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.
Classes and objects can extend traits, but traits cannot be instantiated and therefore have no parameters.
In scala, trait is a collection of abstract and non-abstract methods. You can create trait that can have all abstract methods or some abstract and some non-abstract methods. A variable that is declared either by using val or var keyword in a trait get internally implemented in the class that implements the trait.
Traits in Scala have a lot of similarities with interfaces in Java, but a trait is more powerful than an interface because it allows developers to implement members within it. The trait is a combination of abstract and non-abstract methods. Trait can not be instantiated, thus it has no parameters.
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
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