Java Arrays.binary search returns wrong data even when array is sorted

Question

The documentation writes that "Arrays.binarySearch return a.length if all elements in the array are less than the specified key." So in the following program, I am expecting the value 4 to be printed but it prints -4. Why this anomalous behaviour?

import java.io.*;
import java.math.*;
import java.util.*;
import java.lang.*;

public class Main{ 
    public static void main(String[] args)throws java.lang.Exception{
        int[] a = new int[3];
        a[0] = 3;
        a[1] = 8;
        a[2] = 9;
        System.out.println(Arrays.binarySearch(a, 15));         
    }
}

Answer 1

Quoting from Java Docs..

Returns: index of the search key, if it is contained in the array; otherwise, (-(insertion point) - 1).

where 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, or a.length if all elements in the array are less than the specified key

In your example, all elements are less than 15 and length of the array is 3. So the insertion point is 3 and therefore binarySearch returns -3-1 = -4.