code: public static Observable<String> stringConcat(Observable<String> src) {
          return toString(src.reduce(new StringBuilder(), new Func2<StringBuilder, String, StringBuilder>() {
              @Override
              public StringBuilder call(StringBuilder a, String b) {
                  return a.append(b);
              }
          }));
      }
docs: Gather up all of the strings in to one string to be able to use it as one message. Don't use
  this on infinite streams.
  <p>
  <img width="640" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/St.stringConcat.png" alt="">
  
  @param src
  @return the Observable returing all strings concatenated as a single string