为什么在定义类之前要声明嵌套类的变量

Question

For example,

public class Stack<Item> implements Iterable<Item>
{
 private Node first; // top of stack (most recently added node)
 private int N;      // number of items
 private class Node
 {  // nested class to define nodes
  Item item;
  Node next; 
 }

Why can we declare first to be of type Node before defining what Node is. Also, as an extension, why are instance variables always declared first. What happens if we define some instance variables, then some methods, then more instance variables, and more methods.

Answer 1

Because there is no attempt at resolution until the entire class has been parsed. For the same reason, you can call class methods that haven't been defined yet.

Instance variable are not always declared first. You can declare things in any order you like, subject only to initialization order constraints.