Learning Java, 4th Edition

Case Study: The sort() Method

Poking around in the java.util.Collections class, we find all kinds of static utility methods for working with collections. Among them is this goody—the static generic method sort():

    <T extends Comparable<? super T>> void sort( List<T> list ) { ... }

Another nut for us to crack. Let’s focus on the last part of the bound:

    Comparable<? super T>

This is a wildcard instantiation of the Comparable interface, so we can read the extends as implements if it helps. Comparable holds a compareTo() method for some parameter type. A Comparable<String> means that the compareTo() method takes type String. Therefore, Comparable<? super T> is the set of instantiations of Comparable on T and all of its superclasses. A Comparable<T> suffices and, at the other end, so does a Comparable<Object>. What this means in English is that the elements must be comparable to their own type or some supertype of their own type. This is sufficient to ensure that the elements can all be compared to one another, but not as restrictive as saying that they must all implement the compareTo() method themselves. Some of the elements may inherit the Comparable interface from a parent class that knows how to compare only to a supertype of T, and that is exactly what is allowed here.