concurrentmodification exception -- setting an array element

Question

I read in JAVAdocs that concurrent modification exception comes when you structurally modify a list while iterating through it.In my case I am not changing the size of array but,just replacing some indices in the array. Tried out many things but this doesnt seem to work.

Providing a snippet below.(The code is a recursive function to solve n-queens problem)

    public ArrayList<ArrayList<String>> solve(int a, String[] arr,int beg,
ArrayList<ArrayList<String>> result) {      
    for( int i=beg;i< a*a ;i++){
        String s = arr[i];
        if(s.equals(".")){
            //fill Q and look for valid combs
           arr[i] = "Q";
           if(i== (a*a)-1){
                 String[] t = (String[])(arr.clone());
                 result.add(new ArrayList<String>(Arrays.asList(t)));
           }else{
               ArrayList<Integer> filled = fillX(arr,i,a,"x");//this function fills "X" in queen's path and returns the filled indices

               ArrayList<ArrayList<String>> tmp = solve(a,arr,i+1,result);
               if(!tmp.isEmpty()){
                    for(ArrayList<String> t :tmp){
                        result.add((ArrayList<String>)t.clone());
                     }
               }
                for(int x = 0;x< a*a;x++){                      
                    if(filled.contains(x)){
                        arr[x] = ".";//the exception goes away on removing this line
                    }                         
                }                 
           }
           arr[i] =".";
        }
    }
        return result;  
}

Stacktrace --

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
at java.util.ArrayList$Itr.next(ArrayList.java:851)
at Solution.solve(Solution.java:29)
at Solution.solveNQueens(Solution.java:7)
at Main.main(Main.java:323)

arr[x)] = "."; ... do you really have this in your code? It obviously won't even compile. — Tim Biegeleisen
– Tim Biegeleisen, Commented Jun 20, 2016 at 2:14
In what javadoc did you see a mention of structurally modifying a collection? I don’t see that anywhere in the ConcurrentModificationException javadoc. In general, any modification followed by use of an Iterator created before the modification will throw ConcurrentModificationException, at least in collection classes which are not in the java.util.concurrent package. — VGR
– VGR, Commented Jun 20, 2016 at 2:46
@TimBiegeleisen the parentheses got added while some modifcation for here — code4fun
– code4fun, Commented Jun 20, 2016 at 3:25

xiaofeng.li · Accepted Answer · 2016-06-20 02:16:43Z

1

I believe you hit a typical Concurrent Modification scenario.

ArrayList<ArrayList<String>> tmp = solve(a, arr, i + 1, result);
for (ArrayList<String> t : tmp) { // <-- iterating through the result of a recursive call.
    result.add((ArrayList<String>)t.clone());
}
...
return result; // <-- while the recursive call returns the `result` argument.

It is basically doing something like:

for (ArrayList<String> t : result) {
    result.add((ArrayList<String>)t.clone());
}

IMHO, you either can make resolve a void method, and add results directly to the passed in result argument; or you can remove the result parameter and make each recursive call create a local List and return it.

answered Jun 20, 2016 at 2:16

xiaofeng.li

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2 Comments

code4fun Over a year ago

Thanks ! The exception went away after your suggestion.But I am still not clear why in my code when I was removing this line the exception was going away and coming back on adding it.

code4fun Over a year ago

for(int x = 0;x< a*a;x++){ if(filled.contains(x)){ arr[x] = ".";//the exception goes away on removing this line } }

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