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A class can only extend one superclass, and thus, only one abstract class.
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If you want to compose several classes the Scala way is to use mixin class composition: you combine an (optional) superclass, your own member definitions and one or more traits. A trait is restricted in comparison to classes in that it cannot have constructor parameters (compare the scala reference manual).
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The restrictions of traits in comparison to classes are introduced to avoid
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typical problems with multiple inheritance. There are more or less
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complicated rules with respect to the inheritance hierarchy; it might be best to avoid a hierarchy where this actually matters. ;-) As far as I View more
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scala - Difference between Abstract Class and Trait signature / two variables with the same name from two different traits. Stack Overflow
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understand, it can only matter if you inherit two methods with the same
(/ask/v/topic/scala-Difference-between-Abstract-Class-andYou don't run into complicated scenarios as long as your hierarchy is a Trait/5a92c0501126f453568b6bd7) SCALA ABSTRACT-CLASS TRAITS
tree.
@edit: You can have conflicts without the supertypes inheriting the same 2
If the number of the arguments of a constructor is that large, as you don't trait. Any two methods with same name and parameter (type) list conflict.
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want to repeat the arguments again in the definition, then you should think I think so. It might be noteworthy that Scala resolves such conflicts in a about other pattern for the constructor. well defined way (the "later" trait has preference). Right now, I do not see You can read this SO question and its answer.How many constructor why the same technique can not resolve class multiple inheritance, so I do arguments is too many? The essence of the object-oriented programming not know wether the restrictions of traits are really due to that. does not change whether it's in Java or in Scala. @andershqst That you can abuse something is only a weak reason not to You can also use builder pattern (using a lot of setter methods and call the use something at all. If you want to call something from java code it is build() method), type-safe builder pattern with some amount of trade-off, often a good idea to have it (e.g. the trait) implement a java interface. One or so. advantage of a trait over an interface is that you can extend it later without View more
breaking other code. (You have to recompile it, though.) I have also seen Also, there was (and probably is) a limit on the number of tuple arity, case Scala inheritance argument lists - Stack Overflow nice uses of traits where you can use variations of a basic algorithm by class constructor arguments, function parameters.22 fields limit in case just including a trait. For instance in the standard library 'new TreeSet' (/ask/v/topic/Scala-inheritance-argument-lists/5a767b401126f4a20c8b94e2) SCALA classesWhy are scala functions limited to 22 parameters? I don't know gives you a basic set, 'new Treeset with SynchronizedSet' is synchronized. INHERITANCE PARAMETER-PASSING MULTIPLE-INHERITANCE whether it still exists, but the developers of Scala are trying to remove it. OPTIONAL-PARAMETERS
[SI-7296] Remove arity limit for case classes - Scala Nevertheless, it's a bad habit to make a function that has so many arguments. All classes in Scala have a primary constructor and optionally some At least, you can use/define tuples or case classes for each class to make (/user/login) auxiliary constructors (which must defer to the primary constructor or the constructor simple, but it does not help for optional parameters. Or 0
another auxiliary constructor). maybe curried constructor if you are familiar with functional languages... The issue in your case is that in both cases you've defined the primary constructor as taking no arguments - and then in the second case you try to define an auxiliary constructor with the same signature. This doesn't work, for the same reason that the following wouldn't compile:
// Primary constructor takes a String class Foo(s: String) { View more // Auxiliary constructor also takes a String?? (compile erro r) Scala constructor without parameters - Stack Overflow def this(a: String) { this(a) (/ask/v/topic/Scala-constructor-without-parameters/556d144f2bd273263e8b97b7) } SCALA }
This isn't anything to do with the fact that the constructor is no-args; the 0
You can simply have an overloaded constructor: following compiles for example:
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new Trainer(4) class Foo(s: String) { new Trainer(4, 5.0) // alternative no-arg constructor (with different signature from primary) def this() { this("Default value from auxiliary constructor") //optionally make the constructor protected or private, so the o } nly way to instantiate is using the companion object } class Trainer private(data:Int, model:Double) object Trainer { In particular, in your second example, your comment "the only
def apply(data:Int, model:Double) = new Trainer(data, model) constructor" is wrong. Auxiliary constructors are always secondary to the def apply(data:Int) = new Trainer(data, init(data)) } primary constructor, and cannot ever be the only constructor. View more
constructor - scala: optional default argument as Then you can instantiate using FWIW, the first example is the only option open to you, but it looks fine to function of other ar... me. If you've just started using Scala I'm sure it will start to feel right soon (/ask/v/topic/constructor-scala%3A-optional-default-argument-as-function-of-otherTrainer(4) enough - and it's important to eschew Java-esque ways of doing things arguments/557191402bd273922e8b5b35) SCALA CONSTRUCTOR Trainer(4, 5.0)
when there are more idiomatic alternatives. DEFAULT-VALUE
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Another approach is to use an Option with a default of None, and then Auxiliary constructors can also defer to other auxiliary constructors initialize a private variable in the class body: defined before. The JSON reads macro won't take default constructor values into account
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when generating Reads[Profile]. So if you want to still use the macro, your @jean-phillipe: Thanks, you're right. I just realised that wasn't accurate in class Trainer(data:Int, model:Option[Double] = None) { val modelValue = model.getOrElse(init(data)) only choice is to make sort an Option[Long]. If you want to provide a the broader scope myself; post corrected now. }
default value, then I suggest you go with JSON combinators: Thanks, something bothered me about Scala constructors from the start, new Trainer(5) but it never got in my way too much... it's all clear now. import play.api.libs.json._ new Trainer(5, Some(4.0)) import play.api.libs.functional.syntax._
There is also the possibility of making the primary constructor private. implicit val reads: Reads[Profile] = ( apply The syntax is class Foo private ().
