Question

In this program I keep a list of duelists that kill the best duelist when their random int = 0.
I am having trouble with my loop near the bottom and am getting this error:

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
at java.util.AbstractList$Itr.next(Unknown Source)
at Duelist.main(Duelist.java:74)

Code

import java.util.Arrays;
import java.util.Random;
import java.util.ArrayList;
import java.util.Collections;

public class Duelist implements Comparable{

private String name;
private int chance;
private boolean alive;


public Duelist(String duelistName, int hitChance){
    alive = true;
    name = duelistName;
    chance = hitChance;

    }
public String getName(){
    return name;
}
public int getChance(){
    return chance;
}
public boolean getLife(){
    return alive;
}
public void kill(){
    alive = false;
}

 public int compareTo(Object anotherDuelist) throws ClassCastException {
        if (!(anotherDuelist instanceof Duelist))
          throw new ClassCastException("A Duelist object expected.");
        int anotherDuelistChance = ((Duelist) anotherDuelist).getChance();  
        return this.chance - anotherDuelistChance;    
      }

public static void main(String[] args){

ArrayList<Duelist> duelers = new ArrayList<Duelist>(5);
//ArrayList<Duelist> personToKill= new ArrayList<Duelist>(5);
ArrayList<Duelist> rank = new ArrayList<Duelist>(5);
Random generator = new Random();




Duelist Antoine = new Duelist("Antoine", 3);
Duelist Francis = new Duelist("Francis", 6);
Duelist Juliette = new Duelist("Juliettee", 1);

duelers.add(Antoine); duelers.add(Francis); duelers.add(Juliette);



//System.out.println(duelers);


//for(Duelist element : duelers){
//  System.out.println(element.getName());
//  System.out.println(element.getChance());
//}
Collections.sort(duelers);
Collections.reverse(duelers);
//for(Duelist element : duelers){
    //System.out.println(element.getName());
    //System.out.println(element.getChance());
//}

while(duelers.size() > 1){


    for(Duelist element : duelers){



            System.out.println(element.getName());
            System.out.println("Chance:" + element.getChance());
            int randomInt = generator.nextInt(element.getChance());
            System.out.println("Random int " + randomInt);



            //Destroy target if randomInt equals 0
            if (randomInt == 0){


                //Check to make sure the best duelist is not killing themselves
                if (element.equals(duelers.get(duelers.size()-1))){
                    System.out.println("LASTDUDE");
                    Duelist bestDueler = duelers.get(duelers.size()-2);
                    bestDueler.kill();
                    rank.add(element);
                    duelers.remove(bestDueler);
                }

                else {
                    System.out.println("Killshot");
                    Duelist bestDueler = duelers.get(duelers.size()-1);
                    bestDueler.kill();
                    rank.add(element);
                    duelers.remove(bestDueler);
                }

            }


    }
}
System.out.println(duelers);


}
}

Answer 1

for(Duelist element : duelers){

While you are inside this block, you are iterating over the duelers list, which means that you can’t change the list without causing a ConcurrentModificationException unless you use the iterator’s remove() method.

The problem here is, you don’t have access to the iterator. So you’ll have to change your foreach loop to a while loop with an iterator:

Iterator<Duelist> it = duelists.iterator();
while(it.hasNext()){
    Duelist element = it.next();
    // continue as before

(update after comments)
or better:

for(Iterator<X> it=x.iterator();it.hasNext();){
    Duelist element = it.next();
    //continue as before

and instead of

duelers.remove(element);

write

// the problem here is: you can only remove the current element,
// so you'll have to re-think some of your code
it.remove();

Or: you could create a copy of your list for iterating. That’s a waste of memory (and a code smell) but if your application is small it should not make much of a difference, and it would require the least amount of rewriting on your part.

for(Duelist element : new ArrayList<Duelist>(duelers)){
// continue as above