binarySearch()

Function

This function is used to search the specified object in a list using the binary search algorithm.

The method returns the index of the search key, if it is contained in the list and throws ClassCastException if the list contains elements that are not mutually comparable.

Declaration

public static <T> int binarySearch(List<? extends T> list,T key,Comparator<? super T> c)

Parameters

list − This is the list to be searched.

key − This is the key to be searched for.

c − This is the comparator by which the list is ordered. A null value indicates that the elements’ natural ordering should be used. The list must be sorted into ascending order according to the specified comparator. This parameter is optional.

NOTE!- One important point to keep in mind is that the elements in the Collection must be already sorted, else the results will be something unexpected.

Example (Without Comparator)

import java.util.*;

public class BinarySearch 
{
   public static void main(String args[]) 
   {
   
    ArrayList<Integer> list = new ArrayList<Integer>();

    list.add(30);
    list.add(45);
    list.add(100);
    list.add(250);
    list.add(500);

    int index = Collections.binarySearch(list,45);     

    System.out.println("Position of 45 is "+index);
   }    
}

Output

Position of 45 is 1

Example (With Comparator)

import java.util.*;
 
public class BinarySearchwithComparator
{
    public static void main(String args[])
    {
        ArrayList<Integer> list = new ArrayList<Integer>();
        list.add(500);
        list.add(250);
        list.add(100);
        list.add(45);
        list.add(30);
        int index = Collections.binarySearch(list, 30, 
                                  Collections.reverseOrder());
 
        System.out.println("Position of 30 is " + index);
    }
}

Output

Position of 30 is 4