Question

I noticed the specificaition for Collections.sort:

public static <T> void sort(List<T> list, Comparator<? super T> c) 

Why is the ‘? super‘ necessary here? If ClassB extends ClassA, then wouldn’t we have a guarantee that a Comparator<ClassA> would be able to compare two ClassB objects anyway, without the ‘? super‘ part?

In other words, given this code:

List<ClassB> list = . . . ; Comparator<ClassA> comp = . . . ; Collections.sort(list, comp); 

why isn’t the compiler smart enough to know that this is OK even without specifying ‘? super‘ for the declaration of Collections.sort()?

Answer 1

Josh Bloch had a talk at Google I/O this year, called Effective Java Reloaded, which you may find interesting. It talks about a mnemonic called ‘Pecs’ (producer extends, consumer super), which explains why you use ? extends T and ? super T in your input parameters (only; never for return types), and when to use which.