I think the error is in the example (which should be a documentation, right?)

The command returns a List object NOT an array-list object, so I simply modified to

 

import prohtml.*;

HtmlList htmlList;

void setup(){
  //enter your url here
  htmlList = new HtmlList(" http://forum.processing.org");
 
  List links = htmlList.getLinks();
 
  for(int i = 0;i<links.size();i++){
    println(((Url)(links.get(i))).toString());
  }
}

 

 

That seems to work.