CODEWITHSUNDEEP
scalasorting
Rodrigo Parra
Rodrigo Parra

Reputation: 11

Sort generic unbound Comparable in Scala

As part of a trivial exercise, I am trying to implement the following Java interface:

public interface Sort {

@SuppressWarnings("rawtypes")
public ArrayList<Comparable> sort(ArrayList<Comparable> table);
}

With a Java class it is extremely easy to achieve. However, Scala's type inference seems to be problematic when dealing with Collections.sort(). Here is my Scala implementation:

class SortImpl extends Sort {
  override def sort(table: util.ArrayList[Comparable[_]]): util.ArrayList[Comparable[_]] = {
    util.Collections.sort(table)
    table
  }
}

And here's the error I get when running some tests (that mainly test the method with ArrayList<Double>, ArrayList<Integer>, etc.)

Error:(20, 22) inferred type arguments [Comparable[_]] do not conform to method sort's type parameter bounds [T <: Comparable[_ >: T]]
util.Collections.sort(table)
                 ^
Error:(20, 27) type mismatch;
 found   : java.util.ArrayList[Comparable[_]]
 required: java.util.List[T]
Note: Comparable[_] >: T, but Java-defined trait List is invariant in type E.
You may wish to investigate a wildcard type such as `_ >: T`. (SLS 3.2.10)
    util.Collections.sort(table)
                          ^

I have found examples dealing with this issue, for a specific class only, such as: Sort unbound Comparable in Scala

Moreover, I can't modify the interface. Is there a way to implement this in Scala?

Upvotes: 0

Views: 123

Answers (1)

pedrofurla
pedrofurla

Reputation: 12783

Try that:

scala> import java.{util => u}
import java.{util=>u}

scala> def sort[T <: Comparable[T]](t: u.ArrayList[T]):u.ArrayList[T] = { 
         u.Collections.sort(t); t 
       }
sort: [T <: Comparable[T]](t: java.util.ArrayList[T])java.util.ArrayList[T]

Upvotes: 1

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