今天在編譯Java程序時遇到如下問題：
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).
源代碼為：
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("結果是：" + 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 }
問題解釋：
代碼中，我的ListNode類是定義在PrintListFromTailToHead類中的內部類。ListNode內部類是動態的內部類，而我的main方法是static靜態的。
就好比靜態的方法不能調用動態的方法一樣。
有兩種解決辦法：
第一種：
將內部類ListNode定義成靜態static的類。
第二種：
將內部類ListNode在PrintListFromTailToHead類外邊定義。
兩種解決方法：
第一種：
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("結果是：" + 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 }
第二種：
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 }
