void clear() {
      if (count != 0) {
        lock();
        try {
          HashEntry<V>[] tab = table;
          for (int i = 0; i < tab.length; i++) {
            tab[i] = null;
          }
          ++modCount;
          count = 0; // write-volatile
        } finally {
          unlock();
        }
      }
    }
Remove; match on key only if value null, else match both.
geode-core/src/main/java/org/apache/geode/internal/ObjIdConcurrentMap.java