Use concatMap() to maintain order while performing "Auto-Complete Search"

ConcatumMapum.java

//Use concat-Map() to maintain order while performing "Auto-Complete Search"  
publishSubject
  .debounce(300, TimeUnit.MILLISECONDS)
  .concatMap(new Function<String, ObservableSource<List<Book>>>() {
      @Override
      public ObservableSource<List<Book>> apply(String s) throws Exception {
          Log.d(TAG, "getting books for " + s);
          return dataSource.getBook(s);
          }
      })
  .subscribeOn(provider.io())
  .observeOn(provider.ui())
  .subscribe(new Consumer<List<Book>>() {
      @Override
      public void accept(List<Book> books) throws Exception {
          onBooksFetched(books);
          }
      });