question: public Set < ByteString > keys ( ByteString namespace ) { if ( namespace != null ) { SetMultimap < ByteString , ByteString > namespaceData = contents . get ( namespace ) ; if ( namespaceData != null ) { return namespaceData . keySet ( ) ; } } return Collections . emptySet ( ) ; }
target:  Returns a set view of the keys in the specified namespace that have 1 or more values assigned