Method Bodies in Interfaces -- Why Not? (original) (raw)

Neal Gafter neal at gafter.com
Sat Mar 14 13:32:56 PDT 2009

I think adding extension methods as C# does is both simpler and more powerful than adding method bodies to an interface. See http://www.javac.info/ExtensionMethods.html for a very brief glossy. I do not plan to submit a proposal for this.

On Tue, Mar 10, 2009 at 4:38 PM, David Walend <david at walend.net> wrote:

A lot of us spend energy dealing with the fact that we can not add methods to interfaces. We can't add methods to interfaces because everything that implements those interfaces would break. We could get around that by letting interface-defined methods have method bodies. The resulting problem is that the compiler would not be able to choose between or order method bodies with the same signature. (As I understand it, that's why interfaces can't have method bodies.)

It's modestly rare for classes to inherit two interfaces that define methods with the same signature in the wild. What happens if the JLS were to relax the constraint? I imagine a few options: With multiple inheritance, if different implemented interfaces define bodies for methods with the same signature then: 1) Give the developer a compile error or 2) Inherit neither method body and force the developer to implement something himself or 3) Inherit neither method body, force the developer to implement something himself, and give him some way to access to all the inherited implementations. That would let us out of some of our more painful contortions. What are the holes in that idea? Thanks, Dave david at walend.net

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