Hello there--Can I please have some help here?!

Question about the task

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I have tried everything and nothing seems to work. I keep getting confusing errors every time. Can I please have some help?! I don't see what's wrong. Error:

NullPointerException(java:28)
//I also had errors on lines 27-29.

Thank you!

1. Enter 20 numbers from the keyboard, save them in a list, and then sort them to three other lists:
Numbers divisible by 3 (x%3==0), numbers divisible by 2 (x%2==0), and all other numbers.
Numbers simultaneously divisible by 3 and 2 (for example 6) go into both lists.
The order in which the lists are declared is very important.
2. The printList method should display each list item on a new line.
3. Using the printList method, display these three lists. First, the list for x%3, then the list for x%2, and then the last list.

Requirements:

Declare and immediately initialize 4 ArrayList<Integer> variables. The first list will be the main one. The other lists will be supplementary.
Read 20 numbers from the keyboard and add them to the main list.
Add to the first supplementary list all numbers in the main list that are divisible by 3.
Add to the second supplementary list all numbers in the main list that are divisible by 2.
Add to the third supplementary list all the remaining numbers from the main list.
The printList method should display each element of the passed list on a new line.
The program should display the three supplementary lists using the printList method.

package com.codegym.task.task07.task0713; import java.io.BufferedReader; import java.io.InputStreamReader; import java.util.ArrayList; import java.util.List; /* Playing Javarella */ public class Solution { public static void main(String[] args) throws Exception { //write your code here BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); ArrayList<Integer> mainlist = new ArrayList<Integer>(); ArrayList<Integer> listhree = new ArrayList<Integer>(); ArrayList<Integer> listwo = new ArrayList<Integer>(); ArrayList<Integer> listonone= new ArrayList<Integer>(); while (true) { String s = reader.readLine(); if (s == "") break; String s2 = s.toString(); int x = Integer.parseInt(s2); mainlist.add(x); if (x % 2 == 0) listwo.add(x); if (x % 3 == 0) listhree.add(x); else listonone.add(0, x); } } public static void printList(List<Integer> list) { //write your code here } }

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Gellert Varga

Level 23 , Szekesfehervar, Hungary

20 January 2021, 20:51

solution

The NullPointerException is occured on line 28. Because the check in line 27 is not perfect, and a null-reference (or sometimes some other not a number character) can go through the checking and will cause problems later (if you will try it to convert to string or to integer.) - ReadLine() reads null from the very last non-existent line of the input-list: if the last input-item is not closed with an enter; (It reads the next line, but nothing is there, so it returns a null-reference) - and the ReadLine() reads an empty string: if the last input-element is closed with enter and so one more line exists in the input-list, but that is totally empty. So, you need to do a check in line 27 , to filter both cases above. Important! Never check strings or other objects with the == ! It can give false results. (It checks references.) Use the .equals() method rather. The correct checking looks like this: