Learning Java, 4th Edition

Arrays of Parameterized Types

There is one place where we haven’t yet considered how generic types affect the Java language: array types. After everything we’ve seen, it would seem natural to expect that arrays of generic types would come along for the ride. But as we’ll see, Java has a schizophrenic relationship with arrays of parameterized types.

The first thing we need to do is recall how arrays work for regular Java types. An array is a kind of built-in collection of some base type of element. Furthermore, array types (including all multidimensional variations of the array) are true types in the Java language and are represented at runtime by unique class types. This is where the trouble begins. Although arrays in Java act a lot like generic collections (they change their APIs to adopt a particular type for “reading” and “writing”), they do not behave like Java generics with respect to their type relationships. As we saw in Chapter 6, arrays exist in the Java class hierarchy stemming from Object and extending down parallel branches with the plain Java objects.

Arrays are covariant subtypes of other types of arrays, which means that, unlike concrete generic types, although they change their method signatures, they are still related to their parents. This means that Strings [] in Java is a subtype of Object []. This brings up the aliasing problem that we mentioned earlier. An array of Strings can be aliased as an array of Objects and we can attempt to put things into it illegally ...