private int putForNullKey(int value) {
    for (Entry e = table[0]; e != null; e = e.next) {
      if (e.key == null) {
        int oldValue = e.value;
        e.value = value;
        e.recordAccess(this);
        return oldValue;
      }
    }
    modCount++;
    addEntry(0, null, value, 0);
    return 0;
  }
Offloaded version of put for null keys
geode-core/src/main/java/org/apache/geode/cache/query/internal/ObjectIntHashMap.java