contravariance : Java Glossary


In Java, when you override a method, both parameters and return type must match exactly. In other languages, such as Eiffel, under certain circumstances, they need not precisely match. Here is a definition from Greg Weeks. Suppose I have the following interface:
interface Dog_printer {
void print(Dog d);

Suppose further that a class Animal_printer implements the method:

void print(Animal a);

Can I say that Animal_printer implements Dog_printer? Logically, I should be able to. Anywhere that a Dog_printer is expected, I can supply an Animal_printer and the dog will get printed. (Because all dogs are animals.) So, logically, there is the relation:

Animal_printer is a subtype of Dog_printer.

Although logically correct, this is weird looking. Then again, something that can print any animal extends something that can print any dog, so maybe it isn’t so weird. Anyway, to determine whether Animal_printer is a subtype of Dog_printer, we have used the contravariant argument rule. Similarly, there is a covariant return value rule that would state that:

Dog_creator is a subtype of Animal_creator.

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