code: public final Observable<Integer> sumInteger(Func1<? super T, Integer> valueExtractor) {
          return OperatorSum.sumAtLeastOneIntegers(o.map(valueExtractor));
      }
docs: Returns an Observable that extracts an Integer from each of the items emitted by the source Observable
  via a function you specify, and then emits the sum of these Integers.
  <p>
  <img width="640" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/sum.f.png" alt="">
  
  @param valueExtractor
  the function to extract an Integer from each item emitted by the source Observable
  @return an Observable that emits the Integer sum of the Integer values corresponding to the items emitted
  by the source Observable as transformed by the provided function
  @see <a href="https://github.com/ReactiveX/RxJava/wiki/Mathematical-and-Aggregate-Operators#wiki-suminteger-sumlong-sumfloat-and-sumdouble">RxJava Wiki: sumInteger()</a>
  @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.sum.aspx">MSDN: Observable.Sum</a>