Hey guys,
im working on a project in scala and i encountered very weird problem. This is a part of the code :
class Entity(val id:String){
override def toString = id
}
class RequirementType
case class DependsEither(t:List[(Entity,Int)]) extends RequirementType
case class Depends(t:(Entity,Int)) extends RequirementType
class BuildableEntity(override val id:String,
val mineralCost:Int,
val gasCost:Int,
val buildTime:Int,
val buildCount:Int,
val supplyCount:Int,
val req:List[RequirementType],
val onBuildStart: GameState => GameState,
val onBuildFinish: GameState => GameState
)extends Entity(id)
class SimpleBuilding(id:String,
mineralCost:Int,
gasCost:Int,
buildTime:Int,
req:List[RequirementType]
) extends BuildableEntity(id,mineralCost,gasCost,buildTime,1,0,req:::List(ConsumesOnStart((Drone,1))),{s=>s},{x=>x})
object SpawningPool extends SimpleBuilding("spawningPool",200,0,65,List(DependsEither(List((Hatchery,1),(Lair,1),(Hive,1)))))
object Lair extends SimpleBuilding("lair",150,100,80,List(ConsumesOnFinish(Hatchery,1),Depends(SpawningPool,1)))
object InfestationPit extends SimpleBuilding("infestationPit",100,100,50,List(DependsEither(List((Lair,1),(Hive,1)))))
Now, when i call println(Lair.req), it sometimes prints as
List(ConsumesOnFinish((hatchery,1)), Depends((null,2)), ConsumesOnStart((drone,1)))
and sometimes as
List(ConsumesOnFinish((hatchery,1)),
Depends((spawningPool,2)), ConsumesOnStart((drone,1)))
Please, if anyone has any idea about what could be going wrong, i would love you for ever. I have no clue why is it act as it does. I have more extensions of SimpleBuilding but they seem to be working properly
EDIT:
I should also mention that the outcome changes after compilation. I mean that when i run unit test it sometimes appear as null and sometimes as proper instance.
This is indeed a case of circular dependency and initialization. Here is a shorter version of your issue:
class X(val x: List[X])
object A extends X(List(B))
object B extends X(List(A))
object Main {
def main(args:Array[String]) {
println("A.x: " + A.x)
println("B.x: " + B.x)
}
}
This will print this:
$ scala -cp classes Main
A.x: List(B$#143c8b3)
B.x: List(null)
You can use by names parameter to allow object construction to finish before you use it:
class X(x0: => List[X]) {
lazy val x = x0
}
object A extends X(List(B))
object B extends X(List(A))
The fix works on the small test case:
$ scala -cp classes Main
A.x: List(B$#1feca64)
B.x: List(A$#6d084b)
Based on this you may want to change req:List[RequirementType] to req0: => List[RequirementType] and add a lazy val req = req0.
If that works for you, we should retitle the question to mention object initialization and circular dependencies. Note this is very similar to this question/answer.
Lair use SpawningPool in its constructor and reciprocally. But at that time, the other doesn't exists.
You've got recursive definitions in constructors, and although I believe that is supported, it looks like something's going wrong. Can you try lazy vals instead and see if the problem goes away? That is,
object X extends C("this",that,1) { /* code */ }
becomes
lazy val X = new C("this",that,1) { /* code */ }
Related
I have been bitten a number of times by a situation like this:
trait MyTrait {
val implementMe: String
val upper = implementMe.toUpperCase
}
class MyClass(s: String) extends MyTrait {
override val implementMe: String = s
}
val c = new MyClass("Hello, World")
println(c.upper)
In this example, everything compiles but the last line throws a NullPointerException at runtime, (I assume) because upper is computed before implementMe is actually defined.
I know I can fix it by changing upper to a def or a lazy val but it seems like this should be caught at compile time rather than runtime. Am I doing something wrong/is there a way this sort of problem can be detected at compile time? Is there a reason the compiler allows this?
