Recently, I saw a function declaration as follows:
public static <T extends Comparable<? super T>> void sort(List<T> list) { //.... } Could anyone tell me what limitation is set on T for <T extends Comparable<? super T>> code snippets. Thanks a lot!!!!
I'll take a stab. Start with the simple:
static <T> void sort(List<T> list) Says you're sorting a List of type T. But, we need to be able to sort the Ts, so we have to restrict it to:
static <T extends Comparable> void sort(List<T> list) Which says you're sorting a List of Ts that are a subclass of Comparable. Since Comparable is an interface, this really just means that it's a List of things which implement the Comparable interface. But this isn't sufficient. Foo could implement Comparable where Baz isn't related to Foo at all. So we finally get to:
static <T extends Comparable<? super T>> void sort(List<T> list) Which says you're sorting a List of Ts where T means any type that implements Comparable<? super T> - it can compare two Ts via a method on T, or some ancestor (superclass) of T. This is necessary for sort, because sort needs to compare the items in the process of sorting. Absent an explicit comparator, the only way to do this is for the items to be able to compare themselves. The comparator need not be on T itself - if there's a superclass that implements Comparable, that's fine too.
We can step through this from left to right to figure out what it means.
Start with T extends Comparable, which means that the generic interface that is being used must extend Comparable. Note that because T is extending Comparable, it must be an interface, hence the assumption.
Then, Comparable<? super T>> which means that the type used by Comparable (the <> part), must be a superclass of T.
Put it all together and you get a statement like the following:
A generic interface, T, which extends a Comparable of type higher than T.
Here, "higher" means superclass.
Most limitations on generics can be interpreted this way, except for a few corner cases. If you get stuck again, try reading it out loud - from left to right - and breaking it down verbal to see exactly what is going on.
