Why doesn't java.util.List implement Serializable?

Question

Why is it that java.util.List does not implement Serializable while subclasses like LinkedList, Arraylist do? Does not it seem to be against inheritance principles? For example if we want to send a Linkedlist over a network, we have to write:

new ObjectOutputStream(some inputStream).writeObject(some LinkedList); 

So far so good, but while reading the object on the other side we have to explicity say LinkedList l = (LinkedList)objectInputStream.readObject(); instead of List l = (List)objectInputStream.readObject();. If we were ever to change the writing functionality from LinkedList to say ArrayList, we will also have to change the reading part. Having List implement Serializable would have solved the problem.

Answer 1

List does not implement Serializable because is it not a key requirement for a list. There is no guarantee (or need) that every possible implementation of a List can be serialized.

LinkedList and ArrayList choose to do so, but that is specific to their implementation. Other List implementations may not be Serializable.

Answer 2

List is an interface and making it extend Serializable would mean that any implementation of List should be serializable.

The serializable property is not part of the List abstraction and should therefore not be required for an implementation.