Null pointer Exception during Stream.forEach

Question

I am trying to simulate a median of infinite nos.This is my code

package hard;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Random;
import java.util.stream.Stream;

public class Median
{
static Queue<Integer> minHeap;

static Queue<Integer> maxHeap;

{
    minHeap = new PriorityQueue<>();

    maxHeap = new PriorityQueue<>();
}

private static void add(Stream<Integer> randomNos)
{

    randomNos.forEach(no ->
    {
        if (minHeap.size() == 0 || no < minHeap.peek())
        {
            minHeap.add(no);
        }
            else
            {
                maxHeap.add(no);
            }

            int diff = minHeap.size() - maxHeap.size();

            if (Math.abs(diff) > 1)
            {
                if (diff < 0)
                {
                    minHeap.add(maxHeap.peek());
                }
                else
                {
                    maxHeap.add(minHeap.peek());
                }
            }
        });

}
private static int median()
{
    int median = 0;

    if ((minHeap.size() + maxHeap.size()) % 2 == 0)
    {
        median = (minHeap.peek() + maxHeap.peek()) / 2;
    }
    else
    {
        if (minHeap.size() < maxHeap.size())
        {
            median = maxHeap.peek();
        }
        else
        {
            median = minHeap.peek();
        }
    }

    return median;
}

public static void main(String[] args)
{
    add(new Random().ints(1, 100000).boxed());
    System.out.println(median());
}

}

I get the following exception :

Exception in thread "main" java.lang.NullPointerException
at hard.Median.lambda$0(Median.java:25)
at hard.Median$$Lambda$2/1929600551.accept(Unknown Source)
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)
at java.util.stream.IntPipeline$4$1.accept(IntPipeline.java:250)
at java.util.Random$RandomIntsSpliterator.forEachRemaining(Random.java:1044)
at java.util.Spliterator$OfInt.forEachRemaining(Spliterator.java:693)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150)
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
at hard.Median.add(Median.java:23)
at hard.Median.main(Median.java:75)

I ran the debugger and found that there are 2 values of the same integer repeated after which this exception is flushed out

You might want to point out what line 25 is. (BTW: .peek() might return null). Besides: I dont think this is a correct implementation. You need to sort the values. — eckes
– eckes, Commented Jul 29, 2014 at 2:55

Sotirios Delimanolis · Accepted Answer · 2014-07-29 02:53:34Z

2

This

{
    minHeap = new PriorityQueue<>();

    maxHeap = new PriorityQueue<>();
}

is an instance initializer. You may want

static {
    minHeap = new PriorityQueue<>();

    maxHeap = new PriorityQueue<>();
}

so that it is executed when the class is initialized.

Note however that the PriorityQueue objects will be empty. peek will return null anyway and the < comparison will fail. You may want to add some objects to the PriorityQueue objects before you use them.

answered Jul 29, 2014 at 2:53

Sotirios Delimanolis

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1 Comment

Abhijeet Kushe Over a year ago

Oops !a simple error .Thanks for catching it.I was fooled by the stack trace

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