[Solved] Exception in thread “main” java.util.ConcurrentModificationException

I want to judge whether there is an element “world” in a collection. If there is, I will add a “JavaEE” element

The practice at that time was as follows:

public class ListIteratorDemo1 {

    public static void main(String[] args) {
        // Create a List collection object
        List list = new ArrayList();
        // Add elements
        list.add("hello");
        list.add("world");
        list.add("java");

        // Iterator traversal
        Iterator it = list.iterator();
        while (it.hasNext()) {
            String s = (String) it.next();
            if ("world".equals(s)) {
                list.add("javaee");
            }
        }
　　　　 System.out.println("list:" + list);
    }
}

But the error exception in thread “main” java.util.concurrent modificationexception was reported

Check the API to know:

Concurrent modificationexception: this exception is thrown when a method detects concurrent modifications to an object, but does not allow such modifications

The reasons are as follows

The iterator depends on the collection. After the judgment is successful, a new element is added to the collection, but the iterator doesn’t know it, so an error is reported. This error is called concurrent modification exception

How to solve it
A: the iterator iterates the element, and the iterator modified element
follows the element iterated just now
b: set traversal element, set modification element (common for)
element added at the end

A’s solution:

public class ListIteratorDemo1 {

    public static void main(String[] args) {
        // Create a List collection object
        List list = new ArrayList();
        // Add elements
        list.add("hello");
        list.add("world");
        list.add("java");

        // Way 1: Iterator iterates over elements, iterator modifies elements
        // The Iterator iterator, however, has no added functionality, so we use its sub-interface ListIterator
        ListIterator lit = list.listIterator();
        while (lit.hasNext()) {
            String s = (String) lit.next();
            if ("world".equals(s)) {
                lit.add("javaee");
            }
        }
        System.out.println("list:" + list);
    }
}

Implementation results:

From the result, we can see that the element follows the element of the previous iteration

B’s solution:

public class ListIteratorDemo1 {

    public static void main(String[] args) {
        // Create a List collection object
        List list = new ArrayList();
        // Add elements
        list.add("hello");
        list.add("world");
        list.add("java");

        // way 2: set iterate over elements, set modify elements (normal for)
        for (int x = 0; x < list.size(); x++) {
            String s = (String) list.get(x);
            if ("world".equals(s)) {
                list.add("javaee");
            }
        }

        System.out.println("list:" + list);

    }

}

Implementation results:

The result shows that the element is added at the end

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