How can we add two objects from a ArrayList <Object>?

Question

I recently started studying Java's ArrayList class, and learned about how we can use an ArrayList <Object> to hold elements of different class types. This question, however, confused me.

My general thinking pattern in answering this question was that we can first eliminate answers A and B, because the compiler would complain about using the + operator on two Objects that have not been cast. When I looked at answers C and D, however, it seemed to me that both of them would work correctly, because they both cast the values returns from list.get() to a type where the operator + is defined. This led me to choose answer E, as I believed both C and D could work in this piece of code.

As you can see, however, E was the wrong answer. To compound my confusion, I tested this piece of code on repl.it, where it was confirmed that answer choices A and B result in compilation errors while answer choices C and D both store the correct value of 9 into i . So now, I'm really confused. Is there some rule with Object s that I am glossing over here? Can someone point me in the right direction?

Answer 1

1. Object as type parameter can allow the collection to hold any type of java object
2. Though this simplifies the writers, it places a huge burden on the readers to validate the types to avoid run time ClassCastException s. It behaves like pre-generic world

Regarding why list.get() is allowed in c and d :

1. list.get(0) will be first invoked and the result is casted to int . (since the type of values stored at 0 is Integer , it is perfectly valid to cast as int - actually cast to Integer and then unbox to int )
2. same applies for list.get(2)
3. if you meant about casting the reference before making a method call, then it will look like ((int)list).get(0)) . Unfortunately this is invalid due to the types used.

More

1. Actually it will be interesting to add a long to the list list.add(1L)

List<Object> list = new ArrayList<>();
list.add(1L);
(int) list.get(0) + (int) list.get(0);

2. Casting this to int will result in run time ClassCastException (something like Long cannot be cast to Integer )
3. This is because, even though, we apply the cast as primitive int , it actually does an evaluation of object hierarchy based on the reference(object type) and finds that Integer and Long are siblings and does not have a direct hierarchy and hence throw an ClassCastException