extends clause in Java generics

gistfile1.java

public static int sumInt(List<Integer> intList) {
  int sum = 0;
  for (int i : intList) {
    sum += i;
  }
  return sum;
}

public static double sumDouble(List<Double> doubleList) {
  double sum = 0;
  for (double d : doubleList) {
    sum += i;
  }
  return sum;
}

public static double sumFail(List<Number> numList) {
  double sum = 0;
  for (Number n : numList) {
    sum += n.doubleValue();
  }
  return sum;
}

public static int sumWork(List<? extends Number> numList) {
  double sum = 0;
  for (Number n : numList) {
    sum += n.doubleValue();
  }
  return sum;
}

public static void main(String[] a) {
  Random r = new Random();
  int size = r.nextInt(100) + 10;

  List<Double> doubles = new ArrayList<Double>(size);
  List<Number> numbers = new ArrayList<Numbers>(size);
  List<Integer> integers = new ArrayList<Integers>(size);

  // Fill arrays
  for (;size > 0;size--) {
    int i = r.nextInt();
    doubles.add(new Double(i));
    if (r.nextBoolean()) {
      numbers.add(new Double(i));
    } else {
      numbers.add(new Integer(i));
    }
    integers.add(new Integer(i));
  }

  // Work (not handy)
  int sumInt = sumInt(integers);
  double sumDouble = sumDouble(doubles);
  
  // Don't work, but more handy (one function for all types of number lists)
  double sumFailInt = sumFail(integers);
  double sumFailDouble = sumFail(doubles);
  // Works (as the parameters are matching)
  double sumFailWorkNumbers = sumFail(numbers);

  // Work and handy
  double sumWorkInt = sumWork(integers),
  double sumWorkDouble = sumWork(doubles)
  double sumWorkNumbers = sumWork(numbers);

  /* All sum* variables should be equals! (Maybe some difference due to inaccuracy of double) */
}