A java.lang.NullPointerException

Question

A java.lang.NullPointerException occurs when a Java application or Java applet has been badly coded. Typically, the Java program (and consequently, the programmer) attempted to access the reference or handle to a Java object that did not exist

I have gone round in circles reading . I have made two minimal files - Two.java will compile.

 public class Two {
   public static int width;
   public static int height;

   public static void main(String[] args) {
     int width = 320;
     int height = 100;  
     System.out.println(width + "," + height);
   }
 }

One.java in eclipse will print the string intwo to console. My question is if it will print, Why is it null? I am trying to convert the string to int so I can do math with it. In real life there is a lot more than two numbers arriving in.

import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class One {
  public static int width;
  public static int height;

  public static void main(String[] args) throws IOException, InterruptedException {
    Process p = Runtime.getRuntime().exec("java -jar Two.jar");
    BufferedReader is;
    String intwo;
    is = new BufferedReader(new InputStreamReader(p.getInputStream()));

    while ((intwo = is.readLine()) != null)
      System.out.println(intwo); // Prints to console

    String[] items = (intwo).split(","); //java.langNullPointerException
    int[] results = new int[items.length];

    for (int i = 0; i < items.length; i++) {
      results[i] = Integer.parseInt(items[i]);
    }
    System.out.println(results[1]);
  }
}

Post the whole stack trace. Always post the whole stack trace. — m0skit0, Aug 06 '13 at 13:05
Maroun Maroun, Egor and Sethiel all gave you the correct answer. However, you might want to try stepping through your code line by line in the debugger before posting this sort of question to Stack Overflow. It should be easy to see the behavior they described — BillRobertson42, Aug 06 '13 at 13:28

Arnaud Denoyelle · Answer 1 · 2013-08-06T13:10:00.753

2

while ((intwo = is.readLine()) != null)
  System.out.println(intwo); // Prints to console

//Here intwo is guaranteed to be null!!!!
String[] items = (intwo).split(",");

By the way, it is a bad practice to write if/while statements without curly braces. You should really avoid it.

edited Aug 06 '13 at 13:10

answered Aug 06 '13 at 13:04

Arnaud Denoyelle

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+1 - `intwo` guaranteed to be `null` otherwise the loop wouldn't exit. – Qwerky Aug 06 '13 at 13:06
Maroun Maroun, Egor and Sethiel all gave you the correct answer. However, you might want to try stepping through your code line by line in the debugger before posting this sort of question to Stack Overflow. It should be easy to see the behavior they described. – BillRobertson42 Aug 06 '13 at 13:21
I think you did not post at the right place. – Arnaud Denoyelle Aug 06 '13 at 13:22
Just noticed that. Thanks. – BillRobertson42 Aug 06 '13 at 13:28

score 1 · Answer 2 · answered Aug 06 '13 at 13:05

Here's your while clause:

while ((intwo = is.readLine()) != null)
  System.out.println(intwo); // Prints to console

It exists when intwo is null, right? So it's obvious that on the next line intwo will be null and will cause a NullPointerException:

String[] items = (intwo).split(","); //java.langNullPointerException

score 1 · Answer 3 · answered Aug 06 '13 at 13:05

See your code:

while ((intwo = is.readLine()) != null)

When do you exit the while loop? When intwo is null!

So when you exit the loop, you're doing null.split(","); which of course causes NPE.

Put parenthesis around the while loop and you should be fine.

score 1 · Answer 4 · answered Aug 06 '13 at 13:08

while ((intwo = is.readLine()) != null)
    System.out.println(intwo); // Prints to console
String[] items = (intwo).split(","); //java.langNullPointerException

Of course you get NullPointerException. You basically iterate until intwo is null (so you ensure intwo is null) and then you call a method on that null reference.

score 0 · Answer 5 · answered Aug 06 '13 at 13:03

This:

 while ((intwo = is.readLine()) != null)
 System.out.println(intwo); // Prints to console
 String[] items = (intwo).split(",");

is the same as this:

while ((intwo = is.readLine()) != null){
    System.out.println(intwo); // Prints to console
}

String[] items = (intwo).split(",");

It gets pretty obvious why you get a NullPointerException

A java.lang.NullPointerException

5 Answers5