Java program to find Second Largest number in Array

In this tutorial, you will learn how to write Java program to find second largest number in an array. This challenge appears frequently in coding interviews and practical applications: finding the second largest number in an array using Java. This task not only tests your understanding of array manipulation but also your ability to think algorithmically. Whether you’re a novice eager to learn Java or an experienced programmer brushing up on your skills, this tutorial will guide you through the process with a clear, step-by-step approach. By the end of this post, you’ll have a solid grasp of how to efficiently solve this common problem using Java.

Below are the approach which we will be follow to write our program:

• In this program first we will take an array and take some elements as an input from users.
• Then we will iterate the loop and then will compare elements till the and during comparison we will find the greatest and second greatest elements in a given array and print only second greatest number.
• You can also follow a approach like first short an array in ascending order and then select the second greatest number in array.
• Program is very similar to the find the largest two numbers in a given array. You may refer it

How our program will behave?

As we have already seen above our logic to find the second maximum number of an array.

Our program will take an array as an input.

And on the basis of inputs it will compare each elements and on the basis of comparison it will print second greatest elements from the array.

Example:

For the array elements 4 3 2 6 5 3,

The output should be 5 as 5 is a second largest number.

Java Program to print Second Largest number in an Array

Program 1: Using For loop and Array.sort()

import java.util.*;  
public class Main {  
    public static void main(String[] args) {  
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter the size of array: ");
      int n = sc.nextInt();
      int arr[] = new int[n];
      System.out.println("Enter " +n+ " array elements ");
      for(int i=0; i<n; i++) {
         arr[i] = sc.nextInt();
      }
      Arrays.sort(arr);
      for(int i=n-1;i>1;i--){
       if(arr[i]!=arr[i-1]){
            System.out.println(arr[i-1]+" is second largest element");
            break;
        }
    }
  }
}    

Output:

Enter the size of array: 
8
Enter 8 array elements 
4
3
2
3
4
2
1
2
3 is second largest element

Program Explanations

1. Input Collection: The program starts by prompting the user to enter the size of the array, n, using a Scanner object. It then initializes an array, arr, of this size.
2. Filling the Array: The user is asked to enter n elements to populate the array. These elements are read from the standard input and stored sequentially in arr.
3. Sorting the Array: The Arrays.sort() method is used to sort the array in ascending order. This ensures that the largest number is at the end of the array, and the second largest is just before it, provided all numbers are unique.
4. Finding the Second Largest: After sorting, the program iterates from the last element towards the beginning. It checks each element to see if it is different from the one that follows. When it finds the first element that is different from its successor, it prints this element as the second largest. The iteration starts from the second to last element and stops as soon as a unique element is found, using a break to exit the loop.

Program 2: One-pass Scan

This method uses a single traversal of the array. By maintaining two variables, one for the largest and one for the second largest, we can determine the second largest number in just one pass.

import java.util.*; 
public class SecondLargestArrayElement {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the size of array: ");
        int n = sc.nextInt();
        if (n < 2) {
            throw new IllegalArgumentException("Array must have at least two elements.");
        }
        int arr[] = new int[n];
        System.out.println("Enter " +n+ " array elements ");
        
        for(int i=0; i<n; i++) {
            arr[i] = sc.nextInt();
        }
        int largest = Integer.MIN_VALUE;
        int secondLargest = Integer.MIN_VALUE;

        for (int number : arr) {
            if (number > largest) {
                secondLargest = largest;
                largest = number;
            } else if (number > secondLargest && number != largest) {
                secondLargest = number;
            }
        }
        System.out.println("The second largest number is: " + secondLargest);
    }
}

Output

Enter the size of array: 
6
Enter 6 array elements 
4
3
2
6
5
3
The second largest number is: 5

Program Explanation

• Imports and Setup:
  • import java.util.*; imports the Java Utilities package, which includes the Scanner class for reading user input.
  • public class SecondLargestArrayElement defines the class that contains the program.
• Main Method:
  • public static void main(String[] args) is the entry point where the program starts execution.
• Input Handling:
  • A Scanner object is instantiated to read input from the console.
  • The user is prompted to enter the size of the array n, and the program reads this integer.
• Validation:
  • The program checks if n is less than 2. If true, it throws an IllegalArgumentException because you need at least two numbers to find the second largest.
• Array Initialization:
  • An integer array arr of size n is created.
  • The user is asked to enter n numbers to fill the array.
• Finding the Second Largest Number:
  • Two variables, largest and secondLargest, are initialized to Integer.MIN_VALUE.
  • The program iterates through each number in the array:
    • If a number is greater than largest, then largest is updated to this number, and the old largest value is assigned to secondLargest.
    • If a number is between the current largest and secondLargest, then secondLargest is updated to this number.
• Output:
  • After the loop, the value of secondLargest is printed, representing the second largest number in the array.