In what ways are subtypes different from subclasses in usage?

Question

A subtype is established when a class is linked by means of extending or implementing. Subtypes are also used for generics.

How can I differentiate subtyping from subclasses?

Answer 1

In Java, subclassing is a kind of subtyping.

There are a number of ways Java allows subtyping:

1. When class A extends B, A is a subtype of B because B b = new A(...); is ok.
2. When interface A extends B, A is a subtype of B because B b = new A() { ... } is ok.
3. When class A extends B, A[] is a subtype of B[] because B[] b = new A[0] is ok.
4. When class A implements B, A is a subtype of B because B b = new A(...) is ok.

It sounds like you want a way to distinguish one from the others. The below should do that.

static boolean isSubclass(Class<?> a, Class<?> b) {
  return !b.isArray() && !b.isInterface() && b.isAssignableFrom(a);
}

It won't handle subtyping of generic classes due to type erasure though. Class instances don't carry type parameters at runtime so there is no way to distinguish the runtime type of a new ArrayList<String>() from a new ArrayList<Integer>().