Arrays.binarySearch() returns wrong insertion point

Question

This is the code:

public class Main {

    public static Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) throws IOException {

        int arr[] = {10,50,999,1000};
        int index = Arrays.binarySearch(arr,55);
        System.out.println(index);
    }
}

The output here is '-3', if the output from this formula "(-(insertion point) - 1)" that means the insertion point is '4' and that is not right.

So what i am missing?

Answer 1

The insertion point is 2, not 4.

According to the official documentation:

The insertion point is defined as the point at which the key would be inserted into the array: the index of the first element greater than the key [...]

Your array with indices is

[10, 50, 999, 1000]
  0   1    2     3

The first element greater than 55 is 999 at index 2. Remember that indices start counting at 0.

So the insertion point is 2. With the formula (-(insertion point) - 1) the return value must thus be:

(-(2) - 1) = -3

Which is exactly what you got.