Give an example where upcasting takes advantage of polymorphism.

Here it goes:

class Parent { }

class Child extends Parent { }

Child can be referred as an object of type Parent or Child.

Upcasting works like this:

Child aChildObj = new Child();

Parent aParentObj = (Parent)aChildObj;

As aChildObj can be referred to by the class names Child and
Parent,you can say that this upcasting and type extension is the
implementation of polymorphism in Java.