Implement null object pattern

I found this question which implements a null object pattern in Kotlin in a certain way. I usually do it a little different in Java:

class MyClass {
    static final MyClass INVALID = new MyClass();

    public MyClass() {
       // empty
    }
}

in this way, I can always use MyClass.INVALID as the null object.

How can I achieve this style in Kotlin?

I fiddled with something like this:

data class MyClass(val id: Int) {
    object INVALID: MyClass {
    }
}

but that doesn't even compile.

Solution

One way you could achieve this is by using a companion object. Because the members of the companion object can be called by using simply the class name as the qualifier. You could do;

data class MyClass(val id: Int) {
    companion object {
        @JvmStatic
        val nullInstance = MyClass(0)  //0 or any intended value
    }
}

//invocation
 val a = MyClass.nullInstance
 val b = MyClass.nullInstance
 print(a == b) //prints true because these are equavalent to Java's static instances.

Here I have annotated that nullInstance as @JvmStatic to have it generated as a real static member.

https://kotlinlang.org/docs/reference/object-declarations.html