Detailed explanation of concurrent modification of iterators

Concurrent modification of iterator exception

My understanding

1. Iterator traverses collection objects
. If the collection method is used to modify the collection during the traversal process (for example, the add method of the collection is called to add an element during the traversal process )
, concurrent modification exceptions will occur

2. Iterator traverses collection objects
. If the iterator method is used to modify the collection during the traversal process (such as calling the iterator's remove method to remove an element during the traversal process )
, concurrent modification exceptions will not occur.

Code demo

public void testIteratorException() {
    
    
        List<Integer> list = new ArrayList<Integer>();
        // 快速添加数据 到 list对象
        Collections.addAll(list, 1, 2, 100, 30, 5, 55);
        System.out.println(list);
        //迭代器对象it
        final Iterator<Integer> it = list.iterator();
        //遍历it迭代器中的值
        while(it.hasNext()) {
    
    
            //输出当前it的值
            final Integer curr = it.next();
            System.out.println(curr);

            if (curr == 100) {
    
    
                // 使用迭代器时候 进行 添加操作
                // list.add(101); 在遍历过程中调用集合的add方法对集合进行修改
                list.add(101);
            }
        }
        System.out.println(list);
    }

When we traverse to 100 elements, we call the add method of the collection, then a concurrent modification exception

will occur. This error will not occur when we call the iterator method,
as shown below

public void testListIterator() {
    
    
        List<Integer> list = new ArrayList<Integer>();
        // 快速添加数据 到 list对象
        Collections.addAll(list, 1, 2, 100, 30, 5, 55);
        System.out.println(list);

//        final ListIterator<Integer> listIt = list.listIterator();
        final Iterator<Integer> it = list.iterator();

        while(it.hasNext()) {
    
    
            final Integer curr = it.next();
            System.out.println(curr);
            if (curr == 100) {
    
    
                // 使用迭代器时候 进行 添加操作
                // it.remove(); 在遍历过程中调用迭代器的方法对集合进行修改
                it.remove();
            }
        }
        System.out.println(list);
    }