Anonymous initialization - strange serialization warning [duplicate]

Question

I was wondering why when I use an anonymous instanciation along with an instance initializer block, I get a "serializable class does not declare a static final serialVersionUID field of type long" compile-time warning.

Here's what I mean. Let's say I want to instantiate an ArrayList and at the same time add something to it like so:

ArrayList<Object> arrayList = new ArrayList<Object>(){{add(new Object());}}; 

If I compile this all is ok but I get the serialVersionUID field missing warning. Now ArrayList already implements serializable and has a private static final long serialVersionUID so why is it that when I use it like that it seems that that field "dissapears" and I get a warning for not having it declared?

Answer 1

When you create your anonymous class, you are actually extending ArrayList and therefore, inheriting the Serializable interface.

All Serializable classes are supposed to have a serialVersionUID so that you can distinguish between different serialized versions of the classes. Since the anonymous type is a new class, it would be a good idea to give it an ID so you can distinguish between different versions of it.

Answer 2

Because you're creating what's essentially a subclass. Such a subclass needs its own serial version UID. Same thing happens when you subclass things like JPanel. It's not a terrible problem if you don't require (de)serialization.

Answer 3

new ArrayList<Object>() {

    {
        add(new Object());
    }

};

You are not just instantiating but first defining a subclass (anonymous) of ArrayList and then instantiating the subclass.

Even though there is a private static final long serialVersionUID in ArrayList, since it's static, its not inherited by your anonymous subclass. So it's missing that field.