Java made several mistakes. It also made some questionable (yet often defensible) decisions. It's understandable. Type erasure was one I believe was a mistake. It's talked about in the article. Yes, you kept binary compatibility but you that created so many other problems such as not being able to use value types in generics. Notably, C# looked at that and said "nope". Type erasure is also hurting Valhalla here and the issue of value classes in generics is the second phase so is being pushed far into the future.

But a huge mistake (IMHO) was not having nullability part of the type system. You can still do this with type erasure.

Anyway, I read your comment as "nullability isn't complex" (paraphrased) but that's not the author's point. What's complex is having a value class and a regular class of every class and you don't necessary know which one you're dealing with at the language level.

C++ is a great example of this. You can create an object ont he stack or the heap and that's really what we're talking about with that proposal. And that's a nightmare. Combined with pointers it meant you never knew if you could free something or not and that ownership had to be passed around with vague comments like "// retains ownership".

Anyway, the whole article is a great tale of how difficult it is to retrofit things later and how difficult it can be to fix mistakes later (eg java.util.Date).

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inigyou 4 hours ago | parent [-]

How would a non-nullable class field work in Java when it can be initiqlized by arbitrary imperative code that can read it while it's being initialized?

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tsimionescu 3 hours ago | parent | next [-]

Look at how Go did it. Any value type has a defined 0 value, and any variable of field of that type is initialized to 0 by default. So any un-initialized non-null able value type field could have the corresponding 0 value.

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inigyou 2 hours ago | parent [-]

That's worse than null. Now every object has an invalid state. It works for Go because Go is trying to keep language complexity low. It has no good design principles behind it other than that. Also the zero value of a reference is null so you still haven't answered how a non-nullable reference field would work.

	▲	tsimionescu 2 hours ago \| parent [-]
		Sorry, I misinterpreted your question to be about non-nullable value classes, not non-nullable classes more generally. For reference variables, it seems that the a priori best approach would have been to generate a compiler error if the field was read before it was written; since this ship has long since sailed with final fields, it seems that the same approach would have to be taken - the "not null able" guarantee only applies after the class is fully initialized, and you can observe it as null during initialization. This is probably not a huge problem, given that initialization code always has to deal with a class breaking its invariants, since they are only established at the end of initialization. Regarding the 0 value choice in Go, I don't agree that this is worse than null. It simply applies a design constraint that is not usually very hard to satisfy - that the 0 value of your type must have well defined semantics.

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wiseowise 3 hours ago | parent | prev | next [-]

How can Kotlin do it?

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inigyou 2 hours ago | parent | next [-]

Probably with hacks. Did you know a final field in Java can change its value? And I'm not talking about his reflection to make it non-final. With ordinary code only, you can read a final field before it's been initialized, so it still holds its default zero value. For example "final int x = calcX();" and have calcX print the value of x, it will be zero.

There's a whole bunch of specification language describing how constants aren't actually constant in specific situations.

I don't know Kotlin but I assume it does the same thing: until the non-nullable field gets initialized, it holds null and violates the type system.

	▲	wiseowise an hour ago \| parent [-]
		Nothing stops me from sticking my dick into a blender either, yet it works fine for most of the things (the blender). Both Java’s notion of final and Kotlin’s nullability work great for 99.999 use cases and you really need to go out of your way to break them.

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mike_hearn 3 hours ago | parent | prev [-]

A lot of the language rules are required to make its approach to nullability work. Hence odd keywords like "lateinit var".

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jmyeet 4 hours ago | parent | prev [-]

The type erasure version of this would look a lot like Hack [1]. So generic arguments would simply have a ? if they allow nulls eg List<?Point>. The list itself couldn't be null unless it was ?List<?Point>.

Now, one can argue that this is just smoke and mirrors with type erasure and it is but you can already put a Date into a List<Point> if you're so inclined because the JVM doesn't know the difference, hence type erasure. So this is no different.

I'm no JVM expert but from reading the article it seems like the chosen solution for value classes is to treat them all as a single L-type in the JVM where each primitive type is its own L-type. If I read the correctly, it means that if you have a Point value class then on the JVM level you'll be able to stuff any value class into there if you're so incline, just like with List<Point>.

Obviously we need to be concerned with fuzzing (moreso in C++) but here really we're just trying to have sensible defaults that aren't guaranteed because we can't design the language how we want from the ground up without making a new language.

Oh and there is a prosopal for this [2]. Personally, I prefer the Hack version.

[1]: https://docs.hhvm.com/hack/types/nullable-types/

[2]: https://openjdk.org/jeps/8303099

	▲	inigyou 3 hours ago \| parent [-]
		you seem to have ignored the question and answered a completely different one