The Nature of Asynchronous Method Calls

The nature of asynchronous method calls requires the caller to pass in a callback object that can be notified when an asynchronous call completes, since by definition the caller cannot be blocked until the call completes. For the same reason, asynchronous methods do not have return types they must always return void. After an asynchronous call is made, all communication back to the caller is via the passed-in callback object.