code: private static Key getFinalizeSlotFiller(Barrier finalizeBarrier) {
      Key fillerJobKey = null;
      for (SlotDescriptor slotDescriptor : finalizeBarrier.getWaitingOnInflated()) {
        Key key = slotDescriptor.slot.getSourceJobKey();
        if (null != key) {
          if (null == fillerJobKey) {
            fillerJobKey = key;
          } else {
            if (!fillerJobKey.toString().equals(key.toString())) {
              // Found 2 non-equal values, return null
              return null;
            }
          }
        }
      }
      return fillerJobKey;
    }
docs: Return the unique value of {@link Slot#getSourceJobKey()} for all slots in
  the finalize Barrier, if there is a unique such value. Otherwise return
  {@code null}.
  
  @param finalizeBarrier A finalize Barrier
  @return The unique slot filler for the finalize slots, or {@code null}