question: get the only instance . concode_field_sep Map<IDiagramGraphicalViewer,RequirementDropListener> viewersAndDropListeners concode_elem_sep Map<AbstractRequirementPage,List<IDiagramGraphicalViewer>> requirementPageAndViewers concode_elem_sep DropListenersManager instance concode_field_sep void removeListeners concode_elem_sep boolean registerDropListener
target: DropListenersManager function ( ) { return SingletonHolder . instance ; }