The Java type system is broken

http://wouter.coekaerts.be/2018/java-type-system-broken

by tdurden • 8 years ago

291 155 8 years ago

wouter.coekaerts.be

Froyoh• 8 years ago

Not broken. Working as intended.

lmm5• 8 years ago

Nonsense. There are four bug reports at the bottom of the article; all seem to have been accepted as bugs and one has already been fixed.

viach• 8 years ago

From the examples: "String s = gridWrapper.get(0).get(0).get(0);" - Java type system is broken!

Java is not perfect, but this looks to me like a broken code indeed ".get(0).get(0).get(0)".

Could we rephrase the message of the article as "If you write really broken code in Java, you might be not guarded by the type system". And this is, well, uhm, well... acceptable?

rimunroe• 8 years ago

Is a type system really worth that much if it can't help you in a situation where your code is ugly and hard to follow?

freddie_mercury• 8 years ago

"Is X really worth it if it isn't perfect and doesn't cover every edge case?"

I kinda feel like humanity has repeatedly decided the answer to this question is "yes".

"Being valuable" isn't binary. There are relative amounts of value, I think.

sorokod• 8 years ago

Type system helps those who help themselves.

stickfigure2• 8 years ago

Unequivocally, yes.

Several new languages (eg Typescript, Dart) have optional type systems that still bring significant value to the table.

Sure - all other things being equal, I'd much rather have a type system that is completely sound. Ceylon's type system is beautiful and I'm happy to see its union + intersection types being adopted by Typescript. But honestly, I'm not going to lose much sleep over these particular issues in Java. Bad code gets refactored.

A bigger complaint should be made that generic type erasure makes libraries hard (eg, the number of places you have to pass around Class<?> or TypeReference objects to let the runtime know what to do). But that was a clearly a compromise decision and everyone knows about it.

agumonkey• 8 years ago

It's stupid code, but a type system has to be code agnostic. Otherwise you're back to dynamic languages with local type annotations.

Anderkent2• 8 years ago

And this is, well, uhm, well... acceptable?

It is not. The only purpose of a type system is to prevent broken code from compiling. When it fails to do so, it is broken.

dtech2• 8 years ago

Typescript with its explicitly unsound type system show that a "broken" type system is immensely valuable, especially when you get something in return for that [1]

[1] https://www.typescriptlang.org/docs/handbook/type-compatibil...

bdhess• 8 years ago

The author is trying to write a concise example. You could achieve the same effect by writing three for loops, but that would take additional code and declarations that make his point less clear.

opnitro2• 8 years ago

Is it? The whole point of a type system is that it gives you guarantees over your code. Certain bugs should be inexpressible within the language.

emodendroket3• 8 years ago

Well, think of it this way. A seat belt cannot help you at all if an object flies through your windshield. Nevertheless, driving while wearing a seat belt is, overall, safer than driving without one.

seanmcdirmid• 8 years ago

I don’t think that is true anymore. There are languages like typescript that don’t provide anything near that guarantee but are still seen as very useful.

macspoofing• 8 years ago

If you don't like this example, you can always find cases where the limitations of the type system (or more specifically Generics) become apparent. This isn't quite so obvious during 'normal' application development, but if you start creating libraries or frameworks and you want generic type support, you will run into a wall very quickly.

byte1918• 8 years ago

If he were to show real world example, few would bother to follow it through because it would be tens of lines of code spanning through multiple classes and namespaces.

Doxterpepper• 8 years ago

What's the point of strong typing if it only saves you most of the time?

dtech2• 8 years ago

So a type-system that does the right thing in 99.999999% of the cases is completely useless according to you?

coldtea3• 8 years ago

Could we rephrase the message of the article as "If you write really broken code in Java, you might be not guarded by the type system"

The very idea of the type system is that it makes "broken code" of a certain type a compile error.

cmiles743• 8 years ago

For certain and specific definitions of "broken code".

