Is there any way to extend an object?

In scala, we cannot extend object:

object X 
object Y extends X

gives an error error: not found: type X

In my case someone has defined some functionality in an object and I need to extend it (basically add another method). What would be the easiest way to extend this object?

As so often the correct answer depends on the actual business requirement. Extending from an object would in some sense defy the purpose of that object since it wouldn't be a singleton any longer.

What might be a solution is to extract the behavior into an abstract trait. And create objects extending that trait like so:

trait T{
    // some behavior goes here
}

object X extends T

object Y extends T {
    // additional stuff here
}

If you want use methods and values from another object you can use import.

object X{
  def x = 5
}

object Y{
  import X._
  val y = x
}

You can't actually extend an object, because that would create two of it, and an object by definition exists only once (edit: well, that's not quite true, because the object definition can be in a class or method).

For your purposes, try this:

object X {
}

object Y {
    def a = 5
}

implicit def xToY(x: X.type) = Y

println(X.a)

It doesn't actually extend, but it does allow you to call new methods on it than were originally defined.

The only way to share code between two objects is by having one or more common superclass/trait.

Note that starting in Scala 3, you can alternatively use composition (instead of inheritance) via export clauses which allow defining aliases for selected members of an object:

object X { def f = 5 }

object Y {
  export X._
  def g = 42
  def h = f * g
}

Y.f // 5
Y.g // 42
Y.h // 210

Note that you can also restrict which members you want to export:

object X { def f = 5; def g = 6 }
object Y { export X.f }
Y.f // 5
Y.g
^^^
value g is not a member of Y

You can convert parent into class + companion object, and then have child extend class E.g.

in Parent.scala

class Parent {}

object Parent extends Parent {}

And then in Child.scala

object Child extends Parent {}

Yes, it's more a hack than a solution.