Why does AtomicInteger implements Serializable

Question

Accoriding to javadoc,

public class AtomicInteger extends Number implements java.io.Serializable {

// code for class

}

But,

public abstract class Number implements java.io.Serializable {
//code for class
}

If Number class already implements java.io.Serializable then why do AtomicInteger implements it again?

Edit: Does Serializable being a marker interface makes any difference in this context?

Answer 1

Just to document it more clearly. Same situation with the abstract collection base classes.

Could also have been a mistake initially (that is now carried forward for consistency's sake).

Answer 2

There is no necessity as such, it is just for the sake of documentation.

Answer 3

It is a good practice, and more for readable purpose. The same case with HttpServlet and GenericServlet. There are lot of other implementations in java sdk which follows this.

Answer 4

Does Serializable being a marker interface makes any difference in this context?

No difference at all. There is nothing special about a marker interface at the linguistic level.