Java unbounded wildcards

From Effective Java, 2nd edition:

If a type parameter appears only once in a method declaration, replace it with a wildcard.

For example, given a static method swapping 2 items in a list, prefer

1	public static void swap(List<?> list, int i, int j);

over

1	public static <E> void swap(List<E> list, int i, int j);

However, this implementation won't compile:

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

because you can’t put any value except null into a List<?>. Instead, we will have to use:

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public static void swap(List<?> list, int i, int j) {     swapHelper(list, i, j); } // Private helper method for wildcard capture private static <E> void swapHelper(List<E> list, int i, int j) {     list.set(i, list.set(j, list.get(i))); }