Generic collection as a Java method argument

Question

Is there any way to make this work in Java?

 public static void change(List<? extends Object> list, int pos1, int pos2) {
  Object obj = list.get(pos1);
  list.set(pos1, list.get(pos2));
  list.set(pos2, obj);
 }

The only way I've successfully avoided warnings and errors is this:

 public static <T> T change(List<T> list, int pos1, int pos2) {
  T obj = list.get(pos1);
  list.set(pos1, list.get(pos2));
  list.set(pos2, obj);
  return obj;
 }

but I don't like to be forced to return a value.

Answer 1

Make a generic method

public static <T> void change( List<T> list, int position1, int position2) {
    T obj = list.get(position1);
    list.set(position1, list.get(position2));
    list.set(position2, obj);
}

Answer 2

In java.util.Collections, there's already a swap(List<?> list, int i, int j).

This is how it's implemented in OpenJDK source code:

public static void swap(List<?> list, int i, int j) {
   final List l = list;
   l.set(i, l.set(j, l.get(i)));
}

Interestingly, it uses a raw List type to do the job. The code generates warnings at compile time, but presumably @author Josh Bloch and Neal Gafter think this is tolerable.