Final should be Default for Classes in Java

We were having an interesting discussion the other day, and the issue of final classes came up.  For some reason, it suddenly occurred to me that all classes should be final by default. That is, classes should be implicitly final, rather than requiring an explicit declaration.  For example, the following should be considered invalid in my opinion:

class Parent { ... }
class Child extends Parent {... } // invalid: parent implicitly final

In place of final we would have another modifier, say open.  This would allow us to extend classes like so:

open class Parent { ... }
class Child extends Parent { ... } // valid: parent explicitly open

Now, the question is: why do I think final should be the default? It’s got nothing to do with performance.  The following quotes from Josh Bloch’s excellent talk on API design (see [1][2][3]) gives us a clue:

“When in doubt, leave it out”

“You can always add, but you can never remove”

What does this mean? If you’re not sure whether a function should be included, then don’t include it.  That’s because, once you’ve included a function in a public API, people will depend upon it and you’ll have to maintain it.  If it’s badly designed, you’re stuck with it.  Sure, you can try and deprecate it — but you’ll probably end up keeping it forever anyway.

What has all this got to do with final classes? Well, a non-final class can be extended of course!  Any public or protected methods can be overridden and protected fields read/written.   More importantly, you cannot reverse the decision — i.e. once a public non-final class, always a public non-final class.  In contrast, when using final as the default for classes, you can reverse your decision — i.e. you can always open, but you can never close.

This observation is hardly rocket science!  Josh Bloch in his (also excellent) book Effective Java states* “you should make each class or member as inaccessible as possible”*.  For example, you should always prefer private to protected fields. For some reason though, he doesn’t extend this to include preferring final to non-final classes.

And, it looks like there are at least some like-minded people out there and, of course, there are those who think differently …