defined class implementing interface in groovy but can't call method

Question

I'm using cuke4duke in a grails project. The features/support/env.groovy has

import org.openqa.selenium.htmlunit.HtmlUnitDriver
import org.openqa.selenium.JavascriptExecutor
import com.gargoylesoftware.htmlunit.BrowserVersion
import com.gargoylesoftware.htmlunit.ConfirmHandler
import com.gargoylesoftware.htmlunit.Page
...    
this.metaClass.mixin(cuke4duke.GroovyDsl)
...
public class ConfirmationHandler implements ConfirmHandler {

   boolean handleConfirm(Page page, String message) {
      called = true
      if (text == null || text.length()==0) {
         return answer
      }
      if (message.contains(text)) {
         return answer
      } 
      throw new RuntimeException("Expected '${text}' in confirmation message but got '${message}'")    
   }
   public String text = null
   public boolean answer = false
   public boolean called = false
}
...
Before() {
...
     browser = new HtmlUnitDriver(BrowserVersion.FIREFOX_3_6)
     confirmation = new ConfirmationHandler()
     browser.setConfirmHandler((ConfirmHandler) confirmation) // ERROR !
...
}

It seems the class gets properly compiled, but groovy cannot call setConfirmHandler as it expects a ConfirmHandler... but the provided object's class implements the interface! I checked that "confirmation instanceof ConfirmHandler" prints true.

Note: the HtmlUnit package is written in Java.

Any ideas? (this is the top of the stack trace)

[INFO] org.codehaus.groovy.runtime.InvokerInvocationException: groovy.lang.MissingMethodException: No signature of method: org.openqa.selenium.htmlunit.HtmlUnitDriver.setConfirmHandler() is applicable for argument types: (ConfirmationHandler) values: [ConfirmationHandler@6c08bae7] (NativeException)

Answer 1

Dont you need to do:

browser.getWebClient().setConfirmHandler( confirmation )

As I can't see that HtmlUnitDriver has a setConfirmHandler method...