In http://docs.oracle.com/javase/tutorial/java/generics/capture.html

There is no proposed solution (that involves a helper method) for

public class WildcardErrorBad {

    void swapFirst(List<? extends Number> l1, List<? extends Number> l2) {
      Number temp = l1.get(0);
      l1.set(0, l2.get(0)); // expected a CAP#1 extends Number,
                            // got a CAP#2 extends Number;
                            // same bound, but different types
      l2.set(0, temp); // expected a CAP#1 extends Number,
                            // got a Number
    }
}


Reader feedback suggested rewriting swapFirst to the following as a solution:

public <T extends Number> void swapFirst(List<T> l1, List<T> l2){
   
    T temp = l1.get(0);
    l1.set(0,l2.get(0));
    l2.set(0, temp);
}