Question

Attempting to generics-ify some legacy code, I’m stuck. I have a ParentObject which wraps a ChildObject to provide group-type operations on the ChildObject. Most usefully, it allows iteration over the collection of child objects.
When I try to add some generics to the mix, I can’t work out how to make it play friendly with the iterator method without either a “naming clash” error, or in the example below, a “The return type is incompatible with Iterable.iterator()” error.
Any suggestions? (Bonus question – is there a better way to write the avoid thegetChildObjectByIndex() method to avoid type erasure complier warning other than suppressing the warnings?)
Thanks a lot in advance for any help

public class ParentObject implements Iterable<ChildObject> {  
    protected List<? super ChildObject> theChildObjects;  

    @SuppressWarnings("unchecked")  
    public <T extends ChildObject> T getChildObjectByIndex(int idx) {  
        return (T)theChildObjects.get(idx);  
    }  

    public Iterator<? super ChildObject> iterator() {  
        return java.util.Collections.unmodifiableCollection(this.theChildObjects).iterator();  
    }  

}

Answer 1

If ParentObject only contains one subtype of ChildObject, you could parametrize ParentObject on that type:

public class ParentObject<T extends ChildObject> implements Iterable<T> {
    protected List<T> theChildObjects;

    public T getChildObjectByIndex(int idx) {
        return theChildObjects.get(idx);
    }

    public Iterator<T> iterator() {
        return java.util.Collections.unmodifiableCollection(this.theChildObjects).iterator();
    }
}