@Override
    public void putAll(Map<? extends K, ? extends V> m) {
      for (Entry<? extends K, ? extends V> e : m.entrySet()) {
        put(e.getKey(), e.getValue());
      }
    }
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