ExecutorService is an interface in the JDK. An implementation of the interface can execute a Runnable or Callable class in an asynchronous way. The interface only defines the API for the implementation and does not require that the invocation is asynchronous but, in reality, that is the main point implementing such a service. Invoking the run method of a Runnable interface in a synchronous way is simply calling a method. We do not need a special class for that.

The Runnable interface defines one run method. It has no arguments returns no value and does not throw any exception. The Callable interface is parameterized and the only method it defines, call, has no argument but returns a generic value and may also throw Exception. In our code, we will implement Runnable if we just want to run something, and Callable when we want to return something. Both of these interfaces are functional interfaces, therefore, they are good candidates to be implemented using lambda.

To have an...