I have this code

val pdfDoc = PdfDocument(PdfWriter(fileName))

but the implementation for PdfDocument is

public PdfDocument(PdfWriter writer) {
        this(writer, new DocumentProperties());
    }

what is pdfDoc assigned to with no return value?

This is a constructor

I guess pdfDoc is the object of the class PdfDocument

Is this a Java code snippet, or a Kotlin code snippet?

This looks like Java

I guess it is kotlin code using java lib 🙂

Yup., I'd guess the Java looks like this:

public class PdfDocument {

    public PdfDocument(PdfWriter writer, DocumentProperties documentProperties) {
    }

    public PdfDocument(PdfWriter writer) {
        this(writer, new DocumentProperties());
    }
}

The equivalent Kotlin would be:

class PdfDocument(
  writer: PdfWriter, 
  properties: DocumentProperties = DocumentProperties(),
)

Or if you wanted it to be the basically the same byte code:

class PdfDocument(
  writer: PdfWriter,
  properties: DocumentProperties,
) {
  constructor(writer: PdfWriter): this(writer, DocumentProperties())
}

The way I read the question was that

public PdfDocument(PdfWriter writer)

doesn't seem to return anything - and that is true in the Kotlin world. But in the Java world that's the constructor syntax, so the "return" of that is a

PdfDocument

object

Wait, if you put a default value for an argument in the constructor it won't generate the same bytecode as two constructors with the second one using the default value for the argument ?

not quite, no

Why, and what would be the difference ?

without

@JvmOverloads

Kotlin generates a constructor taking every parameter, a bitmask of which parameters are present (or several, depending on how many arguments there are), and a DefaultConstructorMarker (for overload disambiguation)

Oh yes I remember now that it's doing that

very much not something that is intended to be used from Java

So by default it's not interoperable with Java

it's technically callable… but it involves too many low-level details that you shouldn't