This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
Hi all! I cannot understand this question and answer!
Given a method declared as: public static <E extends Number> List<? super E> process(List<E> nums) A programmer wants to use this method like this: // INSERT DECLARATIONS HERE output = process(input); Which pairs of declarations could be placed at // INSERT DECLARATIONS HERE to allow the code to compile? (Choose all that apply.) A. ArrayList<Integer> input = null; ArrayList<Integer> output = null; B. ArrayList<Integer> input = null; List<Integer> output = null; C. ArrayList<Integer> input = null; List<Number> output = null; D. List<Number> input = null; ArrayList<Integer> output = null; E. List<Number> input = null; List<Number> output = null; F. List<Integer> input = null; List<Integer> output = null; G. None of the above. Answer: � 3 B, E, and F are correct. �˚ The return type of process is definitely declared as a List, not an ArrayList, so A and Dare wrong. C is wrong because the return type evaluates to List<Integer>, and that can'tbe assigned to a variable of type List<Number>. Of course all these would probably cause a NullPointerException since the variables are still null�but the question only asked us to get the code to compile. -------------------------------------------------------- why cannot we return as well an ArrayList, when we can receive an ArrayList as argument to the method?As far as i understand collection reference and object can be polymorphic, but not their type! If List<Integer> cannot be assigned to List<Number>, what does the return <? super E> mean?Why is c answer incorrect?