Benefit of Generic Constructors

Question

What is the benefit of having generic constructor for a non-generic class? Java spec permits the following:

class NonGeneric {
  <T> NonGeneric() { }
  ...
  NonGeneric ref = new <String> NonGeneric();
}

Can one come up with a realistic example of when it enhances the typesafety of the class? How would it be better than using Generic in the first place.

I understand that Java designers wanted the constructors to be more consistent with methods. Given that constructors can have side-effects, generic constructors can use generics to mutate some parameters whose references aren't retained, as in

<T> NonGeneric(T obj, List<T> list) {
  list.add(obj);
  // Don't hold a reference to list
}

Answer 1

The only use that I can think of would be if the constructor needed to use a generic object while it was running, but did not store that object once it was complete.

For example:

<T> NonGeneric(T[] blank, List<T> list) {
    // Sort that list
    T[] array = list.toArray(blank);
    Arrays.sort(array);

    // Pull out the values as strings
    this.list = new ArrayList<String>(array.length);
    for (T value : array) {
        this.list.add(value.toString());
    }
}

It is most likely just something that the language designers decided to do just in case somebody wanted it, since there was no reason to prevent people from doing it.