code: public static <K, R> Observable<R> switchCase(Func0<? extends K> caseSelector,
              Map<? super K, ? extends Observable<? extends R>> mapOfCases) {
          return switchCase(caseSelector, mapOfCases, Observable.<R> empty());
      }
docs: Return a particular one of several possible Observables based on a case
  selector.
  <p>
  <img width="640" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/switchCase.png" alt="">
  
  @param <K>
  the case key type
  @param <R>
  the result value type
  @param caseSelector
  the function that produces a case key when an
  Observer subscribes
  @param mapOfCases
  a map that maps a case key to an Observable
  @return a particular Observable chosen by key from the map of
  Observables, or an empty Observable if no Observable matches the
  key