What is causing the Exception in thread “pool-1-thread-2” (NullPointerException)

[Josef Lundström]

I  am creating a multi-threaded web server. When I run the client I receive a NullPointException. Why is this happening and what can be done to solve it?

Here you can read exception message and the stack trace:

Exception in thread "pool-1-thread-2" java.lang.NullPointerException
at HandlleValidation.run(QuizServer.java:121)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745

Here I am initializing the Executor which are handling the threads:

public class QuizServer {
public static void main(String[] args) throws IOException {

ExecutorService theExecutor = Executors.newFixedThreadPool(10000);
    ServerSocket quizSocket = new ServerSocket(serverPort);

    try {
        while (true) {  //Skriver ut While True
            Socket connection = quizSocket.accept();
            theExecutor.execute(new Broadcaster(connection));
            theExecutor.execute(new HandlleValidation(connection));
        }

The origin of the String questions[1] is in the Broadcaster class:

I am using a get and set method to make it possible for the other inner class to work with

String questions[];

static class Broadcaster extends Thread {

    String quizFile = "src/qa.txt";
    private Socket connection;

    private PrintStream write;
    private String answer;
    private String questions[];
    int points = 0;

    public String[] getQuestions() {
        return questions;
    }

Here are the Strings

questions[0]

and

questions[1]

created:

while (true) {

                    for (String string : questionsList) {
                        String[] questions = string.split("/");
                        write.println(questions[0]);  //Printing out in client.
                        try {
                            Thread.sleep(4000); //"Sleeping" between the lines.
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }

The NullPointerException occurs in the HandleValidation class:

class HandlleValidation extends Thread {

The problem is caused by this line:

  if (answer.equals(questions[1])) {
                points++;
                writer.println("RÄTT SVAR! Du har " + points + " poäng");

The Setter:

public void setQuestions(String[] questions) {
    this.questions = questions;

I have tried to include as little of the code as possible to describe the problem. I may be able to give more information if requested!

[Norm Radder]

Basically with NPE you look at the line where it happened, find the variable with the null value and then backtrack in the code to see why that variable is null.

[H Paul]

If you can not troubleshoot the logic/flow of your App by just reading the code as Norm sugested,
then use the IDE debugger:
1. run QuizServer in debug mode
2. For example:  set a breakpoint at the line where NPE occured: line 121 in HandleValidation class.

See the exception you posted:

at HandlleValidation.run(QuizServer.java:121)

(A side note: I learned many things by using a debugger :-D)