code: private void endFrame() {
          if (previousFrame != null) { // do not write the first frame
              if (stackMap == null) {
                  stackMap = new ByteVector();
              }
              writeFrame();
              ++frameCount;
          }
          previousFrame = frame;
          frame = null;
      }
docs: Checks if the visit of the current frame {@link #frame} is finished, and
  if yes, write it in the StackMapTable attribute.