Java Arrays - binarySearch() Method

The java.util.Arrays.binarySearch() method is used to search the specified array for the specified object using the binary search algorithm. The array must be sorted into ascending order according to the specified comparator (as by the sort(T[], Comparator) method) prior to making this call. If it is not sorted, the results are undefined. If the array contains multiple elements equal to the specified object, there is no guarantee which one will be found.

Syntax

public static <T> int  binarySearch(T[] a, T key, 
                                    Comparator<? super T> c)

Here, T is the class of the objects in the array.

Parameters

Return Value

Returns 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.

Exception

Throws ClassCastException, if the array contains elements that are not mutually comparable using the specified comparator, or the search key is not comparable to the elements of the array using this comparator.

Example:

In the example below, the java.util.Arrays.binarySearch() method is used to search and return the index of the search key in the given array object.

import java.util.*;

public class MyClass {
  public static void main(String[] args) {
    //creating a Byte array
    Byte Arr[] = {10, 5, 25, -10, -30};

    //using comparator as null to sort 
    //using natural ordering
    Comparator<Byte> comp = null;

    //sorting the Byte array, array must be
    //sorted before using binary search
    Arrays.sort(Arr, comp);

    //printing the sorted array
    System.out.print("After sorting, Arr contains:"); 
    for(Byte i: Arr)
      System.out.print(" " + i);

    //returning the index number of searched key
    Byte val = 25;
    int idx = Arrays.binarySearch(Arr, val, comp);
    System.out.print("\nThe index number of 25 is: " + idx);  
  }
}

The output of the above code will be:

After sorting, Arr contains: -30 -10 5 10 25
The index number of 25 is: 4