Java ConcurrentModificationException on ArrayList.

问题:

I am trying to implement merge sort in java, but this causes a ConcurrentModificationException in the line while(ai<la.size()&&bi<lb.size()){. I have seen a lot of answers on Stack about this but they all relate to removing items from a list, but I am not doing that. What is causing this exception, and how can I fix this?

public static <Elem extends Comparable<Elem>> void mergesort(List<Elem> list) {
    if(list.size()>1){
        int hf = list.size()/2;
        List<Elem> la = list.subList(0, hf-1);
        List<Elem> lb = list.subList(hf, list.size()-1);
        mergesort(la);
        mergesort(lb);
        int ai=0;
        int bi=0;
        int oi=0;
        while(ai<la.size()&&bi<lb.size()){
            if(la.get(ai).compareTo(lb.get(bi))>0){
                list.add(oi, la.get(ai));
                ai++;
            }
            else{
                list.add(oi, lb.get(bi));
                bi++;
            }
            oi++;
        }
        while(ai<la.size()){
            list.add(oi, la.get(ai));
            ai++;
            oi++;
        }
        while(bi<lb.size()){
            list.add(oi, lb.get(bi));
            bi++;
            oi++;
        }
    }
}

But testing this with as mergesort(Arrays.asList(3, 2, 1, 6, 5, 4)) causes the following error:

java.util.ConcurrentModificationException
at java.util.ArrayList$SubList.checkForComodification(ArrayList.java:1231)
at java.util.ArrayList$SubList.size(ArrayList.java:1040)
at week04.MergeSort.mergesort(MergeSort.java:17)
at week04.test.MergeSortTest.testMergesortUnsortedList(MergeSortTest.java:42)