Syntax of secondary constructors in Kotlin - kotlin

Question

I've got this code-snippet. It shall demonstrate the order, in which constructors are executed:
fun main(args: Array<String>) {
Sample("T","U")
}
class Sample(private var s : String) {
constructor(t: String, u: String) : this(t) { // I don't get what "this(t)" is!
this.s += u
}
init {
s += "B"
}
}
What's the ": this(t)" in the declaration of the secondary constructor?
That isn't the return-type? Isn't it?

Answer 1

In this particular case this is a keyword that delegates to the primary constructor. It is mandatory in Kotlin when your class has several ones.
The Java equivalent would be:
class Simple {
private String s;
public Simple(String s) { // Here is your primary constructor
this.s = s;
}
public Simple(String t, String u) { // Here is your secondary constructor
this(t);
this.s += u;
}
{
s += "B"; // Here is the init block
}
}

Answer 2

With
this(t)
you call the primary constructor and passes t as an argument for s.
Instead you could even write something like this:
this(s = "a")
Then you set s to "a".
So the order is: Primary constructor, init, secondary constructor. For more detail: https://kotlinlang.org/docs/reference/classes.html

Answer 3

In addition to the answers. This is a required invocation to the default primary constructor:
class Sample(private var s: String) { }
Like in Java:
public Sample(String s) {
}
public Sample(String t, String u) {
this(t); // invoke constructor Sample(String s)
}
To avoid this invocation, you can write like this:
class Sample {
private var s = ""
constructor(t: String) {
s = ...
}
constructor(t: String, u: String) {
s = ...
}
}