The Current State of Java Value Types
infoq.com
infoq.com
I hope that there will be a compiler flag at some point to enable a warning or compiler failure on any bare null usage.
In a few of Java Language Summit talks, they mention about such cases.
"Adventures in Parametric Polymorphism" from Brian Goetz at JVM Language Summit 2016.
http://www.oracle.com/technetwork/java/javase/community/jlss...
It is agreed in general that whether something is a value type is a definition-side attribute. However if code was compiled expecting the Optional value (e.g. does an ==) but it is now an Optional object, what happens? The identity check is different than the value-equals one. The doc goes into more detail on some differing things here.
I haven't studied the arguments or read the doc fully, but I'd say if you make it where it can only migrate one direction, object types to value types, you can solve the two big issues of equality (equals() is intercepted by the JVM for value types to imply ==) and nullability (value is never null, == null becomes always false). There may be other issues with hashCode, use as mutex in synchronized block, etc.
Maybe another approach is to have the value type implement "ObjectLike" if it wants to exist in the L-world. There needs to be a separation between what an "object type" is and what an "object type" can do. The latter causes migration concerns, the former is just about state and copying/reference semantics.
0 - http://cr.openjdk.java.net/~jrose/values/value-type-hygiene....
Sure, classes are generally designed for either being used as value types or OO Objects, you can prevent objects from being copyied/moved and if you really go out of the way even from being created outside of the heap, but in general the value vs reference semantic is left at point of use in C++.
- Object contains a Class that it derrives from (no BaseObject or Modules)
- Class is an Object
- String is an Object
- Boolean is a two-value singleton of true and false
- Number is an abstract subclass of Object
- Decimal and Integer are abstract subclasses of Number
- Float, Double, LongDouble, BigDecimal are concrete subclasses of Decimal
- SignedByte, Byte, Short, UnsignedShort, Int, Unsigned, Long, UnsignedLong, Char, BigInt are concrete subclasses of Integer (or U/I/F## types reminiscent of Rust instead of C type names)
and so on.
Then there is no boxing/unboxing of simple types or literals because they are one-and-the-same, and no there’s no confusion about how to interact with any truly generic type of value.
And the distinction has worked elsewhere. Objective-C for instance has an NSObject in Cocoa but the language is a proper superset of C and has no problem with plain types. (Granted if you use low-level objcruntime functions, it will be more complex to pass in C data types that are not objects but they still work.)
Despite not requiring Object everywhere, early Java had a frustrating tendency to use Object for many basic things, seemingly requiring boxing and casting more than it should. (At least with Objective-C++ you had the option of some std:: container of plain data, if you didn’t want to wrap in some NS<container> type.) Both Java and Objective-C ultimately changed the syntax to allow type specifications in containers to compensate.
Early Java didn’t have Generics. With Generics, using Object instead of genericizing the type would be an anti-pattern in modern Java.
A lot of warts in Java are being removed.
In Eiffel for example, you can declare what is the default behavior (reference or value type), and then any user of the type is able to still override it at declaration site.
Even if it would be your preferred way for a language to do things, not doing so isn't necessarily wrong.
Those early decisions may not have been technically right in some sense, but technical considerations are not the only factor.
One of its descendants, Modula-3 designed around 1986, is equally cited as yet another Java reference.
Both of them, had value types and compiled ahead of time into native code. Although Mesa/Cedar was native in the Lisp Machine sense, given the Xerox PARC microcoded CPU architectures.
There are some old posts from Gosling where he discusses about Eiffel/Sather features.
So the knowledge was there, it was just a bit disappointing not getting them, because as you say technical considerations were not the only factor.
Never mind the machines back when Java was 1.0 — one of the reasons Project Valhalla exists is that "the poor performance of requiring a minimum object overhead" is very much still a significant cost!
The reason Java isn't a 'pure' object-oriented language is simply performance.
Suppose everything - even every int - is heap-allocated. You now need a very sophisticated JIT compiler (as in, better than any we have today), or it's going to run dog slow. Having a huge number of needless allocations happening at every step is going to:
1. Slow things down by doing vastly more heap allocations that you otherwise would (even with Java's ultra-fast allocations)
2. Slow things down by doing violence to your code's locality and cache behaviour, because your ints no longer live in the stack
3. Slow things down by doing violence to your code's locality and cache behaviour, because Java objects are bloated compare to raw ints
4. Slow things down by hugely increasing garbage-collection pressure
If Gosling had taken that route, we wouldn't be talking about Java today.
Oh well, at least in couple of years we will have them.
Java's situation is even worse because it's a compiled language that does not need a JIT or even much sophistication from a compiler to keep a single hierarchy on its type system.
