ConcurrentModificationException 并发修改异常

package com.heima.day01;

import java.util.ArrayList;

public class Demo {
    public static void main(String[] args) {

        ArrayList<String> arrayList = new ArrayList();
        arrayList.add("a1");
        arrayList.add("a2");
        arrayList.add("a3");
        arrayList.add("a4");
        arrayList.add("a5");
        arrayList.add("a6");

        for (String s : arrayList) {
            if (s.equals("a2")) {
                arrayList.remove(s);
            }
        }
    }
}