private long calcFixedOverhead(List<Class<?>> classList) {
    long overhead = 0;
    for (Class<?> clazz : classList) {
      overhead += ClassSize.estimateBase(clazz, false);
    }
    return overhead;
  }
Testing the classes that implements HeapSize and are a part of 0.20. Some are not tested here
for example BlockIndex which is tested in TestHFile since it is a non public class
hbase-server/src/test/java/org/apache/hadoop/hbase/io/TestHeapSize.java