All the Collections classes implementing List are ordered? - question from examlab

[Anuradha Prasanna]

13. public static void search(List<String> list) {
14. list.clear();
15. list.add("b");
16. list.add("a");
17. list.add("c");
18. System.out.println(Collections.binarySearch(list, "a"));
19. }

What is the result of calling search with a valid List implementation?
Answers are: A)0 B)1 C)2 D)The result is undefined

D) is the correct answer.
But as all Lists(ArrayList, Vector, LinkedList) - all these are ordered Collections, the answer should be 1. why D)

[Ankit Garg]

Is there any explanation given to why D is correct?? The only reason that I can think of is that clear method is optional for List implementations. So there's no guarantee that the actual implementation of the list will support the clear method and might throw an UnsupportedOperationException...

[Venu Chakravorty]

A collection must be sorted before it can be searched.
Like this:
Collections.sort(list);
System.out.println(Collections.binarySearch(list, "a"));

Correct me if I am wrong.

[Neha Daga]

Venu is right.
before seacrching a collection it should be sorted otherwise the result will be unpredictable.
read ch-7 K&B it explained there with an example.

[Ankit Garg]

How can I miss that

[Mike Simmons]

There's another reason answer D is correct: a "valid list implementation" may still throw UnsupportedOperationException in response to some methods, such as add() or clear(). Try running this with

search(Collections.unmodifiableList(new ArrayList<String>()));