java - 如何在迭代时删除和添加元素到 TreeMap？

Question

我想写这样的代码 -

for (Map.Entry<Long, Integer> e : map.entrySet()){
    map.remove(k);
    map.put(x, value);
}

但我得到了java.util.ConcurrentModificationException 我也尝试使用Iterator但我得到了相同的Exception

Answer 1

解释为什么会导致ConcurrentModificationException

map.remove(k);
map.put(x, value);

for-each 循环还在内部创建 of 的迭代entrySet器map。在遍历 map 时，您通过将值再次放入 map( map.put(x,value)) 修改了 map 的结构，这导致了这种情况ConcurrentModificationException。

它甚至在文档中得到了很好的解释-

所有此类的“集合视图方法”返回的迭代器都是快速失败的：如果在创建迭代器后的任何时间对映射进行结构修改，除了通过迭代器自己的 remove 方法之外，迭代器将抛出 ConcurrentModificationException . 因此，面对并发修改，迭代器快速而干净地失败，而不是在未来不确定的时间冒任意的、非确定性的行为。

如何解决这个-

您必须在迭代时更改此地图的结构，您可以稍后插入此值，例如保留一个临时地图并在迭代完成后将其添加到主地图中。

Map<Long, Integer> tempMap = new HashMap<>();
for (Map.Entry<Long, Integer> e : map.entrySet()){
    map.remove(k);
    tempMap.put(x, value);
}
map.putAll(tempMap);

Answer 2

迭代一个副本，你可以添加/删除就好了：

for (Map.Entry<Long, Integer> e : new LinkedHashMap<Long, Integer>(map).entrySet()){
    map.remove(k);
    map.put(x, value);
}

它甚至不再是代码行，因为复制 ims 是通过复制构造函数内联生成的。LinkedHashMap选择保留迭代顺序（如果重要的话）。

Answer 3

下面给出了从地图中删除元素的示例代码片段。

for(Iterator<Map.Entry<Long, Integer>> it = map.entrySet().iterator();it.next();)
{
  Map.Entry<String, String> entry = it.next();
 if(//some logic)    
 it.remove();
}

如果您的代码涉及大量添加和删除，您可能只想使用 ConcurrentHashMap。并发哈希映射

Answer 4

You will have to create a copy of your map using copy constructor. Now iterate on 1 and modify second map. I am assuming that you will not need to iterate newly added value as it wont make much sense.

You can achieve your task by creating a copy is because the keys will remain same in both.

EDIT:

I dont think its a good idea to iterate the newly added element to a Hashmap. If you check the api's provided by Iterator then you will find only remove method, there is no add method in it. There is a reason behind this and you can check javadoc for this. Now coming to the point, on how to iterate newly added element.

1. Create a copy of your HashMap. So you will iterate one and modify the the other Map.
2. As the requirement is to both add and remove elements in Map, i would like to use ListIterator for this [this is different from normal Iterator].
3. I will get the keyset of Map1 and convert it to a list using ArrayList(Collection<? extends E> c).
4. Now i will get ListIterator from List created in step 3, and add, remove elements in ListIterator as well as in Map2 [Remeber you need to add , remove both in ListIterator and Map2].

Answer 5

因为你不能那样做。

一个简单的解决方案是使用另一个临时映射，在其中放置所需的值，最后用原始映射切换指针（即 Map = newMap ）

Answer 6

尝试按如下方式浏览地图

while (tree_map.size() > 0){
     // if k is key
     if (tree_map.containsKey()){
        tree_map.remove(k);
     }
     tree_map.put(x, value);
     break;
     // can go through the for loop or other code as per requirements
 }