private int sizeOf(Object obj) {
    int size = 0;
    if (obj == null) {
      return size;
    }
    if (obj instanceof String) {
      size = ObjectSizer.DEFAULT.sizeof(obj);
    } else if (obj instanceof Integer) {
      size = 4; // estimate
    } else if (obj instanceof Long) {
      size = 8; // estimate
    } else {
      size = CachedDeserializableFactory.calcMemSize(obj) - Sizeable.PER_OBJECT_OVERHEAD;
    }
    return size;
  }
Return the EventSequenceID of the Event
geode-core/src/main/java/org/apache/geode/internal/cache/wan/GatewaySenderEventImpl.java