Why is "final" not allowed in Java 8 interface methods?

This question is, to some degree, related to What is the reason why “synchronized” is not allowed in Java 8 interface methods?

The key thing to understand about default methods is that the primary design goal is interface evolution, not "turn interfaces into (mediocre) traits". While there's some overlap between the two, and we tried to be accommodating to the latter where it didn't get in the way of the former, these questions are best understood when viewed in this light. (Note too that class methods are going to be different from interface methods, no matter what the intent, by virtue of the fact that interface methods can be multiply inherited.)

The basic idea of a default method is: it is an interface method with a default implementation, and a derived class can provide a more specific implementation. And because the design center was interface evolution, it was a critical design goal that default methods be able to be added to interfaces after the fact in a source-compatible and binary-compatible manner.

The too-simple answer to "why not final default methods" is that then the body would then not simply be the default implementation, it would be the only implementation. While that's a little too simple an answer, it gives us a clue that the question is already heading in a questionable direction.

Another reason why final interface methods are questionable is that they create impossible problems for implementors. For example, suppose you have:

interface A { 
    default void foo() { ... }
}

interface B { 
}

class C implements A, B { 
}

Here, everything is good; C inherits foo() from A. Now supposing B is changed to have a foo method, with a default:

interface B { 
    default void foo() { ... }
}

Now, when we go to recompile C, the compiler will tell us that it doesn't know what behavior to inherit for foo(), so C has to override it (and could choose to delegate to A.super.foo() if it wanted to retain the same behavior.) But what if B had made its default final, and A is not under the control of the author of C? Now C is irretrievably broken; it can't compile without overriding foo(), but it can't override foo() if it was final in B.

This is just one example, but the point is that finality for methods is really a tool that makes more sense in the world of single-inheritance classes (generally which couple state to behavior), than to interfaces which merely contribute behavior and can be multiply inherited. It's too hard to reason about "what other interfaces might be mixed into the eventual implementor", and allowing an interface method to be final would likely cause these problems (and they would blow up not on the person who wrote the interface, but on the poor user who tries to implement it.)

Another reason to disallow them is that they wouldn't mean what you think they mean. A default implementation is only considered if the class (or its superclasses) don't provide a declaration (concrete or abstract) of the method. If a default method were final, but a superclass already implemented the method, the default would be ignored, which is probably not what the default author was expecting when declaring it final. (This inheritance behavior is a reflection of the design center for default methods -- interface evolution. It should be possible to add a default method (or a default implementation to an existing interface method) to existing interfaces that already have implementations, without changing the behavior of existing classes that implement the interface, guaranteeing that classes that already worked before default methods were added will work the same way in the presence of default methods.)

In the lambda mailing list there are plenty of discussions about it. One of those that seems to contain a lot of discussion about all that stuff is the following: On Varied interface method visibility (was Final defenders).

In this discussion, Talden, the author of the original question asks something very similar to your question:

The decision to make all interface members public was indeed an unfortunate decision. That any use of interface in internal design exposes implementation private details is a big one. It's a tough one to fix without adding some obscure or compatibility breaking nuances to the language. A compatibility break of that magnitude and potential subtlety would seen unconscionable so a solution has to exist that doesn't break existing code. Could reintroducing the 'package' keyword as an access-specifier be viable. It's absence of a specifier in an interface would imply public-access and the absence of a specifier in a class implies package-access. Which specifiers make sense in an interface is unclear - especially if, to minimise the knowledge burden on developers, we have to ensure that access-specifiers mean the same thing in both class and interface if they're present. In the absence of default methods I'd have speculated that the specifier of a member in an interface has to be at least as visible as the interface itself (so the interface can actually be implemented in all visible contexts) - with default methods that's not so certain. Has there been any clear communication as to whether this is even a possible in-scope discussion? If not, should it be held elsewhere.

Eventually Brian Goetz's answer was:

Yes, this is already being explored. However, let me set some realistic expectations -- language / VM features have a long lead time, even trivial-seeming ones like this. The time for proposing new language feature ideas for Java SE 8 has pretty much passed.

So, most likely it was never implemented because it was never part of the scope. It was never proposed in time to be considered.

In another heated discussion about final defender methods on the subject, Brian said again:

And you have gotten exactly what you wished for. That's exactly what this feature adds -- multiple inheritance of behavior. Of course we understand that people will use them as traits. And we've worked hard to ensure that the the model of inheritance they offer is simple and clean enough that people can get good results doing so in a broad variety of situations. We have, at the same time, chosen not to push them beyond the boundary of what works simply and cleanly, and that leads to "aw, you didn't go far enough" reactions in some case. But really, most of this thread seems to be grumbling that the glass is merely 98% full. I'll take that 98% and get on with it!

So this reinforces my theory that it simply was not part of the scope or part of their design. What they did was to provide enough functionality to deal with the issues of API evolution.

It will be hard to find and identify "THE" answer, for the resons mentioned in the comments from @EJP : There are roughly 2 (+/- 2) people in the world who can give the definite answer at all. And in doubt, the answer might just be something like "Supporting final default methods did not seem to be worth the effort of restructuring the internal call resolution mechanisms". This is speculation, of course, but it is at least backed by subtle evidences, like this Statement (by one of the two persons) in the OpenJDK mailing list:

"I suppose if "final default" methods were allowed, they might need rewriting from internal invokespecial to user-visible invokeinterface."

and trivial facts like that a method is simply not considered to be a (really) final method when it is a default method, as currently implemented in the Method::is_final_method method in the OpenJDK.

Further really "authorative" information is indeed hard to find, even with excessive websearches and by reading commit logs. I thought that it might be related to potential ambiguities during the resolution of interface method calls with the invokeinterface instruction and and class method calls, corresponding to the invokevirtual instruction: For the invokevirtual instruction, there may be a simple vtable lookup, because the method must either be inherited from a superclass, or implemented by the class directly. In contrast to that, an invokeinterface call must examine the respective call site to find out which interface this call actually refers to (this is explained in more detail in the InterfaceCalls page of the HotSpot Wiki). However, final methods do either not get inserted into the vtable at all, or replace existing entries in the vtable (see klassVtable.cpp. Line 333), and similarly, default methods are replacing existing entries in the vtable (see klassVtable.cpp, Line 202). So the actual reason (and thus, the answer) must be hidden deeper inside the (rather complex) method call resolution mechanisms, but maybe these references will nevertheless be considered as being helpful, be it only for others that manage to derive the actual answer from that.

I wouldn't think it is neccessary to specify final on a convienience interface method, I can agree though that it may be helpful, but seemingly the costs have outweight the benefits.

What you are supposed to do, either way, is to write proper javadoc for the default method, showing exactly what the method is and is not allowed to do. In that way the classes implementing the interface "are not allowed" to change the implementation, though there are no guarantees.

Anyone could write a Collection that adheres to the interface and then does things in the methods that are absolutely counter intuitive, there is no way to shield yourself from that, other than writing extensive unit tests.

We add default keyword to our method inside an interface when we know that the class extending the interface may or may not override our implementation. But what if we want to add a method that we don't want any implementing class to override? Well, two options were available to us:

Add a default final method. Add a static method.

Now, Java says that if we have a class implementing two or more interfaces such that they have a default method with exactly same method name and signature i.e. they are duplicate, then we need to provide an implementation of that method in our class. Now in case of default final methods, we can't provide an implementation and we are stuck. And that's why final keyword isn't used in interfaces.