question: public static long stop ( @ NonNull String key ) { long now = System . currentTimeMillis ( ) ; Map < String , Long > currentMap = map . get ( ) ; Optional < Long > maybePreviousInstant = Optional . with ( currentMap . get ( key ) ) ; if ( maybePreviousInstant . isEmpty ( ) ) { Log . warn ( Timer . class , "There is no timer started for key '%s'. It might have already been stopped!" , key ) ; return 0 ; } long duration = now - maybePreviousInstant . get ( ) ; currentMap . remove ( key ) ; map . set ( currentMap ) ; return duration ; }
target:  Stops the timer for specified key