Someone• 8 years ago

In Java

  f.get(0).get(0).get(0)

typically is the way to write want in languages that allow operator overloading is written as:

  f[0][0][0]

You will rarely see that with three constants, but

  f[x][y][z]

isn’t uncommon.

lmm5• 8 years ago

The type system should give guarantees. I believe these cases violate the specification, and I think the specification is right to require the typechecker to check all code rather than just most code: a type system that works 100% of the time is much more useful than a type system that works 99.99% of the time, because it means you can blindly rely on it rather than having to double-check it every time you do anything critical and then maintain all those double-checks.

If we take the view that these are bugs in the Java compiler, then of course a bug that's rarely triggered is less bad than a bug that's triggered all the time. But both are bugs all the same.

TazeTSchnitzel2• 8 years ago

My favourite feature of Java's type system is that, because any object type also accepts null, the type system is unsound and you can convert anything to anything else:

https://raw.githubusercontent.com/namin/unsound/master/doc/u...

stingraycharles• 8 years ago

Wouldn't that be a problem in most languages? Even Haskell allows for it through Data.Dynamic

If I can serialize to String, and then deserialize a String to any type of class, I can effectively "cast" anything to anything.

TazeTSchnitzel2• 8 years ago

By “convert” here I mean convince the compiler that any thing of type A is any type B, when it definitely isn't, without using an explicit cast.

lmm5• 8 years ago

The point is not that you can do explicit casts, it's that because you can form a (fake) instance for impossible types you can do invalid type conversions without doing anything that looks unsafe.

In Haskell you can certainly achieve the same thing with various common extensions ( http://okmij.org/ftp/Haskell/impredicativity-bites.html ). Possibly not in vanilla Haskell '98, at the price of being not a very nice language to work in. (A total language like Idris should permit GADT-like functionality without allowing this kind of issue).

J-Kuhn• 8 years ago

Read about Bottom (⊥).

null can be considered bottom (if it were not for primitives)

tathougies• 8 years ago

Data.Dynamic is type safe. It performs a run-time type check.

Haskell can be made to be unsound using 'unsafeCoerce', but as you'll note, there is a giant unsafe in front of it.

Tarean• 8 years ago

The actual correspondence in haskell are GADTs:

    data EqualityProof a b where
        Refl :: (a ~ b) => EqualityProof a b

    -- compiler error because we don't check that the equality proof is actually Refl
    coerce :: EqualityProof a b -> a -> b
    coerce _ a = a

    -- this works
    coerce Refl a = a

    -- here the Refl pattern match catches the undefined
    -- this is like forcing the programmer to do a null check before the equality is in scope
    -- and throwing a runtime exception when it's null
    coerce undefinded 3 :: String

moron4hire2• 8 years ago

It should be noted that soundness is not a common feature for type systems in the wild. It sounds kind of scary, "X programming language's type system is unsound", but that's pretty much been the status quo for most of programming.

kochthesecond• 8 years ago

I was about to say this. Most type systems in practical use are not mathematically sound.

nine_k2• 8 years ago

This is a correct observation, and also something to always remember. There are dark corners where unsound type systems fail, they inevitably exist in most practical languages, so you can wander into one in daily practice.

hinkley• 8 years ago

Our intuition about inheritance is pretty much all wrong.

I make analogies to taxonomy because before I learned software a biology teacher told me that taxonomy is also broken, so when I learned type theory it felt like a similar kind of broken.

We think putting wings to a mammal is adding behavior, when in fact it is subtracting it. Adding wings on a mammal means you have a bat, or a flying squirrel.

You have narrowed the potential of this type, not enhanced it. Anything else a mammal might do that bats can't? You've taken all of that away. Mammals with fly() aren't the fastest land animals. They don't have the record for holding their breath, or hibernation time. They can't dive(), speak() or useTool(). Really they're kinda useless except for vermin control.

wintorez• 8 years ago

* The Java is broken

npstr• 8 years ago

Java generics are broken? What else is new?

buremba• 8 years ago

Welcome to the world of enterprise-ready complex applications. I agree that these cases should be covered by the compiler but man, there are mainstream languages that don't implement generics just because they're complex and pretend that they're not helpful.

