Can I convert the following code to use generics?

Question

I'm converting an application to use Java 1.5 and have found the following method:

  /**
   * Compare two Comparables, treat nulls as -infinity.
   * @param o1
   * @param o2
   * @return -1 if o1<o2, 0 if o1==o2, 1 if o1>o2
   */
  protected static int nullCompare(Comparable o1, Comparable o2) {
    if (o1 == null) {
      if (o2 == null) {
        return 0;
      } else {
        return -1;
      }
    } else if (o2 == null) {
      return 1;
    } else {
      return o1.compareTo(o2);
    }
  }

Ideally I would like to make the method take two Comparables of the same type, is it possible to convert this and how?

I thought the following would do the trick:

protected static <T extends Comparable> int nullCompare(T o1, T o2) {

but it has failed to get rid of a warning in IntelliJ "Unchecked call to 'compareTo(T)' as a member of raw type 'java.lang.Comparable'" on the line:

return o1.compareTo(o2);

Answer 1

Change it to:

protected static <T extends Comparable<T>> int nullCompare(T o1, T o2) {

You need that because Comparable is itself a generic type.

Answer 2

Here's an odd case:

static class A {
    ...
}

static class B extends A implements Comparable<A> {
    public int compareTo(A o) {
        return ...;
    }
}

Luckily code like the one above is rare, but nullCompare() will not support comparison of Bs unless it is stated that Comparable may apply to T or any superclass thereof:

protected static <T extends Comparable<? super T>> int nullCompare(T o1, T o2) {

Even though most people will never benefit from the above tweak, it may come in handy when designing APIs for exported libraries.

Answer 3

Cannot edit so I have to post my answer.

You need to declare nested type parameter since Comparable is generic.

protected static <T extends Comparable<? super T>> int nullCompare(T o1, T o2) {

Please note that Comparable< ? super T >, which makes more flexible. You will see the same method definition on Collections.sort

public static <T extends Comparable<? super T>> void sort(List<T> list) {