question: @ SuppressWarnings ( "unchecked" ) public < T extends SubSystem > T addSystemIfAbsent ( Class < ? > systemApi , T system ) { SubSystem oldService = _systemMap . putIfAbsent ( systemApi , system ) ; if ( oldService != null ) { return ( T ) oldService ; } _pendingStart . add ( system ) ; if ( _lifecycle . isActive ( ) ) { startSystems ( ) ; } return null ; }
target:  Adds a new system.