It would be nice of you to compare Java generics with equivalents instead of just criticizing it. It doesn't help anyone writing "this is broken" not offering any alternatives or suggesting ways to fix the problem.

hyperpape3• 8 years ago

I think it would've been better to write "unsound" rather than broken, but it's pretty common to present little proofs/results like this without saying how the system should fix it. Doing so is helpful, because many times you'll know there's a problem without there being an obvious fix.

pdpi• 8 years ago

Clearly you didn't reach the conclusion. First, because the conclusion is not "Java sucks", but rather "Most of these things, while not kosher, are unlikely to happen in practice." Second, because the author didn't just leave it at criticising the language — he opened a bunch of bugs in the OpenJDK tracker.

bad_user2• 8 years ago

People criticizing constructive criticism are often not constructive.

nurettin• 8 years ago

C# is a similar language in it's goals and architecture. LINQ to Objects type system will throw an error if you try to return conditionally different types from a lambda expression, so it is doable.

humanrebar• 8 years ago

It doesn't help anyone writing "this is broken" not offering any alternatives or suggesting ways to fix the problem.

Clearly stating and detailing problems is absolutely helpful. We can't solve problems if we don't understand them.

coldtea3• 8 years ago

It doesn't help anyone writing "this is broken" not offering any alternatives or suggesting ways to fix the problem.

Actually identifying a problem is of very much help, even without a solution.

For one, it lets people know of the problem, so they can get to an eventual solution.

Second, it lets people know of the problem, and avoid those areas, even if there's no solution available.

projektir• 8 years ago

It would be nice of you to compare Java generics with equivalents instead of just criticizing it. It doesn't help anyone writing "this is broken" not offering any alternatives or suggesting ways to fix the problem.

Hmm. If I knew your coolant was leaking, but had no idea how to fix it, would you prefer I didn't tell you? Just want to keep driving that car around with that coolant leaking?

At the very least, this is educational and helps better understand Java for those of us who work with it.

nik736• 8 years ago

Java is broken ¯\_(ツ)_/¯

hyperpape3• 8 years ago

Here's another fun trick, though it's probably not a flaw in the type system: https://twitter.com/joshbloch/status/1018987317489426432.

BonesJustice• 8 years ago

Yeah, compound types like that are a bit of a hack. During type erasure, the type variable gets reduced to its leftmost type (in this case, `String`), so the `main` signature ends up being a valid entry point. Best the compiler can do to enforce the right-hand type constraints is to insert cast instructions where values are used as insances of those types.

As the tweet shows, you still get (some) type safety from the runtime checks.

dlubarov• 8 years ago

Another flaw we can abuse is Java's covariant array types:

    Object[] objects = new Integer[] {1, 2, 3};
    objects[0] = "hello";

Also compiles but fails at runtime.

bad_user2• 8 years ago

It’s pretty tragic how people respond to such criticism emotionally, instead of acknowledging the issues and working towards fixing them or educating others in the traps and compromises involved.

HN is less prone to toxicity, but this is reminiscent of Reddit and it drives away the people that want to help.

As a piece of advice, some type theory never harmed anyone ;-)

kjeetgill5• 8 years ago

I think it's just a knee jerk reaction to some academic/PL theory types that chime in with a "Huzza! Long live (Haskell/Scala/Ocaml/etc.)!" As if a lack of soundness is dawning on anyone and was anyone's issue to begin with.

It's pretty dramatic so it gets met with preemptive defensiveness.

From the end of article: Everything is broken. Everything is fine.

jerf• 8 years ago

In theory, as soon as a type system is unsound, it is unsound.

In practice, there are ranges of brokenness. If you have to go hunting for the problem, and it takes a combination of 5 obscure features which nobody would ever use together, it's not that big a deal. If it's the sort of thing that you encounter in any non-trivial program, well....

... your language dies.

Part of the reason so many people in this thread can be cavalier about type soundness issues is precisely that all the languages they use, being the languages that did not die because they have a crappy type system, are pretty sound, within the model the language is operating under, so, yeah, of course it doesn't look like a problem to a normal programmer, because it was solved a long time ago. Doesn't mean it's a bad idea, or that people should just sneer contemptuously at the issues and snidely dismiss them as academic frippery, because then these people go on to write new languages that have these sorts of issues.

