CODEWITHSUNDEEP
javamultithreadingsortingsynchronized
user265767
user265767

Reputation: 559

Bubble sort with threads

I have a school assignment where Iam going to use threads in Java to sort a long name list. There is no focus on the speed of the sorting algorithm, but a assignment for understanding threads and how threads affect the speed of the sort.

I feel I'm on the right path, but the code/threads messes up the monitor, I have tried for a while to locate the problem, for me the problem seems to be in class SortThreads run() method. Can folks please give me some tips or hints?

public void run() {

    while (monitor.getSize() > 1) {
        System.out.println("Number of array in list to be sorted: "
                + monitor.getSize());

        String[] f = monitor.getRandom();
        String[] g = monitor.getRandom();

        monitor.add(descSort(f, g));
    }

    System.out.println("*** THREAD DEAD ***");

}

Output:

After delete: 1
Number of array in list to be sorted: 3
Befor delete: 1
After delete: 0
Number of array in list to be sorted: 0
Exception in thread "Thread-101" java.lang.NullPointerException
    at SortThread.append(Sort.java:222)
    at SortThread.descSort(Sort.java:201)
    at SortThread.run(Sort.java:178)
Exception in thread "Thread-96" java.lang.NullPointerException
    at SortThread.append(Sort.java:222)
    at SortThread.descSort(Sort.java:201)
    at SortThread.run(Sort.java:178)
Number of array in list to be sorted: 0
Number of array in list to be sorted: 0

Upvotes: 1

Views: 2682

Answers (1)

assylias
assylias

Reputation: 328618

One of your (possible) problems is that this section is not atomic:

while (monitor.getSize() > 1) {
    System.out.println("Number of array in list to be sorted: "
            + monitor.getSize());

    String[] f = monitor.getRandom();
    String[] g = monitor.getRandom();

    monitor.add(descSort(f, g));
}

Typically, if you use any threads, it is very likely that between the call to monitor.getSize() and monitor.getRandom() the size has actually changed. So I guess that one of the getRandom calls (or both) returns null which triggers the NullPointerException.

You could add a check for null before calling descSort(). If only one of f and g is null, you probably need to put it back into the monitor (just guessing here).

Upvotes: 1

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