迭代器和枚举之间的区别

Question

看下面的代码

import java.util.Enumeration;
import java.util.Vector;

public class Modification_On_Eumeration {
    public static void main(String[] args) {
        Vector<Integer> vector = new Vector<Integer>();
        vector.add(1);
        vector.add(2);
        System.out.println(vector);
        Enumeration<Integer> enumeration = vector.elements();
        while (enumeration.hasMoreElements()) {
            Integer integer = (Integer) enumeration.nextElement();
            System.out.print(integer);
        }
        System.out.println();
        System.out.println("first loop finished");
        vector.add(3);
        while (enumeration.hasMoreElements()) {
            Integer integer1 = (Integer) enumeration.nextElement();
            System.out.println(integer1);
        }
    }
}

上面的代码工作正常,输出是:

[1, 2]
12
first loop finished
3

现在看下面的代码:

import java.util.Iterator;
import java.util.concurrent.CopyOnWriteArrayList;

public class Fail_Safe_Iterator {
    public static void main(String[] args) {
        CopyOnWriteArrayList<Integer> copyOnWriteArrayList=new CopyOnWriteArrayList<Integer>();
        copyOnWriteArrayList.add(6);
        copyOnWriteArrayList.add(2);
        System.out.println(copyOnWriteArrayList);
        Iterator<Integer> iterator=copyOnWriteArrayList.iterator();
        while (iterator.hasNext()) {
            Integer integer = (Integer) iterator.next();
            System.out.println(integer);
        }
        System.out.println("first loop finished");
        copyOnWriteArrayList.add(5);
        while (iterator.hasNext()) {
            Integer integer = (Integer) iterator.next();
            System.out.println(integer);//why not printing 5
        }
    }
}

输出是:

[6, 2]
6
2
first loop finished

我正在学习java ..我怀疑是在添加3之后的第一个例子中,我能够回溯并打印它.但是在第二个例子中,在添加5之后我无法回溯并打印它.原因是什么？是因为迭代器失败安全行为？或者它是枚举和迭代器之间的区别之一？帮我....

Answer 1

根据支持列表的底层数组的当前状态CopyOnWriteArrayList创建一个新Iterator的.

Java来源

 public ListIterator<E> iterator() {
         return new COWIterator<E>(getArray(), 0);
 }

此迭代器不包含创建后添加的元素,因为它通过创建CopyOnWriteArrayList基础数组的新副本来执行变异操作(添加,设置等).因此,在创建迭代器时,它会在该瞬间反射基础数组.

固定

    System.out.println("first loop finished");
    copyOnWriteArrayList.add(5);

    //get the new iterator after adding
    iterator = copyOnWriteArrayList.iterator();

    while (iterator.hasNext()) {
        Integer integer = (Integer) iterator.next();
        System.out.println(integer);//why not printing 5
    }