Find the index of an array element in Java

Given an array of N elements and an element K, find the index of an array element in Java.

Examples:

Input: a[] = { 5, 4, 6, 1, 3, 2, 7, 8, 9 }, K = 5
Output: 0

Input: a[] = { 5, 4, 6, 1, 3, 2, 7, 8, 9 }, K = 7
Output: 6

An element in an array of N integers can be searched using the below-mentioned methods.

Linear Search: Doing a linear search in an array, the element can be found in O(N) complexity.

Below is the implementation of the linear-search approach:

// Java program to find index of 
// an element in N elements 
import java.util.*; 
public class index { 
  
    // Linear-search function to find the index of an element 
    public static int findIndex(int arr[], int t) 
    { 
  
        // if array is Null 
        if (arr == null) { 
            return -1; 
        } 
  
        // find length of array 
        int len = arr.length; 
        int i = 0; 
  
        // traverse in the array 
        while (i < len) { 
  
            // if the i-th element is t 
            // then return the index 
            if (arr[i] == t) { 
                return i; 
            } 
            else { 
                i = i + 1; 
            } 
        } 
        return -1; 
    } 
  
    // Driver Code 
    public static void main(String[] args) 
    { 
        int[] my_array = { 5, 4, 6, 1, 3, 2, 7, 8, 9 }; 
  
        // find the index of 5 
        System.out.println("Index position of 5 is: "
                           + findIndex(my_array, 5)); 
  
        // find the index of 7 
        System.out.println("Index position of 7 is: "
                           + findIndex(my_array, 7)); 
    } 
} 

Output:

Index position of 5 is: 0
Index position of 7 is: 6

Binary search: Binary search can also be used to find the index of the array element in an array. But the binary search can only be used if the array is sorted. Java provides us with an inbuilt function which can be found in the Arrays library of Java which will rreturn the index if the element is present, else it returns -1. The complexity will be O(log n).

Below is the implementation of Binary search.

// Java program to find index of 
// an element in N elements 
import java.util.Arrays; 
  
public class index { 
  
    // Function to find the index of an element 
    public static int findIndex(int arr[], int t) 
    { 
  
        int index = Arrays.binarySearch(arr, t); 
        return (index < 0) ? -1 : index; 
    } 
    // Driver Code 
    public static void main(String[] args) 
    { 
        int[] my_array = { 1, 2, 3, 4, 5, 6, 7 }; 
  
        // find the index of 5 
        System.out.println("Index position of 5 is: "
                           + findIndex(my_array, 5)); 
  
        // find the index of 7 
        System.out.println("Index position of 7 is: "
                           + findIndex(my_array, 7)); 
    } 
} 

Output:

Index position of 5 is: 4
Index position of 7 is: 6

Guava: Guava is an open source, Java-based library developed by Google. It provides utility methods for collections, caching, primitives support, concurrency, common annotations, string processing, I/O, and validations. Guava provides several-utility class pertaining to be primitive like Ints for int, Longs for long, Doubles for double etc. Each utility class has an indexOf() method that returns the index of the first appearance of the element in array.

Below is the implementation of Guava.

// Java program to find index of 
// an element in N elements 
import java.util.List; 
import com.google.common.primitives.Ints; 
public class index { 
    // Function to find the index of an element using 
    public static int findIndex(int arr[], int t) 
    { 
        return Ints.indexOf(arr, t); 
    } 
    // Driver Code 
    public static void main(String[] args) 
    { 
        int[] my_array = { 5, 4, 6, 1, 3, 2, 7, 8, 9 }; 
        System.out.println("Index position of 5 is: "
                           + findIndex(my_array, 5)); 
        System.out.println("Index position of 7 is: "
                           + findIndex(my_array, 7)); 
    } 
} 

Output:

Index position of 5 is: 0
Index position of 7 is: 6

Stream API: Stream is a new abstract layer introduced in Java 8. Using stream, you can process data in a declarative way similar to SQL statements. The stream represents a sequence of objects from a source, which supports aggregate operations. In order to find the index of an element Stream package provides utility, IntStream. Using the length of an array we can get an IntStream of array indices from 0 to n-1, where n is the length of an array.

Below is the implementation of Stream API approach.

// Java program to find index of 
// an element in N elements 
import java.util.stream.IntStream; 
public class index { 
  
    // Function to find the index of an element 
    public static int findIndex(int arr[], int t) 
    { 
        int len = arr.length; 
        return IntStream.range(0, len) 
            .filter(i -> t == arr[i]) 
            .findFirst() // first occurence 
            .orElse(-1); // No element found 
    } 
    public static void main(String[] args) 
    { 
        int[] my_array = { 5, 4, 6, 1, 3, 2, 7, 8, 9 }; 
        System.out.println("Index position of 5 is: "
                           + findIndex(my_array, 5)); 
        System.out.println("Index position of 7 is: "
                           + findIndex(my_array, 7)); 
    } 
} 

Output:

Index position of 5 is: 0
Index position of 7 is: 6

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