The problem is initialization, as you've stated. As far as I know, there is no way to detect this at compile time. There is a bit of a trick, called early initialization, that allows you to work around this while avoiding def or lazy val (if that is your goal):
class MyClass(s: String) extends { override val implementMe: String = s } with MyTrait
I want to list out all the case classes which implements a particular trait. I am currently using Clapper ClassUtil for doing that. I am able to get the case classes that are directly implementing a trait. However, I am not able to get the other classes which are not directly implementing the trait. How can I get all classes which directly or indirectly implements a trait. ?
val finder = ClassFinder()
finder.getClasses().filter(_.isConcrete).filter(_.implements("com.myapp.MyTrait"))
Scala Version : 2.11
Clapper Class Util Version : 1.0.6
Is there any other way I can get these information? Can someone point me to the right direction?
I tried using scala.reflect but could not understand how to get the info.
EDIT:
Sample traits and usages:
trait BaseEntity
trait NamedEntity{ val name:String}
trait MasterDataEntity extends NamedEntity
case class Department(id:Long, override val name:String) extends MasterDataEntity
case class Employee(id:Long, name:String) extends BaseEntity
case class User(id:Long, override val name:String) extends NamedEntity
Now, if I give the trait as NamedEntity, I should be able to get both Department and User since they both are directly or indirectly implementing NamedEntity. With implements method, it will give only User. I also tried by using interfaces method, which will also provide the direct super classes only.
Looking at the source code, the problem seems to be that it doesn't follow the interfaces hierarchy. If you do that, you find all instances:
package foo
import java.io.File
import org.clapper.classutil.{ClassFinder, ClassInfo}
object Main extends App {
val jar = new File("target/scala-2.11/class_test_2.11-0.1.0.jar")
val finder = ClassFinder(jar :: Nil)
val classes = ClassFinder.classInfoMap(finder.getClasses().iterator)
val impl = find("foo.NamedEntity", classes)
impl.foreach(println)
def find(ancestor: String, classes: Map[String, ClassInfo]): List[ClassInfo] =
classes.get(ancestor).fold(List.empty[ClassInfo]) { ancestorInfo =>
val ancestorName = ancestorInfo.name
def compare(info: ClassInfo): Boolean =
info.name == ancestorName ||
(info.superClassName :: info.interfaces).exists {
n => classes.get(n).exists(compare)
}
val it = classes.valuesIterator
it.filter { info => info.isConcrete && compare(info) } .toList
}
}
ClassUtil now contains this functionality (v1.4.0, maybe also in earlier versions):
val finder = ClassFinder()
val impl = ClassFinder.concreteSubclasses("foo.NamedEntity", finder.getClasses())
I have a number of use cases for this, all around the idea of interop between existing Java libraries and new Scala Code. The use case I've selected is the easiest I think.
Use Case:
I working on providing a JUnit Runner for some scala tests (so that I can get my lovely red / green bar in Eclipse)
The runner needs to have a constructor with a java class as a parameter. So in Scala I can do the following:
class MyRunner(val clazz: Class[Any]) extends Runner {
def getDescription(): Description
def run(notifier: RunNotifier)
}
When I use either
#RunWith(MyRunner)
object MyTestObject
or
#RunWith(MyRunner)
class MyTestClass
then the runner is indeed instantiated correctly, and is passed a suitable class object
Unfortunately what i want to do now is to "get hold of" the object MyTestObject, or create a MyTestClass, which are both Scala entities. I would prefer to use Scala Reflection, but I also want to use the standard Junit jar.
What I have done
The following Stackover flow questions were educational, but not the same problem. There were the nearest questions I could find
How to create a TypeTag manually?
Any way to obtain a Java class from a Scala (2.10) type tag or symbol?
Using Scala reflection with Java reflection
The discussion on Environments, Universes and Mirrors in http://docs.scala-lang.org/overviews/reflection/environment-universes-mirrors.html was good, and the similar documents on other scala reflection also helped. Mostly through it is about the Scala reflection.
I browsed the Scaladocs, but my knowledge of Scala reflection wasn't enough (yet) to let me get what I wanted out of them.
