java.util.concurrentmodificationexception: null

import java.util.concurrent.ConcurrentHashMap;

public class Example {
    public static void main(String[] args) {
        ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
        map.put("key1", "value1");
        
        for (String key : map.keySet()) {
            map.put(key + "_new", "newValue"); // 不会抛出异常
        }
    }
}

import java.util.ArrayList;
import java.util.Iterator;

public class Example {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("A");
        list.add("B");
        
        Iterator<String> iterator = list.iterator();
        while (iterator.hasNext()) {
            String item = iterator.next();
            if ("A".equals(item)) {
                iterator.remove(); // 正确方式
            }
        }
    }
}

import java.util.ArrayList;

public class Example {
    private static ArrayList<String> list = new ArrayList<>();

    public static synchronized void addItem(String item) {
        list.add(item);
    }

    public static synchronized void removeItem(String item) {
        list.remove(item);
    }
}