How to fix NullPointerException in array of multithread app

Question

My task: I need to generate BASE (array) of matrix in multiple threads. It can happen that thread won't make matrix at all so I'm try to limit the lifetime of each thread.

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My thought: I made volatile array that contains my matrix generation tasks and made ConcurrentHashMap that contains thread and information class (index in task array and time of thread creation)

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My problem: I got NullPointerException when I tried to deal with array

Thread information class:

public class ThreadInformation {
    private long timeCreated;
    private int automatIndex;

    public ThreadInformation (long timeCreated, int automatIndex) {
        this.timeCreated = timeCreated;
        this.automatIndex = automatIndex;
    }

    public long getTimeCreated() {
        return timeCreated;
    }

    public int getIndex() {
        return automatIndex;
    }
}

MultiThread class:

public class MultiTaskGenerator {
    private final int BASE_SIZE = 10;
    private final Automat[] BASE = new Automat[BASE_SIZE];

    private static final int THREAD_COUNT = 8;
    private final int THREAD_LIVE_TIME_SEC = 10;

    private Map<Thread, ThreadInformation> threads = new ConcurrentHashMap<>();
    private volatile GeneratorTask[] tasks = new GeneratorTask[THREAD_COUNT];

    private int automatReady = 0;

    MultiTaskGenerator() {
        getThreadResult();
    }

    private void initThreads() {
        for (int i = 0; i < THREAD_COUNT; i++) {
            int j = i;
            System.out.println("Thread №" + j + " started");
            threads.put(new Thread(() -> {
                tasks[j] = new GeneratorTask();
            }), new ThreadInformation(System.currentTimeMillis(), j));
        }
        startThreads();
    }

    private void initNewThread(int index) {
        for (int i = 0; i < tasks.length; i++) {
            if (i == index) {
                threads.put(new Thread(() -> {
                    tasks[index] = new GeneratorTask();
                }), new ThreadInformation(System.currentTimeMillis(), index));
            }
        }
    }

    private void startThreads() {
        for (Thread thread : threads.keySet()) {
            Thread.State state = thread.getState();
            if (state == Thread.State.NEW) {
                thread.start();
            }
        }
    }

    private void getThreadResult() {
        initThreads();
        for (int i = 0; i < tasks.length; i++) {
            System.out.println(tasks[i]);
        }
        while (automatReady < BASE_SIZE) {
            Set mapSet = threads.entrySet();
            Iterator iterator = mapSet.iterator();
            while (iterator.hasNext()) {
                Map.Entry<Thread, ThreadInformation> mapEntry = (Map.Entry)iterator.next();

                if (!mapEntry.getKey().isAlive()) {
                    if (automatReady == BASE_SIZE) continue;
                    int currentIndex = mapEntry.getValue().getIndex();

                    BASE[automatReady] = tasks[currentIndex].getAutomat();
                    System.out.println("Добавлен автомат № " + automatReady);
                    automatReady++;
                    System.out.println("Thread №" + mapEntry.getValue().getIndex() + " finished successfully");
                    iterator.remove();
                    initNewThread(mapEntry.getValue().getIndex());
                    startThreads();
                    System.out.println("Thread №" + mapEntry.getValue().getIndex() + " started");
                } else {
                    if ((System.currentTimeMillis() - mapEntry.getValue().getTimeCreated()) < (THREAD_LIVE_TIME_SEC * 1000)) continue;
                    int taskIndex = mapEntry.getValue().getIndex();
                    tasks[taskIndex].stopGeneration(true);
                    System.out.println("Thread №" + taskIndex + " stoped");
                    mapEntry.getKey().interrupt();
                    iterator.remove();
                    System.out.println("Thread №" + taskIndex + " started");
                    initNewThread(mapEntry.getValue().getIndex());
                    startThreads();
                }
            }
        }
        threads.clear();
    }

    public Automat[] getBASE() {
        return BASE;
    }
}

StackTrace:

Thread №0 started
Thread №1 started
Thread №2 started
Thread №3 started
Thread №4 started
Thread №5 started
Thread №6 started
Thread №7 started
null
null
null
null
null
null
null
null
Exception in thread "main" java.lang.NullPointerException
    at ru.stsz.MultiTaskGenerator.getThreadResult(MultiTaskGenerator.java:78)
    at ru.stsz.MultiTaskGenerator.<init>(MultiTaskGenerator.java:19)
    at ru.stsz.MainApp.main(MainApp.java:6)

Can anyone give me advise how to fix this?

Answer 1

You can Add an assertion each time you need to access your arrays! something like that in the case of emty arrays:

public static void notEmpty(@Nullable Object[] array,String message) {
        if (ObjectUtils.isEmpty(array)) {
            throw new IllegalArgumentException(message);
        }
    }