How do I create a class to inherit from a class that can either take an instance of itself or nothin...

# getting-started

[MarkRS]

How do I create a class to inherit from a class that can either take an instance of itself or nothing as an initialiser?

I had assumed

class childClass(child: childClass?) : parentClass(child) {
    constructor() : this( null)

but this breaks when calling it as childClass().
The parent class I'm trying to inherit from is a Java class, "Path" from android.graphics, but I assume that doesn't matter. I've checked that code and it claims to accept a null.

[Rob Elliot]

Do you mean this?

class ChildClass(child: ChildClass? = null) : ParentClass()

or this?

class ChildClass(child: ChildClass?) : ParentClass() {
    constructor(): this(null)
}

[Rob Elliot]

Or do you need to pass child to the ParentClass's constructor:

class ChildClass(child: ChildClass? = null) : ParentClass(child)

[ephemient]

only use primary constructors if every construction can use the same super constructor. doesn't sound like it's the case here, so you may need to

class Child : Parent {
    constructor() : super()
    constructor(p) : super(p)

[MarkRS]

Yes @Rob Elliot, a typo (now corrected) there might've given the wrong impression.
@ephemient, I'll try that now.

[Klitos Kyriacou]

What's wrong with your original code?

[MarkRS]

@Klitos Kyriacou It crashes when I try to create childClass without a parameter ( childClass()" )

[MarkRS]

Many thanks @ephemient, yes, that seems to do the trick!

[Klitos Kyriacou]

Your code works just fine: https://pl.kotl.in/6uAPiAVzB

[MarkRS]

@Klitos Kyriacou, hmmm. That play won't let me modify it by making android.graphics.Path the parent class, but that is my case, and it crashes. Perhaps Path isn't an open class and perhaps that makes the difference?

[Klitos Kyriacou]

Path is a non-final Java class, which means it's an open class. But I think the problem might be that calling Path(null) , albeit valid in itself, causes a crash somewhere down the line. If you want to create an empty Path then you need to call Path() which is very different to Path(null) .

[MarkRS]

The code (that the AS debugger showed me) for path has code to handle null as a parameter, however it crashes. Dunno why, it looks like it shouldn't. However, I can see you're right that Path() is different from Path(null). and @ephemient’s correction makes it work.