Edit:
As asked here is the code of the class that is being created by reflection
#RunWith(classOf[MyRunner])
object Hello2 extends App {
println("starting")
val x= "xxx"
}
So the interesting thing is that the solution proposed below using the field called MODULE$ doesn't print anything and the value of x is null
This solution works fine if you want to use plan old java reflection. Not sure if you can use scala reflection given all you will have is a Class[_] to work with:
object ReflectTest {
import collection.JavaConversions._
def main(args: Array[String]) {
val fooObj = instantiate(MyTestObject.getClass())
println(fooObj.foo)
val fooClass = instantiate(classOf[MyTestClass])
println(fooClass.foo)
}
def instantiate(clazz:Class[_]):Foo = {
val rm = ru.runtimeMirror(clazz.getClassLoader())
val declaredFields = clazz.getDeclaredFields().toList
val obj = declaredFields.find(field => field.getName() == "MODULE$") match{
case Some(modField) => modField.get(clazz)
case None => clazz.newInstance()
}
obj.asInstanceOf[Foo]
}
}
trait Foo{
def foo:String
}
object MyTestObject extends Foo{
def foo = "bar"
}
class MyTestClass extends Foo{
def foo = "baz"
}
I've looked at example of logging in Scala, and it usually looks like this:
import org.slf4j.LoggerFactory
trait Loggable {
private lazy val logger = LoggerFactory.getLogger(getClass)
protected def debug(msg: => AnyRef, t: => Throwable = null): Unit =
{...}
}
This seems independent of the concrete logging framework. While this does the job, it also introduces an extraneous lazy val in every instance that wants to do logging, which might well be every instance of the whole application. This seems much too heavy to me, in particular if you have many "small instances" of some specific type.
Is there a way of putting the logger in the object of the concrete class instead, just by using inheritance? If I have to explicitly declare the logger in the object of the class, and explicitly refer to it from the class/trait, then I have written almost as much code as if I had done no reuse at all.
Expressed in a non-logging specific context, the problem would be:
How do I declare in a trait that the implementing class must have a singleton object of type X, and that this singleton object must be accessible through method def x: X ?
I can't simply define an abstract method, because there could only be a single implementation in the class. I want that logging in a super-class gets me the super-class singleton, and logging in the sub-class gets me the sub-class singleton. Or put more simply, I want logging in Scala to work like traditional logging in Java, using static loggers specific to the class doing the logging. My current knowledge of Scala tells me that this is simply not possible without doing it exactly the same way you do in Java, without much if any benefits from using the "better" Scala.
Premature Optimization is the root of all evil
Let's be clear first about one thing: if your trait looks something like this:
trait Logger { lazy val log = Logger.getLogger }
Then what you have not done is as follows:
You have NOT created a logger instance per instance of your type
You have neither given yourself a memory nor a performance problem (unless you have)
What you have done is as follows:
You have an extra reference in each instance of your type
When you access the logger for the first time, you are probably doing some map lookup
Note that, even if you did create a separate logger for each instance of your type (which I frequently do, even if my program contains hundreds of thousands of these, so that I have very fine-grained control over my logging), you almost certainly still will neither have a performance nor a memory problem!
One "solution" is (of course), to make the companion object implement the logger interface:
object MyType extends Logger
class MyType {
import MyType._
log.info("Yay")
}
How do I declare in a trait that the
implementing class must have a
singleton object of type X, and that
this singleton object must be
accessible through method def x: X ?
Declare a trait that must be implemented by your companion objects.
trait Meta[Base] {
val logger = LoggerFactory.getLogger(getClass)
}
Create a base trait for your classes, sub-classes have to overwrite the meta method.
trait Base {
def meta: Meta[Base]
def logger = meta.logger
}
A class Whatever with a companion object:
object Whatever extends Meta[Base]
class Whatever extends Base {
def meta = Whatever
def doSomething = {
logger.log("oops")
}
}
In this way you only need to have a reference to the meta object.
