Why is the 1st requirement not met?

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I've written this code, it meets all the requirements except the very first one: The run method must run until the thread is terminated (!IsInterrupted). I thought the following while-loop in the run() method was serving that purpose:

public void run() {
            while (!isInterrupted()) {...}
}

Besides, IntellijIDEA produces no output, whereas the output on CodeGym's server is the following: #1:[1, 2, 3, 4, 5, ] #2:[] #3:[] Exception in thread "Thread-2" java.lang.NullPointerException at com.codegym.task.task16.task1628.Solution$ReaderThread.run(Solution.java:49) Exception in thread "Thread-3" java.lang.NullPointerException at com.codegym.task.task16.task1628.Solution$ReaderThread.run(Solution.java:49) That probably means that the 2nd and 3d threads (consoleReader2 and consoleReader3) haven't been filled with values. They probably got interrupted too early. I'm not sure how to fix that and how to make the program first let every thread finish its work and only then stop. I thought the run method completed the whole cycle for each thread...

1. Figure out what the program does.
1.1. Each thread should read lines from the console. Use the existing static BufferedReader reader.
1.2. Use AtomicInteger readStringCount to calculate how many lines all the threads have read from the console.
2. Implement the run method:
2.1. As long as a thread is not interrupted (!IsInterrupted), read lines from the console and add them to List<String> result.
2.2. Use readStringCount to count the strings read from the console.

Requirements:

The run method must run until the thread is terminated (!IsInterrupted).
The run method must NOT create its own InputStreamReaders or BufferedReaders.
The run method must read words from the reader and add them to the result list.
After each string is read, the run method must increment readStringCount by 1.
The program should display the data read by each thread.

package com.codegym.task.task16.task1628; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.ArrayList; import java.util.List; import java.util.concurrent.atomic.AtomicInteger; public class Solution { public static volatile AtomicInteger readStringCount = new AtomicInteger(0); public static volatile BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); public static void main(String[] args) throws IOException { // Read string count int count = Integer.parseInt(reader.readLine()); // Init threads ReaderThread consoleReader1 = new ReaderThread(); ReaderThread consoleReader2 = new ReaderThread(); ReaderThread consoleReader3 = new ReaderThread(); consoleReader1.start(); consoleReader2.start(); consoleReader3.start(); while (count > readStringCount.get()) { } consoleReader1.interrupt(); consoleReader2.interrupt(); consoleReader3.interrupt(); System.out.println("#1:" + consoleReader1); System.out.println("#2:" + consoleReader2); System.out.println("#3:" + consoleReader3); reader.close(); } public static class ReaderThread extends Thread { private List<String> result = new ArrayList<>(); public void run() { while (!isInterrupted()) { try { String s = null; s = reader.readLine(); if (!s.equals(null)) { result.add(s); readStringCount.incrementAndGet(); } } catch (IOException e) { //e.printStackTrace(); } } } @Override public String toString() { return result.toString(); } } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

31 July, 13:44

When a variable's value is null then ANY method that is called from that variable will result in a NullPointerException. This means that line 49 can not be false as it will either be true if 's' has a value or it will throw a null pointer exception. The correct way of testing for null is to use the equality operator: == or !=

// if (!s.equals(null)) { THIS LINE IS NO GOOD
if(s != null) { // this is the correct way to check for null
   result.add(s);
   readStringCount.incrementAndGet();
}

Your code catches IOExceptions but not NullPointerExceptions, that is why you are getting output indicating the Exception even though you have the code in a try-catch block. You could probably pass this task by simply changing the try-catch block to this:

try {
   String s = null;
   s = reader.readLine();
   if (!s.equals(null)) {
      result.add(s);
      readStringCount.incrementAndGet();
   }
} catch (Exception e) { // change to simply Exception to catch everything
      //e.printStackTrace();
}

Now for In IntelliJ vs Codegym the reason you get output on Codegym and not IntelliJ is most likely that IntelliJ is waiting for user input from you to continue. If you are not inputting anything, or not inputting enough, then the lines of BufferedReader.readline() wait until input is received before continuing. This results in user input only ever being either a value or an empty string (never null). When a BufferedReader is used to read from a file or Stream the behavior is different and the each .readline() will pull the next line from the destination and if there are no more lines to pull then result in null. Codegym uses a file or a stream to test student code so be aware of this difference.

Guadalupe Gagnon

Level 37 , Tampa, United States

31 July, 13:53

I found it tedious to always enter user input on tasks when testing my code. You can actually use a Stream to hard code in "user input" so every time you run the code it returns a result. A scanner takes in an InputStream. A BufferedReader takes in an InputStreamReader which takes in an InputStream. Typically you use System.in, which is the InputStream for keyboard input. You need to replace that InputStream with your own InputStream. I use ByteArrayInputStream and pass the String to it that I want for user input. So in this task I would start with the user input from codegyms "input data" on the task page: 5 1 2 3 4 5 In a String that would be "5\n1\n2\n3\n4\n5\n". You just need to code that in like this:

public class Solution {
   public static volatile AtomicInteger readStringCount = new AtomicInteger(0);
   // put a BAIS in and pass your "user input" String in. This takes a byte[] so you need
   // to call .getBytes() on the String
   static ByteArrayInputStream test = new ByteArrayInputStream("5\n1\n2\n3\n4\n5\n".getBytes());
   // pass the BAIS to the ISR
   public static volatile BufferedReader reader = new BufferedReader(new InputStreamReader(test));

   public static void main(String[] args) throws IOException {
      // rest of code

now when you run the code in IntelliJ you get output without having to manually input anything. This will help you test your code, especially on tasks that have you enter something like 20 Strings. If you had to manually enter 20 Strings every time you wanted to test your code then it would take a very LONG time to complete that task. Just make sure that you change it back to System.in before validating the task or your code will fail.

Evgeniia Shabaeva

Level 32 , Budapest, Hungary

7 August, 12:55

Thank you very much for the explanations and advice, all that helped a lot.

Thomas

Level 41 , Bayreuth, Germany

31 July, 13:33

the NullpointerException is thrown because you probably have a null and invoke a method on it (equals() in line 49). null you check like if (s == null) and you never invoke methods on it. An advanced topic would be to use Optional. In addition to that I'd move the while loop inside the try block. If there's an exception you probably want to stop reading values completely. in the end it may look similar to

String input;
try {
    while (!isInterrupted()) {
        if ((input = reader.readLine()) != null) {
            // ...
        }
    }
} catch (IOException e) {
    throw new UncheckedIOException(e);
}

The approach using Optional (as said, that's for later)

Optional.ofNullable(reader.readLine()).ifPresent(s -> {
    result.add(s);
    readStringCount.incrementAndGet();
});

To see the three threads working it may be necessary to slow things down a little bit... for your tests try to add a sout right at the beginning of the run method, like:

public void run() {
    System.out.println("Thread started" + getName());
    // ...

Evgeniia Shabaeva

Level 32 , Budapest, Hungary

7 August, 12:58

Thank you very much for the comment! I actually didn't realize I was invoking a method on sth that has a null value (by using if (!s.equals(null)). Now I understand why I was getting the NullPointer exception all the time, thank you. And I hope I'll sort out how to use Optional, too, in the future. Thank you very much.