Only to do code required in Java

doubleUp 2 public class LinkedIntList { 3 private ListNode front; 4 5 6 Write a method doubleUp that doubles the size of a list by inserting a copy of every integer in the list immediately after its original occurrence in the list. public void doubleUp() { //to do code } 7 8 Suppose a list stores the values: [1, 8, 19, 4, 17] After calling list.doubleUp(), the list should store the values: [1, 1, 8, 8, 19, 19, 4, 4, 17, 17] 9 public static void main(String[] args) { 10 LinkedIntList list = new LinkedIntList(1, 8, 19, 4, 17); 11 list.doubleUp(); 12 System.out.println(list); 13 } 14 15 /////// ////////////// 16 17 // Constructs a list containing the given elements 18 public LinkedIntList(int... elements) { 19 if (elements.length > 0) { 20 front = new ListNode (elements[0]); 21 ListNode current = front; 22 for (int i = 1; i