(__ \ "name").read[String] and (__ \ "type").read[String] and (__ \ "value").read[String] and def apply(data: Int, model: Double) = trainer(data, Some(model)) (__ \ "sort").read[Long].orElse(Reads.pure(0L)) View more )(Profile.apply _) So many great ways of doing this! more scala - How can I use Json.reads to deserialize JSON
into a case class... If sort isn't found within a JSON object, orElse will be applied, which in
(/ask/v/topic/scala-How-can-I-use-Json.reads-to-deserialize-JSON-into-a-case-classthis case I use Reads.pure to provide a fixed default value. with-optional-constructor-parameters/5592d2f57d3563cd1d8b866d) JSON SCALA
Nice, I was dancing around orElse but I couldn't quite figure it out PLAYFRAMEWORK
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A class can only extend one superclass, and thus, only one abstract class.
(/user/login)
If you want to compose several classes the Scala way is to use mixin class composition: you combine an (optional) superclass, your own member definitions and one or more traits. A trait is restricted in comparison to classes in that it cannot have constructor parameters (compare the scala reference manual). The restrictions of traits in comparison to classes are introduced to avoid typical problems with multiple inheritance. There are more or less complicated rules with respect to the inheritance hierarchy; it might be best to avoid a hierarchy where this actually matters. ;-) As far as I View more
understand, it can only matter if you inherit two methods with the same
scala - Difference between Abstract Class and Trait signature / two variables with the same name from two different traits. Stack Overflow
(/ask/v/topic/scala-Difference-between-Abstract-Class-andYou don't run into complicated scenarios as long as your hierarchy is a Trait/559d5e037d3563e2758b81d9) SCALA ABSTRACT-CLASS TRAITS
tree.
@edit: You can have conflicts without the supertypes inheriting the same 0
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Your test method is called in the main constructor first thing. There is no trait. Any two methods with same name and parameter (type) list conflict.
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way another constructor might avoid it being called before its own code I think so. It might be noteworthy that Scala resolves such conflicts in a runs. well defined way (the "later" trait has preference). Right now, I do not see In your case, you should simply reverse which constructor does what. why the same technique can not resolve class multiple inheritance, so I do Have the main constructor have the string parameter, and the auxiliary one not know wether the restrictions of traits are really due to that. set it to null. Added gain, you can declare the var directly in the parameter Interesting reading. Honestly I can't see why one would use traits then, list. even though Odersky et al recommends them..? My reasons: 1. traits are In general, the main constructor should be the more flexible one, typically not always 'callable' from java code. 2. And most importantly. By being assigning each field from a parameter. Scala syntax makes doing exactly able to extend from multiple traits, we can destroy code if we are not View more
that very easy. You can make that main constructor private if need be. careful with our naming.
Scala problem optional constructor - Stack Overflow (/ask/v/topic/Scala-problem-optional-constructor/556d12802bd273263e8b7db0) SCALA CONSTRUCTOR
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You need to make sort an Option but then null or non included value
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would be none . If that does not work you would then have to write your own reads to set default value in case of null / missing sort
scala - How can I use Json.reads to deserialize JSON into a case class... (/ask/v/topic/scala-How-can-I-use-Json.reads-to-deserialize-JSON-into-a-case-classwith-optional-constructor-parameters/5592d2f57d3563cd1d8b8672) JSON SCALA PLAYFRAMEWORK
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