(Like the recent wave of "event based" programming environments that just threw away the benefits of structured programming, because they were so thoroughly immersed in a structured programming world they didn't even realize how thoroughly they'd internalized it, had no idea what the preconditions were for it, and not only had no idea they were throwing it away but snidely berated people who pointed it out. And then spent 5+ years rediscovering it all over again....)

watwut• 8 years ago

Seems like the project acknowledged issues and is working towards fixing them. There is no flame in it and discussion in bugs that were opened is factual.

nine_k2• 8 years ago

Application of type theory, or any theory, or any formal method, requires humility and patience.

To apply a formal method to your work requires to acknowledge that your intuition might me wrong, your past decisions incorrect, and your knowledge of the subject area deficient. Often you have to step back, rethink, and rework.

This is a normal mindset for a scientist, and a pretty common mindset for an experienced engineer.

But this view is not automatically acquired with an engineering diploma, with obvious coding skills, or even with a couple successful open-source projects. It has to be nurtured consciously—or rammed down your throat by unforgiving reality; the latter is pretty traumatic.

pjmlp4• 8 years ago

It is also tribalism.

When one is a polyglot developer, we are like mercenaries, a bag full of tools, each with its caveats, and we care about the solution less how we got there juggling those tools.

Developer X tend to keep searching for external confirmation that they did the right choice. Any attack on tool X triggers defensive attacks, as if they would be a personal attack.

Of course I am exaggerating here, just extrapolating from those that I know on my circle.

MrBuddyCasino• 8 years ago

Everything is broken. Everything is fine.

This is the correct conclusion. The people getting outraged over this are the same that will hold long and boring monologues about how everybody does REST wrong.

bunnymancer2• 8 years ago

Had a client just last month that were doing SOAP wrong somehow...

Required a SOAP message, that contained no XML after the message tag, but instead the entire message body was to be a JSON, and responded with a JSON if something was wrong, and a fully formed SOAP message if things were Ok.

That and the guys that handle your money use the Http status code as their personal error messages. Except when the error is on their end, in which case they return 200 and a JSON with an error message.

There are plenty of ways to do REST wrong, so let's not lump that into the people that make blog posts complaining about X because X is broken, and working just fine...

MaxBarraclough3• 8 years ago

The only thing stopping Tim Berners-Lee spinning in his grave, is that he's still alive.

meddlepal• 8 years ago

Well I hope the money was good at least. Yikes.

cs02rm0• 8 years ago

Had a client just last month that were doing SOAP wrong somehow...

To be fair, I'm not sure there's any other way.

aserafini• 8 years ago

I'm guessing you're talking about Adyen. At least I've had exactly the same experience integrating with their API. It's an awful mismash. I even get responses with bool = 'true' in their json, but only sometimes. That's the string 'true' sometimes and othertimes json true.

bunderbunder3• 8 years ago

I liked the two sentences before those even more:

Java’s type system has taken a beating by the desire for backward compatibility and the interaction of its growing list of features. It needs some love to get it back in shape.

s-shellfish• 8 years ago

This is just what it is to work with computers. Computers get fixed over time. Everything is broken, everything is fine. Mantras.

mcculley8• 8 years ago

The mess that is the current Java type system makes me skeptical when proponents of languages like Go say not to worry, that generics can be added later.

If Java had generics in the beginning, it would look a lot different and the type system would be more powerful and safe.

kjeetgill5• 8 years ago

Interesting. How do you think it would be different? I ask because the go-to is usually genetics erasure. Do you think more could or would have been different?

For what it's worth, Brian Goetz doesn't see type-erasure as a such a failure and defends the design choice[0].

[0] https://m.youtube.com/watch?v=TkpcuL1t1lY

hota_mazi4• 8 years ago

Actually, most people who actually understand in depth the subject and the trade offs of erasure versus reification squarely side on the erasure side.

