code: @Override
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
      super.readExternal(in);
      isLeaf = in.readBoolean();
      numEntries = in.readInt();
      // Entries will be read in subclasses
    }
docs: Reads the id of this node, the numEntries and the entries array from the
  specified stream.
  
  @param in the stream to read data from in order to restore the object
  @throws java.io.IOException if I/O errors occur
  @throws ClassNotFoundException If the class for an object being restored
  cannot be found.