数组列表中的java.util.ConcurrentModificationException

Question

I am pretty new to android and I am having a difficult time using ArrayList .I used an arrayList in my code . Under some condition I update the arraylist value . But when I try to retrieve data from that array list I get java.util.Concurrent modification exception . I tried to solve this problem as far as I can . But i really cannot solve . That's why I am asking here . Help me please .

Declararation :

HashMap<String,List<Player>> subPlayerAndCountForBackArrow = new HashMap<String, List<Player>>();

Here is the code I used to update data in arrayList :

Iterator<Player> iter = subPlayerAndCountForBackArrow.get(String.valueOf(imanopage)).iterator();
while   (iter.hasNext()) {
if (iter.next().getTag().toString().equals(old_Parent.getChildAt(0).getTag().toString())){       
iter.remove();
    }
}
subPlayerAndCountForBackArrow.get(String.valueOf(imanopage)).add((Player)isGoal.getChildAt(0));

This is the code I used to retrieve data from arrayList :

if(v.getId()==R.id.imgRightArrow){
if(subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).size()>0){  //this is the error line 
Log.i("UpArrowUpArrowUpArrow", "UpArrow");
if(imanopage <= subPlayerAndCountForBackArrow.size()){
        subPlayer1.removeAllViews();
subPlayer2.removeAllViews();
subPlayer3.removeAllViews();
subPlayer4.removeAllViews();
for(int j = 0;j<subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).size();j++){

if(j == 0){
subPlayer1.addView(subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).get(j));
RelativeLayout.LayoutParams lp=(RelativeLayout.LayoutParamssubPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).get(j).getLayoutParams();
     lp.addRule(RelativeLayout.CENTER_IN_PARENT);
     lp.width = 50;
     lp.height = 45;

}

if(j == 1){
subPlayer2.addView(subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).get(j));
      RelativeLayout.LayoutParams lp=(RelativeLayout.LayoutParams)subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).get(j).getLayoutParams();
lp.addRule(RelativeLayout.CENTER_IN_PARENT);
lp.width = 50;
lp.height = 45;
}

if(j == 2){
subPlayer3.addView(subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).get(j));
RelativeLayout.LayoutParams lp=(RelativeLayout.LayoutParams)subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).get(j).getLayoutParams();
lp.addRule(RelativeLayout.CENTER_IN_PARENT);
lp.width = 50;
lp.height = 45;
}

if(j == 3){
subPlayer4.addView(subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).get(j));
RelativeLayout.LayoutParams lp=(RelativeLayout.LayoutParams)subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).get(j).getLayoutParams();
lp.addRule(RelativeLayout.CENTER_IN_PARENT);
lp.width = 50;
lp.height = 45;
}
}
if(imanopage == subPlayerAndCountForBackArrow.size()){
}
      else{
imanopage ++;
}
}
}
}

Answer 1

In the first chunk of code, you've iterated of subPlayerAndCountForBackArrow and modified it (by calling iter.remove ).

Iterator<Player> iter = subPlayerAndCountForBackArrow.get(String.valueOf(imanopage)).iterator();
[...]
iter.remove();

A different chunk of code iterates (I assume separately) over the same list:

[...]
if(subPlayerAndCountForBackArrow.get(String.valueOf(imanopage+1)).size()>0){  //this is the error line 
[...]

You can't do that. You can't iterate over a list you modify concurrently. That's what the exception tells you.

Answer 2

You can't remove items from a list while you iterate through it.

The best solution I have seen is to create a second list, add the items you want to remove to the second list. Then iterate over your second list and for each item remove it from the first list. When you're done, clear the second list. At this point there should be no references left to the items you want to remove.

Here is some code, the first loop checks to see if the item should be removed and adds it to the remove list. The next loop removes them completely.

    for (int i = 0; i < myList.size(); ++i) {
        Item item = myList.get(i);
        if (item.isValid()) {
            item.doStuff();
        } else {
            nulledItems.add(item);
        }
    }

    if (!nulledItems.isEmpty()) {
        for (Item item : nulledItems) {
            if (myList.contains(item)) {
                myList.remove(item);
            }
        }
        nulledItems.clear();
    }

Answer 3

This exception occurs when you try to iterate and modify the collection at the same time. So i would suggest to Take another List which holds the item to remove. And the iterate again to remove only.