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Binary Search in Java

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Binary search is one of the searching techniques applied when the input is sorted here we are focusing on finding the middle element that acts as a reference frame whether to go left or right to it as the elements are already sorted. This searching helps in optimizing the search technique with every iteration is referred to as binary search and readers do stress over it as it is indirectly applied in solving questions.

Binary-Search

Binary Search Algorithm in Java

Below is the Algorithm designed for Binary Search:

  1. Start
  2. Take input array and Target
  3. Initialise start = 0 and end = (array size -1)
  4. Intialise mid variable
  5. mid = (start+end)/2
  6. if array[ mid ] == target then return mid
  7. if array[ mid ] < target then start = mid+1
  8. if array[ mid ] > target then end = mid-1
  9. if start<=end then goto step 5
  10. return -1 as Not element found
  11. Exit

Now you must be thinking what if the input is not sorted then the results are undefined.

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

Methods for Java Binary Search

There are three methods in Java to implement Binary Search in Java are mentioned below:

  1. Iterative Method
  2. Recursive Method
  3. Inbuild Method

1. Iterative Method for Binary Search  in Java

Below is the implementation is mentioned below:

Java

// Java implementation of iterative 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)
    {
        while (l <= r) {
            int mid = (l + r) / 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
            // so we decrease our r pointer to mid - 1
            } else if (arr[mid] > x) {
                r = mid - 1;
 
            // Else the element can only be present
            // in right subarray
            // so we increase our l pointer to mid + 1
            } else {
              l = mid + 1;
            
        }
 
        // 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[] = { 2, 3, 4, 10, 40 };
        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


Tip: Geeks you must be wondering out whether there is any function like lower_bound() or upper_bound() just likely found in C++ STL. so the straight answer is that there was no function only till Java 9, later onwards they were added. 

2. Recursive Method for Binary Search

Below is the implementation of the above method:

Java

// Java implementation of
// recursive Binary Search
 
// Driver Class
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;
    }
 
    // main function
    public static void main(String args[])
    {
        BinarySearch ob = new BinarySearch();
 
        int arr[] = { 2, 3, 4, 10, 40 };
        int n = arr.length;
        int x = 10;
        int result = ob.binarySearch(arr, 0, n - 1, x);
 
        if (result == -1)
            System.out.println(
                "Element is not present in array");
        else
            System.out.println(
                "Element is present at index " + result);
    }
}

                    

Output
Element is present at index 3


Complexity of the above method

Time Complexity: O(log N)
Space Complexity: O(1), If the recursive call stack is considered then the auxiliary space will be O(logN)

3. In Build Method for Binary Search in Java

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

Below is the implementation of the above method:

Java

// Java Program to demonstrate working of binarySearch()
// Method of Arrays class In a sorted array
 
// Importing required classes
import java.util.Arrays;
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Declaring an integer array
        int arr[] = { 10, 20, 15, 22, 35 };
 
        // Sorting the above array
        // using sort() method of Arrays class
        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


Binary Search in Java Collections

Now let us see how Collections.binarySearch() work for LinkedList. So basically as discussed above 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.

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

Below is the implementation of the above method: 

Java

// Java Program to Demonstrate Working of binarySearch()
// method of Collections class
 
// Importing required classes
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
 
// Main class
public class GFG {
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an empty ArrayList of integer type
        List<Integer> al = new ArrayList<Integer>();
 
        // Populating the Arraylist
        al.add(1);
        al.add(2);
        al.add(3);
        al.add(10);
        al.add(20);
 
        // 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 found at index = 3
15 Not found


The complexity of the above method

Time complexity: O(log N)
Auxiliary space: O(1)



Last Updated : 28 Dec, 2023
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