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

One of the most useful features of Java 8 are the new default methods on interfaces. There are essentially two reasons (there may be others) why they have been introduced:

From an API designer's perspective, I would have liked to be able to use other modifiers on interface methods, e.g. final. This would be useful when adding convenience methods, preventing "accidental" overrides in implementing classes:

interface Sender {


    // Convenience method to send an empty message
    default final void send() {
        send(null);
    }

    // Implementations should only implement this method
    void send(String message);
}

The above is already common practice if Sender were a class:

abstract class Sender {

    // Convenience method to send an empty message
    final void send() {
        send(null);
    }


    // Implementations should only implement this method
    abstract void send(String message);
}

Now, default and final are obviously contradicting keywords, but the default keyword itself would not have been strictly required, so I'm assuming that this contradiction is deliberate, to reflect the subtle differences between "class methods with body" (just methods) and "interface methods with body" (default methods), i.e. differences which I have not yet understood.

At some point of time, support for modifiers like static and final on interface methods was not yet fully explored, citing Brian Goetz:

The other part is how far we're going to go to support class-building
tools in interfaces, such as final methods, private methods, protected
methods, static methods, etc. The answer is: we don't know yet

Since that time in late 2011, obviously, support for static methods in interfaces was added. Clearly, this added a lot of value to the JDK libraries themselves, such as with Comparator.comparing().

Question:

What is the reason final (and also static final) never made it to Java 8 interfaces?

Answer

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.)