执行并行时出现异常 Stream Java 8

Question

I am writing a function that calculates the sum of squares of digit for a given number. I am calling this function for a number from 1 to 1000 in parallel. But I am getting an exception some time.

public static void main(String[] args) throws Exception {
    IntStream stream = IntStream.rangeClosed(1, 1000);
    Map < Integer, Integer > numbers = new TreeMap < > ();
    stream.parallel().forEach(i - > {
        int result = digitSquareSum(i);
        numbers.put(i, result);
    });
    Files.write(Paths.get("result.csv"), () - > numbers.entrySet().stream()
        . < CharSequence > map(e - > e.getKey() + "," + e.getValue())
        .iterator());
}

private static int digitSquareSum(int i) {
    int result = 0;
    int temp = 0;
    while (i != 0) {
        temp = i % 10;
        result = result + temp * temp;
        i = i / 10;
    }
    return result;
}

I am getting following exception

Exception in thread "main" java.lang.NullPointerException
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:488)
    at java.base/java.util.concurrent.ForkJoinTask.getThrowableException(ForkJoinTask.java:590)
    at java.base/java.util.concurrent.ForkJoinTask.reportException(ForkJoinTask.java:668)
    at java.base/java.util.concurrent.ForkJoinTask.invoke(ForkJoinTask.java:726)
    at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateParallel(ForEachOps.java:160)
    at java.base/java.util.stream.ForEachOps$ForEachOp$OfInt.evaluateParallel(ForEachOps.java:189)
    at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:233)
    at java.base/java.util.stream.IntPipeline.forEach(IntPipeline.java:417)
    at java.base/java.util.stream.IntPipeline$Head.forEach(IntPipeline.java:574)

My aim is to write an efficient method that calculates sum of squares of digit of the number and writes it to the csv file in ascending order manner.

Answer 1

As mentioned in the comments, you are getting this exception, because TreeMap is not thread safe. From javadoc :

Note that this implementation is not synchronized. If multiple threads access a map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with an existing key is not a structural modification.)

I beleive you are using TreeMap to preserve the order of your initial numbers. But it's not necessary (and also introduces side-effects ): you could use Collectors.toList() which collects all the input elements into a List, in encounter order .

This will also allow you to avoid creation of the second stream to write these numbers to a file:

List<String> lines = IntStream.rangeClosed(1, 1000)
            .parallel()
            .mapToObj(i -> i + " -> " + digitSquareSum(i))
            .collect(Collectors.toList());
Files.write(Paths.get("result.csv", lines); // the lines will come in the right order