并发修改列表异常

Question

谁能告诉我我哪里错了?

  File promoCountFile = new File(RaconTours.PATH + "promocodeCount.txt");
            if (promoPlistPath.exists()) {
                try {
                    ObjectInputStream inStream = new ObjectInputStream(new FileInputStream(promoPlistPath));
                    ObjectInputStream promoStream = new ObjectInputStream(new FileInputStream(promoCountFile));
                    promoobj = (ArrayList<HashMap<String, Object>>) promoStream.readObject();
                    obj = (ArrayList<HashMap<String, Object>>) inStream.readObject();
                    for (HashMap<String, Object> tmpObj : obj) {
                        promoTourname = (String) tmpObj.get("promoTour");
                        promocodeID = (String) tmpObj.get("promocode");
                        if (promoTourname.equals(currentTour.getObjtourName())) {
                            //if the condition is met, remove the entry from the file
                            for (HashMap<String, Object> promoTemp : promoobj) {
                                promoTourCount =  (Integer) promoTemp.get("promocodeTourCount");
                            }
                            obj.remove(tmpObj);
                            --promoTourCount; 
                            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(promoPlistPath)); 
                            out.writeObject(obj);
                            out.close();
                            ObjectOutputStream promoout = new ObjectOutputStream(new FileOutputStream(promoCountFile)); 
                            HashMap<String, Object> promoCountDict = new HashMap<String, Object>();
                            promoobj.remove(0);
                            promoCountDict.put("promocodeTourCount",promoTourCount);
                            promoobj.add(promoCountDict);
                            promoout.writeObject(promoobj);
                            promoout.close();

                        }
                    }

                    if (obj.size() == 0 || promoTourCount == 0) {
                        promoPlistPath.delete();
                        promoCountFile.delete();
                    }
                } catch (Exception e) { 
                    e.printStackTrace();
                }
            }

在这里，它给我并发修改异常，而for循环迭代第二次或之后。

我试图在每次循环迭代时更新文件中的promoTourCount值。但我做不到。因为为了避免增加多个对象，我删除了位置0的对象，并在该位置添加了新的对象(promoobj.remove(0);)

请帮帮我

Answer 1

您正在修改要迭代的集合。这将触发一个错误。你在这里迭代obj:

for (HashMap<String, Object> tmpObj : obj) {

但是从这里删除:

obj.remove(tmpObj);

我建议您将要删除的项存储在另一个集合中，只有在完成for循环时才将它们从映射中删除。

编辑: & lt; & lt;添加示例代码>

List<Integer> toRemove = new LinkedList<Integer>();
for (int i = 0; i < obj.size(); i++) {
    HashMap<String, Object> tmpObj = obj.get(i);
    if (/* something */) {
        /* ... */
        /* obj.remove(tmpObj);  replaced with*/
        toRemove.add(0, i); // notice that I add bigger indices first
    }
}
// Here we make the removal from bigger indices to smaller ones
// Notice that we iterate and remove from different collections.
for (Integer indexToDelete : toRemove) {
    obj.remove(indexToDelete);
}

这是想要删除元素时的基本思路。但是，您需要在for循环中立即输出修改后的obj。那么，对索引进行一些修改可能会更好:

for (int i = 0; i < obj.size(); i++) {
    HashMap<String, Object> tmpObj = obj.get(i);
    if (/* something */) {
        /* ... */
        /* obj.remove(tmpObj);  replaced with*/
        obj.remove(i); // We erase the element but this time we do not use enhanced for loop which is ok.
        i--; // we decrease the index because th enumber of elements decreased. Index hack. Ugly.
        System.out.println(obj); // modified obj :)
    }
}