@rrader It is useful for implementations of base classes or interfaces that do not have any state. You can use these objects as

null

objects in a lot of cases. For example:

object NoOperationMouseListener : MouseListener {
    ...
}

The mouse listener member of your class does not have to be nullable in this case:

class MouseEventEmittingClass {
    var mouseListener: MouseListener = NoOperationMouseListener
}

so, is this short hand for

class NoOperationMouseListener : MouseListener {
   ...
}
val noOperationMouseListener = NoOperationMouseListener()

class MouseEventEmittingClass {
   var mouseListener: MouseListener = noOperationMouseListener
}

?

Yes, it is a bit shorter, and communicates that it does not make sense to create multiple instances of this interface implementation more clearly