Generics and sorting in Java

Question

Suppose you write a static function in Java to sort an array, much like Arrays.sort(). The problem with Arrays.sort() is that it receives an array of Object, and throws a ClassCastException if its elements don't implement Comparable.

So you want your function to receive as an argument an array of a subtype of Comparable. Something like that could work:

static <T extends Comparable> void sort(T[] array);

The problem with that signature is that you can still pass an array of Comparables with Integers and Strings for instance, which would cause a RuntimeException.

So, how can you create a function that will receive only an array whose elements implement Comparable and have all the same type (e.g. Integer, String, etc?)

Answer 1

Use

static <T extends Comparable<? super T>> sort(T[] array);

which is the most general specification to accomplish the task. Basically, it asserts, that T is a type which can be compared to itself.

Answer 2

Dirk's answer is the best you can get, but Google Collections used exactly as you wrote to avoid bug in javac:

Why do you use the type <E extends Comparable> in various APIs, which is not "fully generified"? Shouldn't it be <E extends Comparable<?>>, <E extends Comparable<E>> or <E extends Comparable<? super E>>?

The last suggestion is the correct one, as explained in Effective Java. However, we will be using <E extends Comparable<E>> on parameterless methods in order to work around a hideous javac bug. This will cause you problems when you use a very unusual type like java.sql.Timestamp which is comparable to a supertype. (Needs more explanation.)

From: http://code.google.com/p/google-collections/wiki/Faq

Now it's up to you...