I was writing some code and was going to throw a RuntimeException as a default for something, when I noticed that there are two options for RuntimeException - one from java.lang and one from kotlin:
Inside the kotlin version of this, there's an entire list of other methods which work the same way :
So, from my understanding of this, the one from package kotlin is simply an alias for the java equivalent (correct me if I'm wrong) which leads me to the question :
what is the point of having this alias file and when should you use it over the "standard" java equivalent ? Does this simply save a few imports ?
When using the JDK, these map to JDK classes. When using Kotlin for Javascript, they would map to a specific implementation in the Kotlin Javascript library. Documentation about actual and expect here.
To answer your question, if there's a chance of you porting your code to also work on another platform, always use the kotlin. variant. Otherwise, it doesn't matter.
In my opinion typealias is strong feature provided by Kotlin language.
Why? Because it gives ability to extend code to multiple domains like extension to provide interoperability between any other languages and Kotlin.
It also becomes helpful when providing APIs or SDK with semantic versioning without worrying much about changes affecting on lower versions of APIs.
One good example would be Collections derived from Java to Kotlin language with some additional & powerful methods as typealias (Literally it saves lot of development time and efforts).
Another good example would be multiplatform programming support by Kotlin language that helps you create APIs with actual and expect keywords implementation.
So long story short, I prefer using RuntimeException from Kotlin package to extend support amongst variety of Kotlin versions (You can see newly added classes starting from version 1.3, but doesn't affect existing API).
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I am planning on writing a library that will primarily be consumed from Java. However, I would like to write this in Scala. I have read most of the documentation on Java/Scala interop, but it is mostly focused on using existing libraries, rather than best practices to ensure seamless interop when writing libraries.
What are some ways of doing this effectively? Ideally, the consumers would not know the library was written in Scala at all.
Are there any other major libraries that do this?
My current plan is to have an API that exposes everything needed with Scala types/features, then having a smaller layer on top of this that converts to Java types. Are there any issues with this approach?
Is there any reason you would want to do this? It's one thing to write a Scala library that can also be used from Scala, but if you want to target Java specifically it probably just makes sense to use Java to write the library.
Ideally, the consumers would not know the library was written in Scala at all.
The biggest issues with this is going to be transient dependencies. Even if you use no third party libraries from yours, what about the Scala standard library? If your library would be used in a project that wasn't otherwise using Scala, they would need to pull in the entire Scala library as well unless you don't use the standard lib at all in your code.
You are going to want to use Java collections instead of Scala collections, and since that's what your client code will be expecting. You should have no Scala Standard Library classes in your public interface. This also means use Java Functional Interfaces instead of Scala Function Types
Avoid companion objects except for situations where they compile down to static members on a class. Accessing a Scala companion object from Java is cumbersome.
Use Java Bean conventions for getters/setters/case classes instead of the regular Scala properties (I believe Scala has an annotation for this).
As far as existing libraries, the Spark Java API is written in Scala, but Spark is primary build for Scala with some Java support, but maybe worth looking at (especially to see the differences between the Scala and Java apis.
The only concerns I found are needing to included the Scala runtime library, and to append _2.13 to the artifact names.
By publishing via Maven (sbt publishM2), even the Scala library comes automatically to the downstream project.
I am working with Java 8 mainly, but meanwhile also creating a library that uses Java 6, such that other people can use it as well in the future, as it is quite interesting.
The problem I have now is that I could very easily solve some issue by using Java 8's Predicate<T>, however I am unsure how to backport it.
I see the following options available, but they either have issues or I'm unsure how to use them:
Use Google Guava's Predicate<T>, this however introduces a relatively big dependency where I do not really need it, also when a Java 8 user wants to use Predicate, then Google Guava's import for the Predicate class shows up.
Use my own Predicate<T>, no big dependency, still the same issues as mentioned above.
Use a custom name like TessPredicate<T>, as Tess will be relevant name in my project, does not feel that nice either.
Use a name that makes sense in the project setting, such as (tentative) RegexVerficationPredicate, as it is a predicate in addition to using a regular expression, such that you can also do calculations on the elements. Bank codes, etc. usually have some checksum that you need to compute. Implemented as functional interface, this might be most feasible?
Backport java.util.function from Java 8 to Java 6, is this even possible?
How can I solve this?
You can’t backport the java.util.function package due to the heavy use of default and static methods within these interfaces. Such a backport would look quite different.
I recommend creating your own Predicate<T> interface being as minimal as possible, i.e. having that single abstract method with the same signature as the Java 8 Predicate<T>. Having the same interface name and method signature like the well-known acts like a self-documentation.
This implies that programmers using Java 8 can still implement your predicate using a lambda expression or method reference (without even importing your interface). And using a Java 8 predicate is as easy as passing predicate::test to your method.
Adding a dependency to an entire 3rd party library just for one interface looks nasty to me.
