@XmlElementRefs & @XmlElementRef annotatinos in Java

Question

Can some please explain to me what do

@XmlElementRefs

and

@XmlElementRef

annotations mean in Java and what is their use..

EDIT: @skaffman

okay, suppose I have one such collection

@XmlElementRefs({
    @XmlElementRef(name="ElementA", type=ClassA),
    @XmlElementRef(name="ElementB", type=ClassB) }
)

List<Object> items;

Now how do I access every individual element of this list? Is the following code correct?

for (int j = 0; j < items.size(); ++j) {
    if (items.get(i).getClass().equals(ClassA)) {
        // perform ClassA specific processing:
    } else if (items.get(i).getClass().equals(ClassB)) {
        // perform ClassB specific processing:
    }
}

Is this correct approach? Is there a better way to perform each class specific processing? I mean is there a way to avoid those if else constructs?

Answer 1

These are used to annotate a collection which can contain various different types. The java binding for such a list is:

@XmlElementRefs({
   @XmlElementRef(name="ElementA", type=ClassA),
   @XmlElementRef(name="ElementB", type=ClassB)
})
List<Object> items

Here, items can contain an arbitrary mix of ClassA and ClassB, and since that can't be expressed in List's type signature, it has to be expressed using annotations.