"Runnable::run" - How is this creating an Executor instance?

Question

I'm working on a project where the following line is used to create a test Executor member variable instance:

private Executor executor = Runnable::run;

The code runs and compiles but I don't understand how Runnable::run creates an instance of Executor since both are different interfaces.

Is anyone able to explain? In particular:

• Where does the implementation of Runnable come from?
• How is it assigned to an Executor implementation (since Executor is a different interface)?
• What kind of Executor is created? e.g. single threaded or pooled
• How would this be written before Java 8?

Thanks.

Answer 1

Executor is a @FunctionalInterface:

 public interface Executor {
     void execute(Runnable command);
 }

You can re-write it like this to actually understand it better probably:

 Executor executor = (Runnable r) -> r.run(); // or Runnable::run