這樣寫是出于兼容性考慮。

舊版本的JDK中，ArrayList的實現有所不同，會對length字段進行序列化。

而新版的JDK中，對優化了ArrayList的實現，不再序列化length字段。

這個時候，如果去掉s.writeInt(size)，那么新版本JDK序列化的對象，在舊版本中就無法正確讀取，

因為缺少了length字段。

因此這種寫法看起來多此一舉，實際上卻保證了兼容性。

附上官方解釋：defaultReadObject()?and?defaultWriteObject()?should?be?the?first?method?call?inside?readObject(ObjectInputStream?o)?and?writeObject(ObjectOutputStream?o).?It?reads?and?writes?all?the?non-transient?fields?of?the?class?respectively.?

These?methods?also?helps?in?backward?and?future?compatibility.?If?in?future?you?add?some?non-transient?field?to?the?class?and?you?are?trying?to?deserialize?it?by?the?older?version?of?class?then?the?defaultReadObject()?method?will?neglect?the?newly?added?field,?similarly?if?you?deserialize?the?old?serialized?object?by?the?new?version?then?the?new?non?transient?field?will?take?default?value?from?JVM?i.e.?if?its?object?then?null?else?if?primitive?then?boolean?to?false,?int?to?0?etc….