为什么java中的基本堆栈代码没有运行?

因此,我试图编写一个堆栈数据结构的基本代码,当我在sample\ u stack中运行这个类时,它只是不运行,不打印单词,而是只打印“null”:(有人知道为什么吗?如果这是显而易见的,我道歉

堆栈JAVA类:

import java.util.NoSuchElementException;
public class Stack {

// private inner class node

private class Node{
    
    private String item;
    private Node link;
    
    public Node() {
        item = null;
        link = null;
    }
    
    public Node(String item, Node link) {
        item = this.item;
        link = this.link;
    }
    
}  // end of inner class 

private Node head;

public Stack() {
    head = null;
}

// method: PUSH into stack          (like addToStart)

public void push(String itemName) {
    head = new Node(itemName, head);            // so head is the top of the stack ????
}

// method: POP out of stack

public String pop() {
    if (head == null) throw new IllegalStateException();
    
    else {
        String returnItem = head.item;
        head = head.link;   // the second top item becomes the new head
        return returnItem;
    }
}

// method: is it empty?

public boolean isEmpty() {
    return ( head == null );
}
}

使用堆栈JAVA类初始化:

public class Stack_Example {

public static void main (String[] args) {
    
    Stack message = new Stack();
    
    message.push("Hi");
    
    System.out.println(message.pop());
    
    message.push("my");
    message.push("name");
    message.push("is");
    message.push("JARVIS");
    
    
    while (!message.isEmpty()) {            // while true
        String s = message.pop();
        System.out.println(s);
    }           
}
    
}

提前谢谢!