Thread: Interfaces Executor and ExecutorService

From my understanding, interfaces aren't concrete and hence can't be instantiated.
You must implement every method of an interface you implemet or declare the class an absract one.
My nightmare is with the Executor servuce interfcae used for multithrreading. I understand clearly that to excute a task concurrently, it must implemnt interface Runable, ( in the process override the run() method). And and object of type Executorservice must be create and method execute called on the runnable object.

An Executor service is and Executor being a sub interface of Executor so how and why does the compiler allow an object to be created from an interface? And

Why don't we over ride the execute() method of interface Executor since this an abstract one with no default implementation, aren't we supposed to provide implemetation details for this abstract method?