Java解决No enclosing instance of type PrintListFromTailToHead is accessible问题

今天在编译Java程序时遇到如下问题：

No enclosing instance of type PrintListFromTailToHead is accessible. Must qualify the allocation with an enclosing instance
of type PrintListFromTailToHead (e.g. x.new A() where x is an instance of PrintListFromTailToHead).

源代码为：

 1 public class PrintListFromTailToHead {
 2 
 3     public static void main(String[] args) {
 4         ListNode one = new ListNode(1);
 5         ListNode two = new ListNode(2);
 6         ListNode three = new ListNode(3);
 7         one.next = two;
 8         two.next = three;
 9         
10         ArrayList<Integer> result = printListFromTailToHead(one);
11 
12         System.out.println("结果是：" + result);
13     }
14     
15      class ListNode {
16 
17         public int val;
18         public ListNode next;
19 
20         public ListNode() {
21 
22         }
23 
24         public ListNode(int val) {
25             this.val = val;
26         }
27     }
28 
29     public static ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
30 
31         Stack<Integer> stack = new Stack<Integer>();
32         while (listNode != null) {
33             stack.push(listNode.val);
34             listNode = listNode.next;
35         }
36 
37         ArrayList<Integer> arrayList = new ArrayList<Integer>();
38         while (!stack.isEmpty()) {
39             arrayList.add(stack.pop());
40         }
41         return arrayList;
42     }   
45 }

问题解释：

代码中，我的ListNode类是定义在PrintListFromTailToHead类中的内部类。ListNode内部类是动态的内部类，而我的main方法是static静态的。

就好比静态的方法不能调用动态的方法一样。

有两种解决办法：

第一种：

将内部类ListNode定义成静态static的类。

第二种：

将内部类ListNode在PrintListFromTailToHead类外边定义。

两种解决方法：

第一种：

 1 public class PrintListFromTailToHead {
 2 
 3     public static void main(String[] args) {
 4         ListNode one = new ListNode(1);
 5         ListNode two = new ListNode(2);
 6         ListNode three = new ListNode(3);
 7         one.next = two;
 8         two.next = three;
 9         
10         ArrayList<Integer> result = printListFromTailToHead(one);
11 
12         System.out.println("结果是：" + result);
13     }
14     
15     static class ListNode {
16 
17         public int val;
18         public ListNode next;
19 
20         public ListNode() {
21 
22         }
23 
24         public ListNode(int val) {
25             this.val = val;
26         }
27     }

第二种：

 1 public class PrintListFromTailToHead {
 2 
 3     public static void main(String[] args) {
 4         ListNode one = new ListNode(1);
 5         ListNode two = new ListNode(2);
 6         ListNode three = new ListNode(3);
 7         one.next = two;
 8         two.next = three;
 9     }
10 
11         public static ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
12 
13         Stack<Integer> stack = new Stack<Integer>();
14         while (listNode != null) {
15             stack.push(listNode.val);
16             listNode = listNode.next;
17         }
18 
19         ArrayList<Integer> arrayList = new ArrayList<Integer>();
20         while (!stack.isEmpty()) {
21             arrayList.add(stack.pop());
22         }
23         return arrayList;
24     }
25 }
26 
27 class ListNode {
28 
29     public int val;
30     public ListNode next;
31 
32     public ListNode() {
33 
34     }
35 
36     public ListNode(int val) {
37         this.val = val;
38     }
39 }