Given the code fragment: List lst = new ArrayList<>(); lst.add("e1"); lst.add("e3"); lst.add("e2"); int x1 = Collections.binarySearch(lst, "e3"); System.out.println(x1); Collections.sort(lst); int…

The correct answer is B. 2. Option B (2) is correct because it represents x2 - the only binary search call that follows the required precondition: the list must be sorted first. After Collections.sort(lst), the list becomes ["e1", "e2", "e3"], and a binary search for "e3" correctly returns index 2…

Working with Arrays and Collections

Question

Given the code fragment:
List lst = new ArrayList<>();
lst.add("e1");
lst.add("e3");
lst.add("e2");
int x1 = Collections.binarySearch(lst, "e3");
System.out.println(x1);
Collections.sort(lst);
int x2 = Collections.binarySearch(lst, "e3");
System.out.println(x2);
Collections.reverse(lst);
int x3 = Collections.binarySearch(lst, "e3");
System.out.println(x3);
What is the result?

Options

How the community answered

(63 responses)

A
5% (3)
B
68% (43)
C
10% (6)
D
2% (1)
E
16% (10)

Explanation

Option B (2) is correct because it represents x2 - the only binary search call that follows the required precondition: the list must be sorted first. After Collections.sort(lst), the list becomes ["e1", "e2", "e3"], and a binary search for "e3" correctly returns index 2 (zero-based).

The other options map to common misconceptions:

D (1) tempts students who think x1 is valid - but binary search on the unsorted list ["e1", "e3", "e2"] violates the method's contract, making x1's result undefined (it may accidentally return 1, but this is not guaranteed behavior).
E (-1), C (-2) tempt students who expect x3 to return a "not found near index 0 or 1" result - but since Collections.reverse() produces a descending-order list, the contract is violated again and the actual return value is unpredictable/implementation-defined.
A (0) has no strong basis in the logic but could trap students who miscount the sorted index.

Memory tip: Think "Sort Before Search" - Collections.binarySearch() requires a sorted list and returns the zero-based index on success, or a negative insertion-point value on failure. Any search on an unsorted or reverse-sorted list is undefined behavior and should be treated as wrong on exams.

Topics

#Collections.binarySearch#Collections.sort#sorted collections#preconditions

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