Learning Java, 4th Edition

Parameterized Type Relationships

We know now that parameterized types share a common, raw type. This is why our parameterized List<Date> is just a List at runtime. In fact, we can assign any instantiation of List to the raw type if we want:

    List list = new ArrayList<Date>();

We can even go the other way and assign a raw type to a specific instantiation of the generic type:

    List<Date> dates = new ArrayList(); // unchecked warning

This statement generates an unchecked warning on the assignment, but thereafter the compiler trusts that the list contained only Dates prior to the assignment. It is also permissible, albeit pointless, to perform a cast in this statement. We’ll talk about casting to generic types a bit later.

Whatever the runtime types, the compiler is running the show and does not let us assign things that are clearly incompatible:

    List<Date> dates = new ArrayList<String>(); // Compile-time Error!

Of course, the ArrayList<String> does not implement the methods of List<Date> conjured by the compiler, so these types are incompatible.

But what about more interesting type relationships? The List interface, for example, is a subtype of the more general Collection interface. Is a particular instantiation of the generic List also assignable to some instantiation of the generic Collection? Does it depend on the type parameters and their relationships? Clearly, a List<Date> is not a Collection<String>. But is a List<Date> a Collection<Date>? Can a List<Date> be a Collection<Object> ...