下面是一个“关于集合的迭代删除问题”

public class  TestIterator{

        public static void main(String[] args) {

                List<String> aList = new ArrayList<String>();

                aList.add("aaa");

                aList.add("bbb");

                aList.add("asdf");

                aList.add("jvaa");

                aList.add("aaaa");

                aList.add("itcast");

                System.out.println(aList);

                delElements1(aList);

                delElements2(aList);

                delElements3(aList);

                System.out.println(aList);

        }

        public static void delElements1(List<String> aList) {

                for (String str : aList) {

                        if (str.equals("aaa")) {

                                aList.remove(str);

                        }

                }

        }

        public static void delElements2(List<String> aList){

                Iterator<String> it = aList.iterator();

                 while(it.hasNext()){

                        String str = it.next();

                        if(str.equals("aaa")){

                                aList.remove(str);

                        }

                 }

        }

       

        public static void delElements3(List<String> aList){

                        Iterator<String> it = aList.iterator();

                         while(it.hasNext()){

                        String str = it.next();

                        if(str.equals("aaa")){

                                it.remove();

                        }

                 }

        }

}

以上代码中delElements1,delElements2都会报异常：Exception in thread "main" java.util.ConcurrentModificationException,而delElements3则不会报异常，请问这是为什么呢？

答案由读者来回答。 方便 你们来思考。