How to fix: Java Collection Error: Collection traversal exception

How to solve: Java Collection Error: Collection Traversal Exception

Introduction:
In Java development, collections are very commonly used data structures for storing and Manipulate a set of data. However, when using collections for traversal operations, collection traversal exceptions are often encountered. This article describes the cause and solution of this problem, and provides corresponding code examples.

1. Reasons for collection traversal exceptions:
When we use iterators or for-each loops to traverse the collection, if the collection is modified during the traversal process (such as adding or deleting elements), then ConcurrentModificationException will be thrown. This is because, during the traversal process, the iterator or for-each loop will maintain a counter to detect whether the structure of the collection has changed, and will throw an exception once a change is found.

2. Solution:

1. Use Iterator to traverse the collection, and use Iterator's remove() method to delete elements during the traversal process. An example is as follows:

List list = new ArrayList<>();
// 添加元素
list.add("A");
list.add("B");
list.add("C");

Iterator iterator = list.iterator();
while (iterator.hasNext()) {
    String item = iterator.next();
    if (item.equals("B")) {
        iterator.remove(); // 删除元素
    }
}

2. When using an ordinary for loop to traverse a collection, you can use the remove() method of List to delete elements, and you need to traverse the collection in reverse order to avoid changes in the index. An example is as follows:

List list = new ArrayList<>();
// 添加元素
list.add("A");
list.add("B");
list.add("C");

for (int i = list.size() - 1; i >= 0; i--) {
    String item = list.get(i);
    if (item.equals("B")) {
        list.remove(i); // 删除元素
    }
}

3. Use concurrent collection classes, such as ConcurrentHashMap or CopyOnWriteArrayList, which allow the collection to be modified during traversal without throwing exceptions. Examples are as follows:

ConcurrentHashMap map = new ConcurrentHashMap<>();
// 添加元素
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

for (Map.Entry entry : map.entrySet()) {
    if (entry.getValue() == 2) {
        map.remove(entry.getKey()); // 删除元素
    }
}

Conclusion:
Collection traversal exceptions are caused by modifications to the collection during the traversal process. To solve this problem, we can use Iterator to traverse the collection and use the remove() method, use a normal for loop to traverse the collection and use the remove() method of List, or use a concurrent collection class. Choosing the appropriate solution can effectively avoid the problem of collection traversal exceptions and ensure the normal operation of the program.

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