-1

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.

enter image description here

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 Objects that I am glossing over here? Can someone point me in the right direction?

Improve this question
6
  • 2
    I can confirm that both c and d should work. Looks like an error with the test. Commented Feb 6, 2021 at 16:40
  • Was this a multiple choice answer or were you supposed to select only one option? And what does "more than one of these" mean? Are you supposed to select e when you know other ways to sum both numbers that is not listed there? If yes, then e is correct as well, because there are several other ways to fill the "blank". Commented Feb 6, 2021 at 16:45
  • I would say that 'c' is the more correct answer as the cast to int takes place on the entire list.get() operation. The key phrase being more correct Commented Feb 6, 2021 at 17:58
  • @BigGuy the two statements are perfectly equivalent, because the derefence (.) has higher precedence than the cast. That's the reason you need to do ((Whatever)x).y() if you want to cast x to Whatever before invoking the y method Commented Feb 6, 2021 at 18:18
  • @BigGuy sorry, method call, not derefernce. Too late to edit my comment Commented Feb 6, 2021 at 18:29

1 Answer 1

Reset to default
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 ClassCastExceptions. 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);
  1. Casting this to int will result in run time ClassCastException (something like Long cannot be cast to Integer)
  2. 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
Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.