Java Reflection, why are the supertype type arguments preserved at runtime

Question

So java has type erasure, which deletes the type arguments (no generic types at runtime). Can someone explain why the supertype type arguments are preserved then?

class StringList extends ArrayList<String> {
    public Type getType() {

        Type[] typeArguments = ((ParameterizedType) this.getClass()
                .getGenericSuperclass()).getActualTypeArguments();
        return typeArguments[0];
    }
}

What this means basically is:

new ArrayList<String>()// I cannot use reflection to find out the generified type String

but

new StringList()//I can use reflection to see the generic type String

So type erasure is for backwards compatibility, but why are the supertype type arguments preserved then? Is the metadata updated to support it?

Why the one and not the other?

Answer 1

It's done for compile time type safety. If the super class of StringList was erased to ArrayList, there would be no way to protect against

StringList stringList = new StringList();
stringList.add(new Integer(123456));

If the type remains in the .class file for StringList, the compiler can correctly refuse the source code above since new Integer(123456) is not of type String expected by ArrayList<String>#add(String).