Java : ConcurrentModificationException while iteration over list

Question

I'm currently working on an AI that will take decisions for every individual of a list. But while iterating over this list, I get ConcurrentModificationException. I verified if my removeDeads function could cause such a thing but as it isn't inside the loop, it shouldn't. Here is the error stack :

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:473)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:372)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:945)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:973)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:198)
    at java.base/java.lang.Thread.run(Thread.java:844)
Caused by: java.util.ConcurrentModificationException
    at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:939)
    at java.base/java.util.ArrayList$Itr.next(ArrayList.java:893)
    at IA.RLAI.play(RLAI.java:48)
    at Game.Game.play(Game.java:61)
    at Main.start(Main.java:136)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:919)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$11(PlatformImpl.java:449)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$9(PlatformImpl.java:418)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:417)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:277)
    ... 1 more
Exception running application Main

The problem is that it never happens at the same moment. I could have iterated on 3 individuals as well as 15.

Here is my code :

    public void play(Population population, Game game) throws Exception {
        //looking if some individuals died from the other player
        ArrayList<Individual> deads = new ArrayList<>();
        for(Individual i: population.getIndividuals()){
            if(i.getCurrentHP()==0){
                deads.add(i);
            }
        }
        population.getIndividuals().removeAll(deads);
        for (Individual dead: deads) {
            learn(dead);
        }

    //Normal individuals
    for (Individual me: population.getIndividuals()) {
        int size = Math.max(0, me.getMemory().size()-1);

        if(me.getMemory().isEmpty()){
            System.out.println(me.getMemory().toString());
        }else {
            System.out.println(me.getMemory().get(size).toString());
        }

        double[] inputs = new double[153];
        int i = 0;

        double[] toInsert = getAllyMap(me);
        System.arraycopy(toInsert, 0, inputs, i, 49);
        i+=49;

        toInsert = getEnemyMap(me, game);
        System.arraycopy(toInsert, 0, inputs, i, 49);
        i+=49;

        toInsert = getFoodMap(me);
        System.arraycopy(toInsert, 0, inputs, i, 49);
        i+=49;

        inputs[i++] = me.getPosition().getX()-population.getFoodBase().getX(); //Relative position to foodBase X
        inputs[i++] = me.getPosition().getY()-population.getFoodBase().getY(); //Relative position to foodBase Y
        inputs[i++] = me.getCurrentHP();
        inputs[i++] = me.getCurrentFoodLvl();
        inputs[i++] = me.getCurrentFoodStock();
        inputs[i++] = population.getMapping().getCase(population.getFoodBase()).getFoodQuantity();

        double[] outputs = network.forward(inputs);
        //calculus of the probability of the outputs
        double sumExp = 0;
        for(int j = 0; j<outputs.length; j++){
            sumExp += Math.exp(outputs[j]);
        }
        double[] proba = new double[outputs.length];
        for (int j = 0; j<outputs.length; j++){
            proba[j] = Math.exp(outputs[j])/sumExp;
        }
        //determination of the chosen output
        double[] probCuml = new double[outputs.length+1];
        probCuml[0] = 0;
        for (int j = 0; j < outputs.length; j++){
            probCuml[j+1] = probCuml[j]+proba[j];
        }
        double rand = Math.random();
        int choice = 1;
        for(int j = 1; j<probCuml.length; j++){
            if(rand < probCuml[j]){
                choice = j-1;
                break;
            }
        }
        //Action
        switch(choice){
            case 0:
                me.move(Actions.MoveUp);
                break;
            case 1:
                me.move(Actions.MoveDown);
                break;
            case 2:
                me.move(Actions.MoveRight);
                break;
            case 3:
                me.move(Actions.MoveLeft);
                break;
            case 4:
                Population toAttack = game.getOtherPopulation(population);
                me.attack(toAttack);
                break;
            case 5:
                me.collect();
                break;
            case 6:
                me.store();
                break;
            case 7:
                me.eat();
                break;
            case 8:
                me.copulate();
                break;
            default :
                throw new Exception("Out of Possible Choice");
        }
        me.regen();
        if(me.getCurrentHP()<=0){me.addMemory(Reinforcement.Die);}
        learn(me);
    }
    //looking for individuals that die of hunger but as they already learned, 

    no need to do it again
    population.areDead();
    System.out.println(" ");
}

And here is my areDead function:

public ArrayList<Individual> areDead(){
        ArrayList<Individual> areDead = new ArrayList<>();
        for (Individual individual: individuals) {
            if(individual.getCurrentHP()<=0){
                areDead.add(individual);
            }
        }
        individuals.removeAll(areDead);
        return areDead;
    }

The problem is that I don't know where this exception comes from. It says it comes from the loop on individuals but there is nothing that modifies the list in it.

Do you call your `play(...)` method within some loop structure? — Hovercraft Full Of Eels, May 08 '18 at 13:12
It looks like error is at `areDead.add(individual);` since you are modifying the array structure. — Harshit, May 08 '18 at 13:13
@HarshitShrivastava no, because `areDead` isn't the collection being iterated. — Andy Turner, May 08 '18 at 13:14
@HarshitShrivastava: that can't be the problem. I'm not sure that the question as currently written is solvable by us. — Hovercraft Full Of Eels, May 08 '18 at 13:14
`areDead()` can be more efficiently and concisely implemented as `individuals.removeIf(individual -> individual.getCurrentHP()<=0)`. — Andy Turner, May 08 '18 at 13:14
If it isn t solvable in this form, then what information would you need? The error is throws at the line : ' for(Individual me : population.getIndividuals()){' — Timothée Neitthoffer, May 08 '18 at 13:20
We would need you to post a valid [mcve] program in your question to prove that it's not a duplicate (but 99% chance that it is). And you never answered my first comment's question. — Hovercraft Full Of Eels, May 08 '18 at 13:23
And are you 100% sure that that is the line that throws the exception? It looks unusual to do this. — Hovercraft Full Of Eels, May 08 '18 at 13:24
I found the answer. Even if the error is thrown in the foreach loop, it doesn't come from it. In fact, the function "copulate()" is the one to throw the error as it adds an Individual to the list I am iterating on. Thanks for your answers and sorry, as i wasn't understanding from where the problem rose, i thought it wasn't a duplicate. Thanks for your time and have a good afternoon — Timothée Neitthoffer, May 08 '18 at 13:40

Java : ConcurrentModificationException while iteration over list

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