of implemented class.