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 }