Runnable::new vs new Runnable()

Question

Why doesn't the first of the following examples work?

• run(R::new); method R.run is not called.
• run(new R()); method R.run is called.

Both examples are compiled-able.

public class ConstructorRefVsNew {

  public static void main(String[] args) {
      new ConstructorRefVsNew().run(R::new);
      System.out.println("-----------------------");
      new ConstructorRefVsNew().run(new R());
  }

  void run(Runnable r) {
      r.run();
  }

  static class R implements Runnable {

      R() {
          System.out.println("R constructor runs");
      }

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

The output is:

  R constructor runs
  -----------------------
  R constructor runs
  R.run runs

In the first example, the R constructor is called, it returns lambda (which is not object):

But then how is it possible, that the example is compiled successfully?

Answer 1

Your run method takes a Runnable instance, and that explains why run(new R()) works with the R implementation.

R::new is not equivalent to new R(). It can fit the signature of a Supplier<Runnable> (or similar functional interfaces), but R::new cannot be used as a Runnable implemented with your R class.

A version of your run method that can takeR::new could look like this (but this would be unnecessarily complex):

void run(Supplier<Runnable> r) {
    r.get().run();
}

Why does it compile?

Because the compiler can make a Runnable out of the constructor call, and that would be equivalent to this lambda expression version:

new ConstructorRefVsNew().run(() -> {
    new R(); //discarded result, but this is the run() body
});

The same applies to these statements:

Runnable runnable = () -> new R();
new ConstructorRefVsNew().run(runnable);
Runnable runnable2 = R::new;
new ConstructorRefVsNew().run(runnable2);

But, as you can notice, the Runnable created with R::new does just call new R() in its run method body.

---

A valid use of a method reference to execute R#run could use an instance, like this (but you'd surely rather use the r instance directly, in this case):

R r = new R();
new ConstructorRefVsNew().run(r::run);

Answer 2

The first example:

new ConstructorRefVsNew().run(R::new);

is more or less equivalent to:

new ConstructorRefVsNew().run( () -> {new R();} );

The effect is you just create an instance of R but do not call its run method.