code: public static AsyncOutputStream channelToOutputStream(final AsynchronousFileChannel asynchronousFileChannel) {
          notNull("asynchronousByteChannel", asynchronousFileChannel);
          return new AsyncOutputStream() {
              private int position = 0;
              @Override
              public void write(final ByteBuffer src, final SingleResultCallback<Integer> callback) {
                  asynchronousFileChannel.write(src, position, null, new CompletionHandler<Integer, Object>() {
                      @Override
                      public void completed(final Integer result, final Object attachment) {
                          position += result;
                          callback.onResult(result, null);
                      }
  
                      @Override
                      public void failed(final Throwable exc, final Object attachment) {
                          callback.onResult(null, exc);
                      }
                  });
              }
  
              @Override
              public void close(final SingleResultCallback<Void> callback) {
                  try {
                      asynchronousFileChannel.close();
                      callback.onResult(null, null);
                  } catch (IOException e) {
                      callback.onResult(null, e);
                  }
              }
          };
      }
docs: Converts a {@link AsynchronousFileChannel} into a {@link AsyncOutputStream}
  
  @param asynchronousFileChannel the AsynchronousFileChannel
  @return the AsyncOutputStream