Dom4j: error "incompatible types" on compilation

Question

I am developing a small desktop application in Java. I came across a point where i need to read data from XML file, for this i am using Dom4j library. While coding i am facing the following error could anyone guide me in resolving this error:

public void FromXML(String sXMLFileURI)
    {//Reads the XML File and Stroe Data in Calling Object
      Document document = getDocument( sXMLFileURI );
      String xPath = "myXpath";
      List<Node> nodes = document.selectNodes( xPath );//This line gives the followiing error:

//error "incompatible types
//required: java.util.List<org.dom4j.Node>
//found:    java.util.List<capture#1 of ? extends org.dom4j.Node>"
          for (Node node : nodes)
          {   
             //some processing here
          }
        }

Answer 1

Since the method signature is

List<? extends Node> selectNodes(String)

your variable nodes should be of type List<? extends Node>, and not of type List<Node>.

A List<Node> accepts any Node instance as element. Whereas a List<? extends Node> is a List<Node>, or a List<Element>, or a List<Attribute>, or a list of some other subclass of Node. You just don't know which one.

Answer 2

It's a consequence of the fact that java generic collections are not "covariant". Method selectNodes() returns a list of Objects all of which implement Node. But this is not a List<Node>. You have to change the declaration to

List<? extends Node> nodes=...