Merge two linked list of integers in ascending order

MergeLinkedListASC.java

package com.company;

import java.io.*;
import java.util.*;

public class Solution {

    static class SinglyLinkedListNode {
        public int data;
        public SinglyLinkedListNode next;

        public SinglyLinkedListNode(int nodeData) {
            this.data = nodeData;
            this.next = null;
        }

    }

    static class SinglyLinkedList {
        public SinglyLinkedListNode head;
        public SinglyLinkedListNode tail;

        public SinglyLinkedList() {
            this.head = null;
            this.tail = null;
        }

        public void insertNode(int nodeData) {
            SinglyLinkedListNode node = new SinglyLinkedListNode(nodeData);

            if (this.head == null) {
                this.head = node;
            } else {
                this.tail.next = node;
            }

            this.tail = node;
        }
    }

    public static void printSinglyLinkedList(SinglyLinkedListNode node, String sep, BufferedWriter bufferedWriter) throws IOException {
        while (node != null) {
            bufferedWriter.write(String.valueOf(node.data));

            node = node.next;

            if (node != null) {
                bufferedWriter.write(sep);
            }
        }
    }

    // Complete the mergeLists function below.

    /*
     * For your reference:
     *
     * SinglyLinkedListNode {
     *     int data;
     *     SinglyLinkedListNode next;
     * }
     *
     */
    static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
        if (head1 == null) {
            return head2;
        }
        if (head2 == null) {
            return head1;
        }
        List<Integer> allDetails= new ArrayList<>();
        allDetails.add(head1.data);
        allDetails.add(head2.data);
        while (head1.next!=null){
            allDetails.add(head1.next.data);
            head1=head1.next;
        }
        while (head2.next!=null){
            allDetails.add(head2.next.data);
            head2=head2.next;
        }
        Collections.sort(allDetails);
        System.out.println(Arrays.toString(allDetails.toArray()));
        SinglyLinkedListNode mStart = new SinglyLinkedListNode(allDetails.get(0));

        SinglyLinkedListNode mCopy = mStart;
        for (int i = 1; i < allDetails.size(); i++) {
            mCopy.next= new SinglyLinkedListNode(allDetails.get(i));
            mCopy=mCopy.next;
            System.out.println("Position:"+allDetails.get(i));
        }

        return  mStart;
    }

    private static final Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) throws IOException {
        BufferedWriter bufferedWriter = null;
        try {
            File f = new File("s.txt");
            if (!f.exists()) {
                System.out.println(f.createNewFile());

            }
            bufferedWriter = new BufferedWriter(new FileWriter(f.getName()));
        } catch (IOException e) {
            e.printStackTrace();
        }

        int tests = scanner.nextInt();
        scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

        for (int testsItr = 0; testsItr < tests; testsItr++) {
            SinglyLinkedList llist1 = new SinglyLinkedList();

            int llist1Count = scanner.nextInt();
            scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

            for (int i = 0; i < llist1Count; i++) {
                int llist1Item = scanner.nextInt();
                scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

                llist1.insertNode(llist1Item);
            }

            SinglyLinkedList llist2 = new SinglyLinkedList();

            int llist2Count = scanner.nextInt();
            scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

            for (int i = 0; i < llist2Count; i++) {
                int llist2Item = scanner.nextInt();
                scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

                llist2.insertNode(llist2Item);
            }

            SinglyLinkedListNode llist3 = mergeLists(llist1.head, llist2.head);

            printSinglyLinkedList(llist3, " ", bufferedWriter);
            bufferedWriter.newLine();
        }

        bufferedWriter.close();

        scanner.close();
    }
}