Java Program to Compute the Sum of Numbers in a List Using Recursion

// Java Program to Compute Sum of Numbers in a List
// by converting to arrays and applying recursion

// Importing java input/output classes
import java.io.*;
// Importing List and ArrayList class from
// java.util package
import java.util.ArrayList;
import java.util.List;

// Class
public class GFG {

    // Method to calculate sum recursively
    public static int sumOfArray(Integer[] a, int n)
    {
        if (n == 0)
            return a[n];
        else
            return a[n] + sumOfArray(a, n - 1);
    }

    // Method- main()
    public static void main(String[] args)
    {
        // Creating a List of Integer type
        // Declaring an object- 'al'
        List<Integer> al = new ArrayList<Integer>();

        // Adding elements to the List
        // Custom inputs
        al.add(1);
        al.add(2);
        al.add(3);
        al.add(4);
        al.add(5);

        // Converting above List to array
        // using toArray() method
        Integer a[] = new Integer[al.size()];
        al.toArray(a);

        // Display message
        System.out.print("Elements in List : ");

        // Printing array of objects
        // using for each loop
        for (Integer obj : a) {
            System.out.print(obj + " ");
        }

        // Recursion math to calculate sum snd
        // storing sum in a variable
        int sum = sumOfArray(a, a.length - 1);

        // Next line
        System.out.println();

        // Print the sum returned above
        System.out.println("Sum of elements  : " + sum);
    }
}

// Java Program to Compute the Sum of Numbers in a List 
// using Recursion via ArrayList.add() method

// Importing all classes of 
// java.util package
import java.util.*;

// Class
public class GFG
{   
  // Declaring variables outside main class
  int sum = 0, j = 0;

  // Main driver method
  public static void main(String[]args)
  { 
    /*
    // Taking the input from the user 
    int n;
    Scanner s = new Scanner(System.in);

    // Display message 
    System.out.print("Enter the no. of elements :");
    
    // Reading integer elements using nextInt() method
    n = s.nextInt();

    // Display message 
    System.out.println("Enter all the elements you want:");
    */

    // Creating an object of List of Integer type 
    List < Integer > list = new ArrayList < Integer > ();

    // Adding elements to object of List
    // Custom inputs to show sum
    list.add(10);
    list.add(90);
    list.add(30);
    list.add(40);
    list.add(70);
    list.add(100);
    list.add(0);
    
    System.out.println("Elements in List : " + list);
    /*
    // If input is through user than
    
    // For loop to add elements inside List
    for (int i = 0; i < n; i++)
    {
      // Adding integer elements in the list
      list.add(s.nextInt()); 
    }
    */

    // Converting List to Array
    Integer[] a = list.toArray(new Integer[list.size()]);

    // Initialising object of Main class
    GFG elem = new GFG();

    // Finding sum of elements in array 
    // via add() method using recursion
    int x = elem.add(a, a.length, 0);
    
    // Print the sum of array/elements initially in List
    System.out.println("Sum of elements in List :" + x);
  }

  // add() method to add elements in array
  // using recursion
  int add(Integer arr[], int n, int i)
  {
    if(i < n)
    {
        return arr[i] + add(arr, n, ++i);
    }
    else
    {
        return 0;
    }
  }
}