@Override
    public V get(Object key) {
      V v0 = comparedMap.get(key);
      if (v0 != null) {
        return v0;
      }
      return identityMap.get(key);
    }
Specialized class which can compare by identity or value; based on the key type.
ql/src/java/org/apache/hadoop/hive/ql/plan/mapper/PlanMapper.java