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

ArrayList is a part of the Collection framework and is present in java.util package. It provides us with dynamic arrays in Java. Though, it may be slower than standard arrays but can be helpful in programs where lots of manipulation in the array is needed. This class is found in java.util package. 

Illustration:

Input  : [1, 3, 9]  
Output : 13

Here the naive method can be to add up elements of List and maintain a counter in which sum is stored while traversing List. The step ahead method can be to convert the List to the array and do the same. Now more optimal method can be to use recursion while doing so in which a subpart of a List or array is automatically computed by recursion principles. Here this optimal approach is described and implemented as shown. 

Methods:

1. Converting ArrayList to arrays and using recursion principles over arrays.
2. Using ArrayList.add() method

Method 1: Converting ArrayList to arrays and using recursion principles over arrays.

It is achieved by converting ArrayList to arrays and using recursion principles over arrays. Recursion in the list to array conversion and computing sum of elements using add() method.

Approach: 

1. Take the elements of the list as input from the user.
2. Convert the list into an array of the same size.
3. Add the elements to it.
4. Compute the sum of arrays using recursion principles.

Example

Java

// 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);
    }
}

Output

Elements in List : 1 2 3 4 5 
Sum of elements  : 15

The time complexity is O(n), where n is the size of the input list. 

The auxiliary space is also O(n), where n is the size of the input list. 

Method 2: Using ArrayList.add() method

This method appends the specified element to the end of this list

Syntax:

public boolean add(E element) ;

Parameter: Object to be appended to this list.

Return Type: It will always return a boolean true and the signature is as so because other classes in collections family need a return type.

Exceptions: NA

Example:

Java

// 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;
    }
  }
}

Output

Elements in List : [10, 90, 30, 40, 70, 100, 0]
Sum of elements in List :340

Time complexity: O(n) where n is size of given list of numbers

Auxiliary space: O(n) because using extra space