There are two ways to do binary search in Java.

Arrays.binarysearch() works for arrays which can be of primitive data type also.

// Java program to demonstrate working of Arrays.

// binarySearch() in a sorted array.

import java.util.Arrays;

public class P_AI {

public static void main(String[] args)

{

int arr[] = { 11, 19, 16, 22, 42 };

Arrays.sort(arr);

int key = 22;

int res = Arrays.binarySearch(arr, key);

if (res >= 0)

System.out.println(key + " found at index = "

+ res);

else

System.out.println(key + " Not found");

key = 40;

res = Arrays.binarySearch(arr, key);

if (res >= 0)

System.out.println(key + " found at index = "

+ res);

else

System.out.println(key + " Not found");

}

}

**Output:**

22 found at index = 3

40 Not found

Collections.binarysearch() works for objects Collections like ArrayList and LinkedList.

// Java program to demonstrate working of Collections.

// binarySearch()

import java.util.List;

import java.util.ArrayList;

import java.util.Collections;

public class P_AI

{

public static void main(String[] args)

{

List<Integer> al = new ArrayList<Integer>();

al.add(2);

al.add(3);

al.add(6);

al.add(15);

al.add(22);

// 10 is present at index 3.

int key = 10;

int res = Collections.binarySearch(al, key);

if (res >= 0)

System.out.println(key + " found at index = "

+ res);

else

System.out.println(key + " Not found");

key = 15;

res = Collections.binarySearch(al, key);

if (res >= 0)

System.out.println(key + " found at index = "

+ res);

else

System.out.println(key + " Not found");

}

}

**Output:**

10 Not found

15 found at index = 3

**What if input is not sorted?**

If input list is not sorted, the results are undefined.

**What if there are duplicates?**

If there are duplicates, there is no guarantee which one will be found.

**How does Collections.binarySearch work for LinkedList?**

This method runs in log(n) time for a “random access” list like ArrayList. If the specified list does not implement the RandomAccess interface and is large, this method will do an iterator-based binary search that performs O(n) link traversals and O(log n) element comparisons.

**What is significant value of negative value returned by both functions?**

The function returns an index of the search key, if it is contained in the array; otherwise, (-(insertion point) – 1). 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. Note that this guarantees that the return value will be >= 0 if and only if the key is found.

**How to implement our own Binary search in Java?**

// Java implementation of recursive Binary Search

class BinarySearch

{

// Returns index of x if it is present in arr[l..

// r], else return -1

int binarySearch(int arr[], int l, int r, int x)

{

if (r>=l)

{

int mid = l + (r - l)/2;

// If the element is present at the

// middle itself

if (arr[mid] == x)

return mid;

// If element is smaller than mid, then

// it can only be present in left subarray

if (arr[mid] > x)

return binarySearch(arr, l, mid-1, x);

// Else the element can only be present

// in right subarray

return binarySearch(arr, mid+1, r, x);

}

// We reach here when element is not present

// in array

return -1;

}

// Driver method to test above

public static void main(String args[])

{

BinarySearch ob = new BinarySearch();

int arr[] = {4,6,9,10,30};

int n = arr.length;

int x = 10;

int result = ob.binarySearch(arr,0,n-1,x);

if (result == -1)

System.out.println("Element not present");

else

System.out.println("Element found at index " +

result);

}

}

**Output:**

Element found at index 3