Generics (List) typing question

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

All a wildcard type means is that the actual type parameter T of the List that you pass as the second argument to parseObjectsToList is going to be a subtype of Object. It does NOT mean that the same List will be parameterized with different types.

So now you have a List<T> (called targetList) and you are trying to call targetList.add(Object). This is illegal because Object is not necessarily a subtype of T.

Because you are adding to the List rather than extracting elements from it, use List<Object> and make sure that's exactly what you pass in.

Answer 2

The problem you are running into is that, with the bounded wildcard that you have defined, you will be unable to add any element to the collection. From this tutorial:

List<? extends Shape > is an example of a bounded wildcard. The ? stands for an unknown type, just like the wildcards we saw earlier. However, in this case, we know that this unknown type is in fact a subtype of Shape. (Note: It could be Shape itself, or some subclass; it need not literally extend Shape.) We say that Shape is the upper bound of the wildcard.

There is, as usual, a price to be paid for the flexibility of using wildcards. That price is that it is now illegal to write into shapes in the body of the method

Answer 3

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);

Answer 4

Think about what you are asking the compiler to do:

1. Given a list of "something that is a subtype of Object
2. Let me insert an Object into it

This doesn't make sense. Suppose your list is a list of Integer. Suppose that createObjectFromXml returns a String. It wouldn't make sense to allow inserting a String into a list typed for Integers.

So, your options are either to make your List a List<Object> or to find some way to make createObjectFromXml return a specific type, that you can then tie to the type of your list.