Arrays.binarySearch

Learn binary search with Arrays.binarySearch.

Binary search: Arrays.binarySearch

The fast way to search a value in a sorted array is Arrays.binarySearch. The method returns the index of the value; if it cannot find it, it returns a negative number.

int[] nums = {1, 4, 7, 9};
int i = Arrays.binarySearch(nums, 7);
// i = 2

Important: if the array is not sorted, the result is undefined. That is why you call Arrays.sort first. Binary search is very fast because it halves the search range at each step.

Return value when not found

When binarySearch cannot find the value, it returns a negative number. It does not just say "not found"; it also tells the position where the value would belong.

int[] nums = {1, 4, 7, 9};
int i = Arrays.binarySearch(nums, 5);
// i = -3  (5 should sit between 4 and 7)

The rule is -(insertionPoint) - 1. In practice you check i < 0, conclude the value is missing, and move on.

Exercise Look up a student id

Sort the ids below, then search for 103 with Arrays.binarySearch and print the found index.

Show example solution

Arrays.sort(ids);
int i = Arrays.binarySearch(ids, 103);
System.out.println(i);

Test yourself

Order the steps: binary search flow

1

Sort the array
2

Pick the middle element
3

Compare with the target
4

Halve the range
What is the precondition of binarySearch?
- The array must be sorted
- The array must have at least 10 elements
- The array must contain only even numbers
- The array must have no duplicate elements
Show answer

The array must have at least 10 elements — On an unsorted array the result is undefined.

Fill in the blank: sort and search

int[] nums = {9, 1, 7, 4};
Arrays.sort(nums);
int i = Arrays.____(nums, 7);

Show answer

int[] nums = {9, 1, 7, 4};
Arrays.sort(nums);
int i = Arrays.binarySearch(nums, 7);

Next lesson: Arrays.copyOf

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies