Java lambda method and new Object

Question

I have the following code:

public class RefDemo {

    static class Demo implements Runnable {

        public Demo() {
            System.out.println(this.toString() + "-----");
        }

        @Override
        public void run() {
            System.out.println("run");
        }
    }

    public static void main(String[] args) {
        Runnable runnable = Demo::new; // lambda ref constructor method
        runnable.run(); // does not do anything
        System.out.println(runnable);
        Runnable demo = new Demo(); // using new to create a Demo instance
        demo.run();
        System.out.println(demo);
    }
}

Which prints:

RefDemo$Demo@7291c18f----- // lambda run constructor method print
RefDemo$$Lambda$1/793589513@34a245ab // main method print
RefDemo$Demo@7cc355be-----
run
RefDemo$Demo@7cc355be

I don't know why the code does not print run when calling runnable.run();

Why does that happen?

Answer 1

This code

Runnable runnable = Demo::new;

Is equivalent to this code

Runnable runnable = new Runnable() {
    @Override 
    public void run() {
        new Demo();
    }
};

So you are not referring to the run method of Demo but to the constructor.

Answer 2

You are simply using Runnable in too many places and confusing yourself. The following code makes it a bit clearer what is happening:

public class RefDemo {

    static class Demo {

        public Demo() {
            System.out.println(this.toString() + "-----");
        }

        public void something() {
            System.out.println("something");
        }
    }

    public static void main(String[] args) {
        Runnable runnable = Demo::new; 
        runnable.run();

        System.out.println(runnable);

        Demo demo = new Demo();
        demo.something();

        System.out.println(demo);
    }
}

Runnable runnable = Demo::new; means that you now have a reference to the constructor of Demo (which still works after dropping the conformance to the Runnable interface). And you store that reference in a variable of type Runnable which only works because their functional interfaces are compatible. Calling run on that reference then simply calls the constructor, not the run / something method of Demo.