Extending Java Class with Scala Trait

Question

I would like to define a class ContextItem as the extension of java class Predicate with a trait Confidence.

Confidence is a simple trait which simply adds a confidence field to whatever it extends.

trait Confidence{
  def confidence:Double
}

I am defining my ContextItem class by simply stating:

class ContextItem extends Predicate with Confidence{}

But attempting to compile this yields...

com/slug/berds/Berds.scala:11: error: overloaded method constructor Predicate with     alternatives:
  (java.lang.String,<repeated...>[java.lang.String])com.Predicate <and>
  (java.lang.String,<repeated...>[com.Symbol])com.Predicate <and>
  (java.lang.String,java.util.ArrayList[com.Symbol])com.Predicate <and>
  (com.Predicate)com.Predicate <and>
  (com.Term)com.Predicate <and>
  (java.lang.String)com.Predicate
 cannot be applied to ()
class ContextItem(pred:Predicate) extends Predicate with Confidence{
             ^

This seems like a trivial example, so what's going wrong?

Predicate (which is not mine) looks like:

/** Representation of predicate logical form. */
public class Predicate extends Term implements Serializable {
    public Predicate(String n) {
        super(n);
    }
    public Predicate(Term t) {
        super(t);
    }
    public Predicate(Predicate p) {
        super((Term)p);
    }
    public Predicate(String n, ArrayList<Symbol> a) {
        super(n, a);
    }
    public Predicate(String n, Symbol... a) {
        super(n, a);
    }
    public Predicate(String n, String... a) {
        super(n, a);
    }
    @Override
    public Predicate copy() {
        return new Predicate(this);
    }
}

Neither Predicate nor any of its ancestors implements confidence.

Could we see the Predicate class? Does it implement the confidence method? — Nick H
– Nick H, Commented Apr 17, 2013 at 20:55

Nick H · Accepted Answer · 2013-04-17 21:00:00Z

6

I think it is listing all the constructors of Predicate, and informing you that you're not using any of them. The default is to use a parameterless constructor, which doesn't exist here. The syntax to call, for example, the (String) super-constructor, would be

class ContextItem extends Predicate("something") with Confidence

or

class ContextItem(str: String) extends Predicate(str) with Confidence

Also, at the moment your def confidence is an abstract method, so won't compile until you give it a definition. If you intended the trait to add a writable confidence field then this is what you want instead:

var confidence: Double = 0.0

answered Apr 17, 2013 at 21:00

Nick H

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