Erasure is the safest way to implement generics for a long list of reasons. Reification comes with a lot of downsides, which is why most languages that support parametric polymorphism use erasure.

mcculley8• 8 years ago

I have not watched the video. All of the defenses of type erasure I have read boiled down to backwards compatibility. If generics had been there from the start, the JVM and the compiler could do more with them, eliminating casts and allowing things like List<int>, getting rid of arrays entirely as a special case and allowing the compiler and JVM to optimize accordingly.

cmiles743• 8 years ago

I agree that generics should probably be a strong consideration right from the beginning. I disagree that this would mean that it would necessarily be done correctly or that it's impossible to add it later. That was the case with .Net and C#, it wasn't until the 2.0 release that generics were added. [0]

[0] http://mattwarren.org/2018/03/02/How-generics-were-added-to-...

monocasa5• 8 years ago

Unlike Java, Go doesn't have to worry about bytecode backwards compatibility. That was the main issue that had them reaching for type erasure.

Take a look at C# which added it without many issues (because they were willing to break back compat of their bytecode). Go fits this model a whole lot better.

enitihas• 8 years ago

Did C# really break backwards compatibility of their bytecode? What I understand is that C# generic and non generic APIs are completely different, and are not inter operable.

pjmlp4• 8 years ago

The missing part every time this story gets told, is that Microsoft already had a generics implementation ongoing being lead by Don Syme of F# fame.

They just decided to release 1.0 without generics, instead of waiting for it to be 100% ready.

As Don clearly describes on his blog.

pron2• 8 years ago

added it without many issues

Except for the fact that all CLR languages must adopt the same variance model, enforced by the runtime (or choose not to interoperate well).

alexeiz• 8 years ago

Take a look at C# which added it without many issues

I wouldn't say "without many issues." It took a major surgery in the CLR codebase to add generics. The difference between CLR 1.1 without generics and CLR 2.0 with generics was huge. No area of code was untouched.

mnd999• 8 years ago

Except they didn’t - they introduced annotations at the same time and bumped the class file version for that.

mcculley8• 8 years ago

Go doesn’t have an ABI? Does every library ship only as source code? I didn’t know that.

I was thinking of more fundamental changes to Java idiom and the standard library had generics been part of the design (e.g., how arrays work).

MaxBarraclough3• 8 years ago

If Java had generics in the beginning, it would look a lot different and the type system would be more powerful and safe.

And conceivably a good deal faster, too. My understanding is that the JVM is required to constantly perform runtime type checks.

monocasa5• 8 years ago

Yeah, it either has to cast from Object all the time, or spend JIT compiler time and memory figuring out if it can elide those checks (probably with exception code for when it guesses wrong).

leshow• 8 years ago

and pointer chasing too right? unless the runtime can optimize, every generic sits behind a pointer.

icedchai3• 8 years ago

Good deal faster? I seriously doubt it. My understanding are generics are purely a compile time feature and does not effect run time.

I remember Java before generics: tons of casting from Object. The compiler is doing the same thing.

piinbinary• 8 years ago

Since there is no subclassing [0] in Go, many of those cases are not expressible in Go.

[0] Interfaces can contain other interfaces, but I think that is still different because you can't do

    var a []subinterface = []superinterface{...}

tabtab3• 8 years ago

Just use dynamic or better yet type-tag-free languages. Types are hard to get right and hard to reason about. Sure, there are some Sheldons out there who master it even under pasta applications, but most of us are not Sheldons. C# also bleeped up types by adding nullable types, making reflection into a scavenger hunt.

For dynamic or tag-free languages, type indicators could be used to parse-check scalar (base) values to see if they are interpretable as the intended type:

   function foo(int a, date b) {...}

This would be equivalent to:

   function foo(a, b) {
     if (! parsableAsInt(a)) throwTypeError(...);
     if (! parsableAsDate(b)) throwTypeError(...);
   }

And don't overload operators, such as how some languages use "+" to mean both arithmetic addition and string concatenation. Use a different symbol for concatenation like PHP does.

recursive• 8 years ago

C# also bleeped up types by adding nullable types, making reflection into a scavenger hunt.

