Restrictions on Generics in java and reflection

Question

I was reading this about generics and their restrictions.

Cannot Create Instances of Type Parameters

You cannot create an instance of a type parameter. For example, the following code causes a compile-time error:

public static <E> void append(List<E> list) {
    E elem = new E();  // compile-time error
    list.add(elem);
}

As a workaround, you can create an object of a type parameter through reflection:

public static <E> void append(List<E> list, Class<E> cls) throws Exception {
    E elem = cls.newInstance();   // OK
    list.add(elem);
}

You can invoke the append method as follows:

List<String> ls = new ArrayList<>();
append(ls, String.class);

I am somewhat confused. I understand why it isn't allowed because that new E() will just turn into new object() because of type erasure, but why doesn't the same thing happen in class.newInstance? I mean it uses the type parameter E, so wouldn't it end up with the same problem? Could someone please explain (I know I have to add the try-catch to get it to compile).

Thanks in advance!

Answer 1

As you have noted, the type parameter E is just something used by the compiler, it's not instantiated with the generic and cannot be used at run-rime. The parameter cls, on the other hand, is a real thing. It is an object passed in at run-time that has all its type information there to be reflected upon. The E in cls's type (recall Class<E> cls) will be type-erased (after all, Class is just another generic) but the object cls will carry on the information about its type.