Why it is allowable to create Thread instance with NON Runnable argument?

Question

I encountered code like this(I simplidied it a bit) in the article:

public class Main {


    static class A {        
    }

    public static void main(String[] args) {
        new Thread(A::new).start();
    }
}

I surprised about that code because from my point if view it must produce compile time error because Thread constructor accepts Runnable but A doesn't have methid run but it compiles and even starts without any errors/exceptions. I checked it on my PC in several variations and it works anyway.

So I have following questions:

Why there no compilation errors?
Which method executes instead of run method?

Answer 1

A Runnable is a FunctionalInterface which can also be represented in lambda expression as in your case:

new Thread(() -> new A())

which is nothing but a similar representation of method-reference

A::new

that in your code is equivalent to

new Runnable() {
    @Override
    public void run() {
        new A(); // just creating an instance of 'A' for every call to 'run'
    }
}