I'd recommend use of Guava. When I first used it, I was thinking the same way (too big dependency) then over time I started to use other features provided by guava and now I dont understand how do I do without them (those are my must have tools right now), code is clearer, faster and easily maintainable.
The fact that java SDK (especially java 8) is taking lot of features from Guava tells a lot... thus even tough you can write your own implementation, in a long term use of the library is more preferable...
I'm developing an open source library in Java and would like to ensure that it is convenient for Java 8 users, and takes advantage of new concepts in Java 8 wherever possible (lambdas etc.)
At the same time I absolutely need to maintain backwards compatibility (the library must still be usable for people using Java 6 or 7).
What useful features from Java 8 can I adopt that would be beneficial for library users without breaking library compatibility for users of older Java versions?
I don't know about your library, this advice might be slightly off.
Lambdas: Don't worry. Any functional interface can be implemented using a Lambda expression.
Method references: Same as lambdas, they should just be usable.
Streams: If this fits your library, you should use them, but keeping compatibility is harder here. The backwards compatibility could be achieved using a second library part, wrapping around the base library and hooking into the public API of it. It could therefore provide additional sugar/functionality without abandoning Java 6/7.
Default methods: By all means, use these! They are a quick/cheap/good way to enhance existing implementations without breaking them. All default methods you add will be automatically available for implementing classes. These will, however, also need the second library part, so you should provide the base interfaces in your base library, and extend the interfaces from the companion-library.
Don't fork the library, abandoning the old one, as there are still many developers who cannot use Java 8, or even Java 7. If your library is sensible to use on e.g. Android, please keep that compatibility.
If you want your code to be usable by Java 6 consuming VMs, you have to write using Java 6 language compatibility. Alas. The bytecode format critically changed from 6 to 7, so code for 7 and 8 won't load into 6. (I found this with my own code migrating to 7; even when all I was using was multi-catch — which should be writable in the 6 bytecode — I couldn't build it for a 6 target. The compiler refused.)
I've yet to experiment with 8, but you'll have to be careful because if you depend on any new Java packages or classes, you will still be unable to work with older versions; the old consuming VMs simply won't have access to the relevant classes and won't be able to load the code. In particular, lambdas definitely won't work.
Well, can we target a different classfile version? There's no combination of source and target that will actually make javac happy with this.
kevin$ $JAVA8/bin/javac -source 1.8 -target 1.7 *.java
javac: source release 1.8 requires target release 1.8
So there's simply no way to compile Java source with lambdas into a pre-Java 8 classfile version.
My general take is that if you want to support old versions of Java, you have to use old versions of Java to do so. You can use a Java 8 toolchain, but you need Java 7 or Java 6 source. You can do it by forking the code, maintaining versions for all the versions of Java you want to support, but that's far more work than you could ever justify as a lone developer. Pick the minimum version and go with that (and kiss those juicy new features goodbye for now, alas).
If you use any new language features in Java 8, it requires also using Java 8 bytecode.
$ javac -source 1.8 -target 1.7
javac: source release 1.8 requires target release 1.8
That means your options are quite limited. You cannot use lambdas, method references, default methods, Streams, etc. and maintain backwards compatibility.
There are still two things you can do that users of Java 8 will benefit from. The first is using Functional Interfaces in your public API. If your methods take Runnables, Callables, Comparators, etc. as parameters, then users of Java 8 will be able to pass in lambdas. You may want to create your own Single-Abstract-Method interfaces as well. If you find you need Functions and Predicates, I'd suggest reusing the ones that ship with GS Collections or Guava instead of writing your own.
The second thing you can do is use a rich collections API that benefits from using Functional Interfaces. Again, that means using GS Collections or Guava. For example, if you have a method that would return List, return MutableList or ImmutableList instead. That way, callers of the method will be able to chain usages of the rich API exposed by these interfaces.
As said by others, providing and using interfaces with a single method such that they can be implemented using lambdas or method references when using Java 8 is a good way of supporting Java 8 without breaking Java 7 compatibility.
This can be complemented by providing methods by your library which fit into one of the standard function types of Java 8 (e.g. Supplier, (Bi)Consumer, (Bi)Function) so that Java 8 developers can create method references to them for Java 8 API methods. This implies that their signature matches one of these functional interfaces and they don’t throw checked exceptions. This often comes naturally, e.g. getFoo() may act as a Function and isBar() as a Predicate but sometimes it’s possible to improve methods by thinking about possible Java 8 use scenarios.
For example, if you provide a method taking two parameters, it’s useful to choose the order where the first parameter is the one that is more likely to be a key in a Map. So it is more likely to be useful for Map.forEach with a method reference.
And avoid method with ambiguous signatures. E.g. if you have a class Foo with an instance method ReturnType bar() and a static method ReturnType bar(Foo) neither of them can be used as method reference anymore as Foo::bar would be ambiguous. Eliminate or rename one of these methods.
