Question

What are the differences between the following three signatures?

static <T> void foo(List<T>,           Comparator<? super T>);
static <T> void bar(List<? extends T>, Comparator<T>        );
static <T> void baz(List<? extends T>, Comparator<? super T>);

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I know what extends and super mean in Generics. My question is whether there is a difference between foo, bar and baz. Should I make one of the parameters invariant and the other one variant in the appropriate direction, or should I make both of them variant? Does it make a difference?

Answer 1

The only difference is whether T represents the type parameter of the List, the Comparator, or something in between.

As far as the caller is concerned, the three method signatures are equivalent, i.e. whenever one of them can be used, the others can be used as well.

For the method implementation foo is probably most convenient, as it allows modifying the list without needing an additional capture conversion, which would require delegating to a helper method.