java.util.ConcurrentModificationException in Non Multithreaded Program

Question

Hey SO Guru's im having one heck of a job with this code

public void kill(double GrowthRate, int Death)
{
    int before = population.size();
    for (PopulationMember p : population)
    {
        int[] probs = ProbablityArrayDeath(GrowthRate,Death,(int)p.fitness());
        if (probs[RandomNumberGen.nextRandomInt(0, 99)]==0)
        {
            population.remove(p);
        }
    }
    System.out.println("Intial Population: "+before+", Deaths:"+(before-          population.size())+", New Population: "+population.size());
}

When I run my program the first time it tries to run the code it hits this error

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
    at java.util.HashMap$KeyIterator.next(HashMap.java:828)
    at Genetics.Population.kill(Population.java:181)
    at Genetics.Population.run(Population.java:47)
    at Control.Main.main(Main.java:35)

Having goggled around a bit this seems to be an error that normally happens with threads why they try and access the same resource concurrently, but this is what getting me im not multithreading at all in this system.

Can someone explain why this is happening, or think of a hack to get around it

Many thanks ^_^

Answer 1

You can modify the underlying Collection of the Iterator (which is hidden in the for-each loop). The proper way to do this is:

for (Iterator<PopulationMember> it = population.iterator(); it.hasNext();) {
    PopulationMemeber p = it.next();
    int[] probs = ProbablityArrayDeath(GrowthRate,Death,(int)p.fitness());
    if (probs[RandomNumberGen.nextRandomInt(0, 99)] == 0) {
        it.remove();
    }
}

Answer 2

You can't use the for each loop if you remove things from the collection.
You have to use an Iterator and to remove the current item call Iterator.remove.

Otherwise, the underlying iterator that the for-each loop creates for you behind the scenes doesn't understand how come the collection it's going through is changing, tells you that it is being changed while you iterate it.

Answer 3

You've got an iterator over population hidden under a for loop. You are removing an item from population in the middle of iterator working. Iterator can't work no more because you changed the collection in the middle of it iterating.

It's not related to multithreading.

Answer 4

A workaround can be copy a collection. Iterate over the copy and remove elements from the original collection.

public void kill(double GrowthRate, int Death) {
    int before = population.size();
    Collection<PopulationMember> forIteration = new HashSet<PopulationMember>(population); 
    for (PopulationMember p : forIteration) {
        int[] probs = ProbablityArrayDeath(GrowthRate,Death,(int)p.fitness());
        if (probs[RandomNumberGen.nextRandomInt(0, 99)]==0) {
            population.remove(p);
        }
    }
    System.out.println("Intial Population: "+before+", Deaths:"+(before - population.size())+", New Population: "+population.size());

}