Passing Interfaces by Value
Consider the following definitions:
interface Time {
idempotent TimeOfDay getTime();
// ...
};
interface Record {
void addTimeStamp(Time t); // Note: Time t, not Time* t
// ...
};
Note that addTimeStamp accepts a parameter of type Time, not of type Time*. The question is, what does it mean to pass an interface by value? Obviously, at run time, we cannot pass an an actual interface to this operation because interfaces are abstract and cannot be instantiated. Neither can we pass a proxy to a Time object to addTimeStamp because a proxy cannot be passed where an interface is expected.
However, what we can pass to addTimeStamp is something that is not abstract and derives from the Time interface. For example, at run time, we could pass an instance of the TimeOfDay class we saw earlier. Because the TimeOfDay class derives from the Time interface, the class type is compatible with the formal parameter type Time and, at run time, what is sent over the wire to the server is the TimeOfDay class instance.