" ? extends Classifier " not assignable to Classifier

Question

I am just trying to understand this
My method is like this
public List<? extends Classifier> getClassifiers(){
Now if I use the loop like this I get the error "Type mismatch: cannot convert from element type ? extends Classifier to Classifier"
for( Classifier c : getClassifiers()){
I was hoping that a subtype of Classifier is assignable to Classifier.

Answer 1

bounded wildcard is useful as an argument type so for example a method that processes List<Shape> can be passed a List<Rectangle>
-- quote -- from http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf
Any drawing will typically contain a number of shapes. Assuming that they are
represented as a list, it would be convenient to have a method in Canvas that draws
them all:public void drawAll(List<Shape> shapes) {
    for (Shape s: shapes) {
        s.draw(this);
}[i]Now, the type rules say that drawAll() can only be called on lists of exactly Shape:
it cannot, for instance, be called on a List<Circle>. That is unfortunate, since all
the method does is read shapes from the list, so it could just as well be called on a
List<Circle>. What we really want is for the method to accept a list of any kind of
shape:public void drawAll(List<? extends Shape> shapes) {
}[i]There is a small but very important difference here: we have replaced the type
List<Shape> with List<? extends Shape>. Now drawAll() will accept lists of
any subclass of Shape, so we can now call it on a List<Circle> if we want.
-- end quote --
But for a return value, I cannot see any benefit of using a bounded wildcard List.
But there is a problem, because you cannot do the assignment you are expecting. Here's why http://blogs.sun.com/roller/page/ahe/20040610
I would suggest making your method signature
public List<? extends Classifier> getClassifiers()