Why do you extend Serializable in Scala?

Question

I'm going through some Sapark projects in Scala, and I see that all of the objects are extending Serializable .

It's like :

object someName extends Serializable {
//some code
}

I understand that serialization is often used for storing or communicating data structures, so that the data can be easily loaded into memory in the original form, from the serialized form. However, in this case the object is more like a Java class. So, what's the point or advantage of extending Serializable ? When do you do this? Is it necessary to always do this?

Answer 1

You only actually need it when objects are used other than calling methods directly on them. E.g. something like

val x = someName

rdd.map { ... x ... }

or

object SomeName extends Serializable, SomeTrait { ... }

def f(x: SomeTrait) = ...

rdd.map { ... f(SomeName) ... }

For "global" objects not extending any classes/traits it's basically useless because no one ever does what the first snippet shows, but it doesn't hurt either. Note that Nil and None are examples of the second and they do extend Serializable (even without Spark).