I suspect the reason Java did it was to not surprise C++ programmers. Solely dictated by marketing, not by technical reasons.
Sure, but that doesn't excuse the well-documented 'sufficiently-smart-compiler fallacy'.
The performance improvements that can be had by the escape-analysis/object-inlining family of JIT compiler optimisations, are considerable, but even today, production JVMs don't do a very good job. It's not an easy problem to solve well.
> I suspect the reason Java did it was to not surprise C++ programmers. Solely dictated by marketing, not by technical reasons.
I sincerely doubt it. You're wrong to dismiss the performance question.
Java isn't just a language but also a VM specification.
In both Java and Java bytecode, primitives are not objects.
The situation is almost the opposite of what you described it. If anything one of the major design points was to solve the surprises inherent to C/C++.
Take integers for example. In java they're defined to be 2s compliment; the processor architecture doesn't matter. C/C++ left the spec open to let the language be 1 to 1 with the medium, Java did not.
You can see the proposals to close this headache: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p090... http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2218.htm
This is similar to all the issues with how threads interact with memory. Java lend the way in making the memory model a part of the language specification rather than allow it to be implementation (and architecture) defined.
http://www.theregister.co.uk/2011/06/11/herb_sutter_next_c_p...
There is a historical reason for this. In the early 1970s, when C was first designed, non-twos complement machines (such as CDC and UNIVAC machines) were still an important part of the industry and so it made sense for C to be designed to allow supporting those machines. By the 1990s, when Java was designed, non-twos complement machines were much less relevant, so it made sense to exclude support for them from the design of Java. Now finally, in the late 2010s, when the relevance of those machines has shrunk even further (although ones-complement Unisys mainframes still exist even today), it makes sense to remove that support from the C standard, even though it made a lot of sense when C was first designed.
Sure, but you're speaking past each other.
* Java was designed to be more predictable and have fewer dark corners than C++ (no undefined behaviour, precisely defined primitives and generally far less platform-specific behaviour, etc)
* Java was designed to feel familiar to C++ developers in order to aid adoption (specifically its syntax)
These aren't in contradiction.
C++ is actually more uniform than Java in this respect because it allows one to define new value types, and also allows heap-allocation of "primitive" types such as int.
java reference vs value distinction is extremely surprising for C++ programmers
Most dynamic language implementations (JavaScript, Lisp, Smalltalk, even Ruby) use a tagged pointer representation allowing integers (and sometimes floats) to be encoded directly in the reference, avoiding heap allocation in this common case.
Another alternate model is to pass the type of a value separately from the value itself, and allow the value to be of variable size.
Java simply made the wrong tradeoff, and while it wasn't fully apparent at the time, there's no good defense of that decision today.
Code that deals with a lot of values which are small but larger than a machine word can be made a lot more efficient if there is a way to treat those values as not objects.
I don't see how that could work for Java. Every Java object can be used as a mutex. This strikes me as very silly, but it's part of Java. Incidentally .Net went the same way, and I'm not the only person who thinks it was a silly decision for both frameworks [0]
Java also tags objects with type information for runtime checking, but that would play ok with tagged pointers as you're describing, as far as I can see.
Seems to me .Net generally has the right idea on types. Primitives are not objects, but you can do List<int> without autoboxing.
> Another alternate model is to pass the type of a value separately from the value itself, and allow the value to be of variable size.
Wouldn't that bloat the stack considerably? Wouldn't it be better to have a type-system that eliminates the need for that sort of thing?
> Java simply made the wrong tradeoff, and while it wasn't fully apparent at the time, there's no good defense of that decision today.
Are there any modern frameworks at all similar to Java, that do as you describe? Dog-slow dynamic languages aren't really the same beast.
It's simply performance/space. It's why value types still exist in Java, C#, etc.
> A type hierarchy, patterned similar to Ruby’s as an example, makes the most sense:
The type hierarchy doesn't matter. That's what C# does. It has one unified type system where even int, double, etc all ultimately derived from Object and it still have value types. It's a language/compiler design issue.
> Then there is no boxing/unboxing of simple types or literals because they are one-and-the-same, and no there’s no confusion about how to interact with any truly generic type of value.
In ruby there is no unboxing, but you could argue that all simple types are "lightly boxed" because they are objects by default.
C++ allows user-defined value types that will behave like fundamental types.
And it has raw pointers/references to them.
Among all, there was a user-defined value type invented: the shared pointer (which works kinda like a GC, or exactly like a GC if you implement it and throw RAII out of the window).
Java took only the fundamental types and a shared pointer. No wonder that there are some parts missing.
The options we have right now are bad: the long constructor arguments are too verbose, the setters are causing non-immutability and builders are worst due to lack of discoverability and standards.