We can use the Whatever class like this.
object Sample {
def main(args: Array[String]) {
val whatever = new Whatever
whatever.doSomething
}
}
I'm not sure I understand your question completely. So I apologize up front if this is not the answer you are looking for.
Define an object were you put your logger into, then create a companion trait.
object Loggable {
private val logger = "I'm a logger"
}
trait Loggable {
import Loggable._
def debug(msg: String) {
println(logger + ": " + msg)
}
}
So now you can use it like this:
scala> abstract class Abstraction
scala> class Implementation extends Abstraction with Loggable
scala> val test = new Implementation
scala> test.debug("error message")
I'm a logger: error message
Does this answer your question?
I think you cannot automatically get the corresponding singleton object of a class or require that such a singleton exists.
One reason is that you cannot know the type of the singleton before it is defined. Not sure, if this helps or if it is the best solution to your problem, but if you want to require some meta object to be defined with a specific trait, you could define something like:
trait HasSingleton[Traits] {
def meta: Traits
}
trait Log {
def classname: String
def log { println(classname) }
}
trait Debug {
def debug { print("Debug") }
}
class A extends HasSingleton[Log] {
def meta = A // Needs to be defined with a Singleton (or any object which inherits from Log}
def f {
meta.log
}
}
object A extends Log {
def classname = "A"
}
class B extends HasSingleton[Log with Debug] { // we want to use Log and Debug here
def meta = B
def g {
meta.log
meta.debug
}
}
object B extends Log with Debug {
def classname = "B"
}
(new A).f
// A
(new B).g
// B
// Debug
I need access to the Class of the object being constructed in its own constructor (for various detailed reasons I don't think are relevant to my question).
I want something like this
class Foo(val i:Int)
class Bar extends Foo(this.getClass.getName.length)
val b = new Bar
println(b.i)
to print 3 ("Bar".length). But it doesn't. If the code above is inside some other object, "this" refers to that object. If the code above is not inside some other object (just in some package), the compiler complains with
error: this can be used only in a class, object, or template
class Bar extends Foo(this.getClass.getName)
^
CLARIFICATION: I can't change Foo to use a val in its body instead of its constructor because Foo's API already exists and is fixed (so, yes, i must be a constructor parameter). It needs an integer argument at constructor time, but that integer can only be calculated with access to the Class.
(I know the example above is still silly and degenerate. If people care, I can explain in detail why I need the class in my real project, http://code.google.com/p/factorie)
Surely, the class of the object being constructed is known to the compiler and runtime during construction. With what syntax can I get it? (Is there no such syntax? If not, I wonder why. I'm surprised it there doesn't seem to be a simple, standard way to get this.)
A lazy val solves this problem:
object Early
{
abstract class Foo { val name: String }
class Bar extends Foo { lazy val name = getClass.getName }
def
main(args: Array[String]): Unit = {
val b = new Bar
println(b.name)
}
}
Yields:
% scala Early
Early$Bar
Not sure whether this is possible in a clean way. If you like hacks you could do
class Bar extends Foo((new Exception).getStackTrace.apply(0).getClassName)
However I strongly advertise against it!
This appears to satisfy your requirement without using a lazy val and without altering the base class:
scala> class Base(val name: String)
defined class Base
scala> class Derived extends Base(classOf[Derived].getName)
defined class Derived
scala> new Derived name
res0: String = Derived
You are going to have to explain your motivation for wanting to do this. Does name have to be a constructor parameter of Foo or can it be an abstract member? Does it have to be a val or will a def do.
You can do this
class Foo(val name: String)
class Bar extends Foo("") {
override val name = getClass.getName
}
and new Bar().name will give Bar
But I suspect that if your true motivation is known then there is a better way to do what you really want.
How about
class Bar extends Foo(classOf[Bar].getName.length)
What about this:
class Foo(otherName: Option[String] = None) {
val name = otherName.getOrElse(this.getClass.getName)
}
class Bar extends Foo()
val b = new Bar
println(b.name)