Question

I am trying to use a common technique to create objects from Xml. (Xml is legacy, so although there are already libraries to do this, it seemed faster to write this myself.)

I don’t understand the compiler’s complaint about the generic usage. Code sample:

public void createObjects() {
  List<Object1> objectOnes = new ArrayList<Object1>();
  List<Object2> objectTwos = new ArrayList<Object2>();

  parseObjectsToList("XmlElement1", objectOnes);
  parseObjectsToList("XmlElement2", objectTwos);
}

private void parseObjectsToList(String xmlTag, List<? extends Object> targetList) {
   // read Xml and create object using reflection
   Object newObj = createObjectFromXml(xmlTag);
   targetList.add(newObj)  

/* compiler complains: "The method add(capture#2-of ? extends Object) in the type List<capture#2-of ? extends Object> is not applicable for the arguments (Object)" 
*/

/* If I change method signature to parseObjectsToList(String xmlTag, List targetList)
it works fine, but generates compiler warning about raw type */

}

Thanks for any enlightenment on the subject!

Answer 1

Using a List<Object> will work, but you might want keep your more precisely typed List<Object1> and List<Object2> for type-safety elsewhere. In that case, you’ll need to check the type of each object before adding it to the List.

private void parseObjectsToList(String tag, List<T> list, Class<? extends T> c) {
   // read Xml and create object using reflection
   Object newObj = createObjectFromXml(tag);
   list.add(c.cast(newObj))  ;
}

The cast() operation is a reflective equivalent to the static cast operator: (T) newObj

Using the altered method would look something like this:

parseObjectsToList("XmlElement1", objectOnes, Object1.class);