Why does this heap sort method work with <T> but not with a wildcard <?>?

Alexander Dragnev :

I have implemented it with the Priority queue because of the task. My question is why it is working like this:

public <T> void heapSort(List<T> elements){
    PriorityQueue<T> q = new PriorityQueue<>(elements);

    elements.clear();
    while (!q.isEmpty()){
        elements.add(q.poll());
    }
}

But it stops working when I change it with a wildcard?

public void heapSort(List<?> elements){
    PriorityQueue<?> q = new PriorityQueue<>(elements);

    elements.clear();
    while (!q.isEmpty()){
        elements.add(q.poll());
    }
}

Andy Turner :

Please, someone, explain to me why?

In general, you can't add elements from a PriorityQueue<?> into a List<?>, because the queue might be a PriorityQueue<String> and the list might be a List<Integer>.

The compiler doesn't "remember" that the type parameter is the same in both cases, unless you create a type variable to tell it they are the same.

and how to fix it.

You know how to fix it: use a type variable, as in the first code example.

If you mean how not to have the type variable in the method signature, you can delegate to a private method:

public void heapSort(List<?> elements) {
  heapSortPrivate(elements);
}

private <T> void heapSortPrivate(List<T> elements){

    PriorityQueue<T> q = new PriorityQueue<>(elements);
    elements.clear();
    while (!q.isEmpty()){
        elements.add(q.poll());
    }
}