How do arrays “remember” their types in Java?

Question

Consider the following code:

class AA { }

class BB extends AA { }

public class Testing {

    public static void main(String[] args) {
        BB[] arr = new B         


        

Answer 1

1. The type information for arrays, unlike for generics, is stored at runtime. This has been part of Java since the beginning of it. At runtime, a AA[] can be distinguished from a BB[], because the JVM knows their types.

2. An ArrayList (and the rest of the Collections framework) uses generics, which is subject to type erasure. At runtime, the generic type parameter is not available, so an ArrayList is indistinguishable from an ArrayList; they are both just ArrayLists to the JVM.

3. The compiler only knows that arr2 is a AA[]. If you have a AA[], the compiler can only assume that it can store an AA. The compiler will not detect a type safety issue in that you are placing an AA in what's really a BB[] there, because it only sees the AA[] reference. Unlike generics, Java arrays are covariant, in that a BB[] is an AA[] because a BB is an AA. But that introduces the possibility of what you just demonstrated - an ArrayStoreException, because the object referred to by arr2 is really a BB[], which will not handle an AA as an element.