binarySearch collections using ArrayList

Question

I'm sorry for the stupid question, I have been searching about how to use binarysearch with my ArrayList like this :

List<Integer> arrList = new ArrayList<Integer>();       
        arrList.add(3); 
        arrList.add(5); 
        arrList.add(7);
        arrList.add(2);

The problem is when I use :

Collections.sort(arrList);
Collections.reverse(arrList);
int indeks = Collections.binarySearch(arrList, 7);

the value of indeks is always -5, I thought it should be 2 because after reversing myArrList the output looks like this :

[7, 5, 3, 2]

So what should I do here in order to get the right indeks of 7...? Thanks in advance

What does the javadoc of binarySearch say?

Sotirios Delimanolis
– Sotirios Delimanolis

2016-06-07 02:18:40 +00:00
Commented Jun 7, 2016 at 2:18 — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Jun 7, 2016 at 2:18
mmmm.. .I didn't read it ... :v

Maryadi Poipo
– Maryadi Poipo

2016-06-07 02:28:43 +00:00
Commented Jun 7, 2016 at 2:28 — Maryadi Poipo
– Maryadi Poipo, Commented Jun 7, 2016 at 2:28

shmosel · Accepted Answer · 2016-06-07 02:21:33Z

2

Collections.binarySearch() expects elements to be in ascending order:

The list must be sorted into ascending order according to the natural ordering of its elements (as by the sort(List) method) prior to making this call. If it is not sorted, the results are undefined.

If you want to do a binary search on a descending list, use Comparator.reverseOrder():

int indeks = Collections.binarySearch(arrList, 7, Comparator.reverseOrder());

indeks is now 0, corresponding to the first element of the list.

Note that you can use the same comparator to sort the list descending, instead of sorting ascending and then reversing:

Collections.sort(arrList, Comparator.reverseOrder());

answered Jun 7, 2016 at 2:21

shmosel

51.1k7 gold badges81 silver badges148 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Maryadi Poipo Over a year ago

Thanks Mr. Shmosel... :)

Robby Cornelissen · Accepted Answer · 2016-06-07 02:19:13Z

0

binarySearch() results are undefined if the list is not sorted in ascending order. By reversing your list, it's sorted in descending order, hence the results you're seeing.

From the Javadoc:

The list must be sorted into ascending order according to the natural ordering of its elements (as by the sort(List) method) prior to making this call. If it is not sorted, the results are undefined.

answered Jun 7, 2016 at 2:19

Robby Cornelissen

98.6k23 gold badges154 silver badges180 bronze badges

1 Comment

Maryadi Poipo Over a year ago

Thank you for your answer Mr.Robby.. :)

Collectives™ on Stack Overflow

binarySearch collections using ArrayList

2 Answers 2

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related