It’s important that such methods do not have undocumented internal state that causes surprising behavior when used by multiple threads. Otherwise they can not be used by parallel streams.
Another opportunity that should not be underestimated is to use names for classes, interfaces and members conforming to patterns introduced by the Java 8 API. E.g. if you have to introduce some sort of filter interface with a test method for your library that ought to work with Java 7 as well, you should name the interface Predicate and the method test to associate it with the similar named functional interface of Java 8.
I'm looking for a Java macro language that provides for convenient ways of doing closures (that compile to anonymous inner classes) and list comprehension (that compiles down to basic java loops).
An example of the kind of thing I'm looking for would be Xtend2 http://www.eclipse.org/Xtext/#xtend2
But I want something for general purpose programming (Xtend2 is very specific DSL for Xtext and has a ton of dependencies). Maybe even something that would let me define multiple classes in a single file (which would then get split up into two separate files by the pre-processor).
Does anything like this exist?
Edited to add:
I'm doing Android development so any alternatives have to generate either valid Java source or the byte code has to be compatible with the dalvik recompiler.
Mmm, there used to be the JSE, which was tremendous fun, back in the day.
Mirah is cool, but not ready for primetime, IMO.
You can do a lot with smart templating, although your source view is the Java.
There's a post on SO about using XTend on Android from a few days ago, too.
Frege produces java source code.
I do not know whether dalvik would like it. (But I would be interested to hear ...)
And, of course, you have some runtime library code.
That being said, there are a number of other projects that do closures etc. in java, for example: lambdaj
After reading some OpenJDK mailinglist entries, it seems that the Oracle developers are currently further removing things from the closure proposal, because earlier design mistakes in the Java language complicate the introduction of the Java closures.
Considering that Scala closures are much more powerful than the closures planned for Java 8, I wonder if it will be possible to e. g. call a Java method taking a closure from Scala, defining a closure in Java and giving it to a Scala function etc.?
So will Java closures be represented like their Scala counterparts in bytecode or differently?
Will it be possible to close the gap of functionality between Java/Scala closures?
I think it's more complicated than assuming there's two groups of stakeholders here. The Project Lambda people seem to be working mostly independently of the Oracle people, occasionally throwing something over the wall that the Project Lambda people find out indirectly. (Scala, is of course the third stakeholder.)
Since the latest Project Lambda proposal is to eliminate function types all together, and just create some sort of fancy inference for implementing interfaces that have a single astract method (SAM types), I foresee the following:
Calling Scala code that requires a Scala closure will depend entirely on the implementation of the Function* traits (and the implementation of traits in general) -- whether it appears to the Java compiler as a SAM (which it is in Scala-land) or whether the non-abstract methods also appear abstract to the JVM. (I would think they currently do look like they're abstract since traits are implemented as interfaces, but I'm know almost nothing about Scala's implementation. This could be a big hurdle to interperability.)
Complications with Java generics (in particular how to expressInt/int/Integer, or Unit/Nothing/void in a generic interface) may also complicate things.
Using Scala functions to implement Java SAMs will not be any different than it now -- you need to create an implicit conversion for the specific interface you wish to implement.
If the JVM gets function types (and Oracle seems not to have eliminated that possibility), it may depend how it's implemented. If they're first class objects implementing a particular interface, then all that Scala needs to do to be compatible is make Function* implement the new interface. If a new kind of type is implemented in the JVM entirely, then this could be difficult -- the Scala developers may wrap them using magic like they currently do for Arrays, or they may create create implicit conversions. (A new language concept seems a bit far-fetched.)
I hope that one of the results of all of this discussion is that all of the various JVM languages will agree on some standard way to represent closures -- so that Scala, Groovy, JRuby, etc... can all pass closures back and forth with a minimum of hassle.
What's more interesting to me is the proposals for virtual extension methods that will allow the Java Collections API to use lambdas. Depending on how these are implemented, they may greatly simplify some of the binary compatibility problems that we've had to deal with when changing Scala code, and they may help to more easily and efficiently implement traits.
I hope that some of the Scala developers are getting involved and offering their input, but I haven't actually seen any discussion of Scala on the Project Lambda lists, nor any participants who jump out to me as being Scala developers.
You would likely be able to do this extremely easily using implicit conversions a la collection.JavaConversions whether or not they come out of the box.
Of course, this is not obviously so, because it may be the case that Java closures are turned into types which get generated by the JVM at runtime - I seem to recall a Neal Gafter presentation saying something along these lines
Note: 5 years later, SCALA 2.12.0-M3 (Oct. 2015) did include this enhancement:
Scala 2.12 emits closures in the same style as Java 8.
For each lambda the compiler generates a method containing the lambda body.
At runtime, this method is passed as an argument to the LambdaMetaFactory provided by the JDK, which creates a closure object.
Compared to Scala 2.11, the new scheme has the advantage that the compiler does not generate an anonymous class for each lambda anymore. This leads to significantly smaller JAR files.