Serializable and transient

Question

To make a class serializable we do the following:

class A implements Serializable {
    transient Object a;
}

And not this:

Answer 1

Serializable is a marker interface. Interfaces are a standard way (in Java and in some other languages) of indicating features of a class; an "is a" relaionship. Making Serializable an interface means we can declare methods that accept or return Serializables just like we can methods that work with other interfaces. Anything else would have required syntax changes to the language (at the time; now we have annotations, but I think an interface would still be used).