CODEWITHSUNDEEP
scalagenerics
calvin
calvin

Reputation: 2965

How can I initialize a generic variable in scala

As we all know, we can initialize a object of template parameter T in C++.

template <typename T>
struct Cont{
    T t;
}

I want to do the same in scala, so I write the following codes. There are some other classes which extends Evaluator, and EvaluateComponent is something like a wrapper. I can write code like new EvaluateComponent[BinaryClassificationEvaluator].evaluate(df)

abstract class Evaluator extends Serializable{
  var predCol = "prediction"
  var trueCol = "label"
  def evaluate(dataset: DataFrame) : String
  def evaluateRDD(dataset: RDD[(Double, Double)]) : String
}

class EvaluateComponent[E <: Evaluator : ClassTag] {
  val evaluator = new E
}

class BinaryClassificationEvaluator extends Evaluator {
  ...
}

However it doesn't compile with error class type required but E found. The post does not solve my question because I want evaluator to be initialized.

Upvotes: 0

Views: 481

Answers (1)

SwiftMango
SwiftMango

Reputation: 15304

The limitation in Scala is the same as Java - generic type is erased at compile time, and no direct access to a class constructor like C++. But it can be done, through Java reflection:

abstract class Evaluator extends Serializable{}

class EvaluateComponent[E <: Evaluator : ClassTag] {
  val evaluator = implicitly[ClassTag[E]].runtimeClass.newInstance().asInstanceOf[E]
}

class BinaryClassificationEvaluator extends Evaluator {}

new EvaluateComponent[BinaryClassificationEvaluator].evaluator // Returns a BinaryClassificationEvaluator

This being said doesn't mean it is a good approach. The takeaways are:

  • The passed class in the generic parameter must have a default constructor. There will be runtime exception if it doesn't
  • Reflection is slower than normal code

Different language program in different mentality. It is better to implement this using type class in Scala but it's another topic.

EDIT:

Class.newInstance is deprecated now. Use Class.getDeclaredConstructor().newInstance() instead

Upvotes: 2

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