Null Pointer Exception

Question about the task

Under discussion

Why am I continuing to get a NullPointerException? I've tried s.equals(""), s == null, s.length() == 0, !s.equals(""), !s.equals(null) and no matter what, I've continued to receive the NPE error message. Why?

Task: The program takes in strings until the user enters an empty string (by pressing Enter). Then it converts the strings to uppercase ("mother" becomes "MOTHER") and displays them.

New task: The program takes in strings until the user enters an empty string (by pressing Enter).
Then the program builds a new list. If a string has an even number of letters, then the string is duplicated. If the number of letters is odd, then the string is repeated three times.
The program displays the contents of the new list.

Example input:
Cat Cats I

Example output:
Cat Cat Cat Cats Cats I I I

Requirements:

The program must read data from the keyboard.
The program should display as many strings as are entered.
If a string has an even number of letters, then you need to duplicate the string (as in the example).
If a string has an odd number of letters, then you need to repeat the string three times (as in the example).
The program should stop reading data from the keyboard as soon as the user enters an empty string (by pressing Enter).

package com.codegym.task.task07.task0727; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.ArrayList; /* Changing functionality */ public class Solution { public static void main(String[] args) throws IOException { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); ArrayList<String> list = new ArrayList<String>(); String s = ""; while (true) { s = reader.readLine(); if(s.equals("")) { break; } else { list.add(s); } } ArrayList<String> oddList = new ArrayList<String>(); String word = ""; for(int i = 0; i < list.size(); i++) { word = list.get(i); if(word.equals("")) { oddList.add(word); } int length = word.length(); if(length % 2 == 0) { oddList.add(word + " " + word); } else { oddList.add(word + " " + word + " " + word); } } for(String words : oddList) { System.out.println(words); } // ArrayList<String> listUpperCase = new ArrayList<String>(); // for (int i = 0; i < list.size(); i++) { // String s = list.get(i); // listUpperCase.add(s.toUpperCase()); // } // for (int i = 0; i < listUpperCase.size(); i++) { // System.out.println(listUpperCase.get(i)); // } } }

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Pablo Souza

Level 22 , Brisbane, Australia

26 March 2020, 22:32

I think these exercises were meant for IntelliJ. I had the same problem and it works fine over there.

Hannah Software Architect

23 January 2020, 11:18

Have you solved this meanwhile? I have the same problem and also tried endless possibilities and still get the NPE-error.

Mike McKenna

Level 25 , Wilmington, United States

10 January 2020, 22:04

Hi Anthony, This task is a research gem. lines 29 to 39 u have the information . code i used follows . < for (int i = 0; i < list.size(); i++) { String a = list.get(i); if(list.get(i).length()%2==0){ System.out.println((a+" "+a)); } else{ System.out.println((a+" "+a+" "+a)); } } Line 43-44 code i used is another for loop. for (int i = 0; i < listEvenOdd.size(); i++) { System.out.println(listEvenOdd.get(i)); } hope i helped

oli blaustrom

Level 18 , Luzern, Switzerland

24 February 2020, 09:03

Hi Mike. The NPE doesn't have anything to do with this and the error actually occurs at 21 in the above.

Vesa

Level 41 , Kaliningrad, Russia

10 January 2020, 18:41

s.equals("")

When s is null you will receive NPE, because null have no any methods. Try

s == null || s.isEmpty()

If s will be null the second part of the || operator will not execute, and NPE will not appear.