I'm not familiar with java beans framework, in the below scenario, I got the IndexedPropertyDescriptor for the method getFooWithX, could someone explain why?

public class IntrospectorTest {
public static void main(String[] args) throws IntrospectionException {
    BeanInfo info = Introspector.getBeanInfo(SubClass.class);
    PropertyDescriptor[] descriptors = info.getPropertyDescriptors();
    for (int i = 0; i < descriptors.length; i++) {
        System.out.println(descriptors[i].getClass().getName() + ":" + descriptors[i].getName());
    }
}

}

abstract class BaseClass {
public abstract Object getFoo();

}

abstract class SubClass extends BaseClass {
public Object getFooWithX(int x) {
    return null;
}

}

and the result will be:

java.beans.PropertyDescriptor:class
java.beans.PropertyDescriptor:foo
java.beans.IndexedPropertyDescriptor:fooWithX

Why?