private void init() {
      assert directSize == -1;
      directSize = memoryModel.object();
    }
Creates size estimators for java objects. The estimators attempt to do most of the reflection
work at initialization time, and also take some shortcuts, to minimize the amount of work done
during the actual estimation.
TODO: clean up
storage-api/src/java/org/apache/hadoop/hive/ql/util/IncrementalObjectSizeEstimator.java