Having used reflection in C# with both nullable types and non-, I'm curious what you mean by "scavenger hunt".

dep_b• 8 years ago

String concatenation got a lot nicer in C# lately but the old method is still there

erik_seaberg• 8 years ago

PHP only got this halfway right because it borrowed "." from Perl. It should have also borrowed "eq" instead of making "==" unusable.

_skel• 8 years ago

I prefer my type errors at compile time.

You have to reason about types in any language because passing the wrong type to a function can result in errors. You have to do all of that mental work by yourself in a dynamic language because there isn't a system to do it for you.

AnimalMuppet• 8 years ago

That would solve the problem with adding generics, yes. It would also destroy the thing people appreciate about static typing - that you get told at compile time when you've done something wrong.

tdsamardzhiev• 8 years ago

True, but I think that for bigger applications static typing is better even if you don't get it quite right.

rs86• 8 years ago

The thing I most resent is that dynamic languages are so widespread. It is so pleasurable to write haskell or ocaml!

sqrt17• 8 years ago

Go's type system with limited polymorphism looks to me like someone took the (horrible) collections from Java 1.2 and applied a fix that solves 85% of the problem. Compared to the 95% solution we get with Java, it's not as refined but also plenty good enough in most cases where concrete code deals with concrete problems (i.e. not framework-level code where you'd curse Java for lacking Scala's types-and-implicit-driven compile time contorsions).

microtherion• 8 years ago

My impression (having not tried this part of the language much) is that Swift underwent similar growing pains with their generics (but with the benefit of being willing to break backward compatibility). After 40 years or so of generics, it seems language designers STILL think "all these languages before me are overcomplicating the matter, I bet I can do this in a much simpler way".

slavapestov• 8 years ago

The implementation of generics has undergone major changes and new features were added, but the conceptual model is pretty much what it was in Swift 1. This was one part of the language that they almost got right from the start.

kelnos• 8 years ago

I was reading something a while ago (can't find it now, unfortunately) that suggested that the use of type erasure for generics actually opens up the JVM to be able to do some useful things that (for example) the CLR can't do. One thing I recall was that the Scala developers gave up on their CLR/.NET backend.

I also read, more generally, that adding reified generics to the JVM might make it hard or impossible to implement some dynamic languages on top of the JVM.

So yes, erasure has its drawbacks, but it's not all bad.

chrisseaton• 8 years ago

Yes I've heard this from multiple people targeting new languages to the JVM.

Maybe it's why the JVM has a larger ecosystem of new languages?

hyperpape3• 8 years ago

One source: https://olabini.com/blog/2010/07/questioning-the-reality-of-...

bunnymancer2• 8 years ago

Oh good, another opinion about how this one thing in this one language is the worst thing ever, and also works just fine.

Find something that bothers you, complain publicly, do absolutely nothing after that, repeat.

raj7749• 8 years ago

I was trying to justify the type safety of list in java to one of my senior, i am glad that i was right, it is indeed broken

whatyoucantsay• 8 years ago

This fact may be hacker but it is not news.

kazinator2• 8 years ago

Disagree with bullshit term: heap pollution.

Pollution is the proliferation of superfluous objects creating some sort of undesirable mixture. For instance "namespace pollution".

An inconsistency between the run-time type of an object in the heap, and the type of the expression in the program which refers to that object, isn't a good fit for the word "pollution".

Quit trying to pollute the computing lexicon with nonsense.

kazinator2• 8 years ago

Looks like this is a thing in Java culture; good grief.

exabrial• 8 years ago

Website is struggling, here's a cache: http://archive.is/k2YbA

olliej• 8 years ago

It missed my favorite: contravariant arrays (or is it covariant, I last did work on this more than a decade ago.

Specifically you can do (forgive syntax it’s also been a long time since I wrote java)

X = new String[10] Ovject[] o = X

o[0] = 1; // this will auto box, and then fail at runtime

Note that this isn’t a “bug” as it predates any version of generics so collection types especially are much saber if you support this. That said it does result in a logical error in which you can’t assign something that looks like it should be fine.

.Net unfortunately picked up this design decision in order to support java on their vm.

The Java type system is broken

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