Java – 链接堆栈实现,无法弄清楚为什么始终打印相同的数字

所以我一直在努力完成以下大学实验.大部分代码都提供给我们,但我们被告知要填写LinkedStack. java代码的方法.这个概念非常简单,每个Node()对象都包含一个Object类型变量(在本例中为Integer)和另一个Node()对象. LinkedStack()类用于初始化这些节点并设置它们的数量限制.问题是,在执行StackTest.java之后,返回以下内容：

18 18 18 18 18 18 18 18 18 18
6 6 6 6 6 6

当它应该返回以下内容：

18 16 14 12 10 8 6 4 2 0
6 5 4 3 2 1

附：代码被广泛评论,所以我相信你将能够找出我没有提到的任何内容.此外,StackADT只是一个简单的接口,它只是陈述LinkedStack中使用的方法.

当然,非常感谢任何帮助过的人！

这是所有的代码(对不起,我是新来的,不知道如何防止它分组)：

// ***************************************************************
    // LinkedStack.java
    //
    // A linked implementation of an Object stack class with operations push,// pop,and isEmpty and isFull.
    //
    // ***************************************************************
    public class LinkedStack implements StackADT {

    private Node top; // reference to top of stack
    private int size;
    private int counter = 0;

    // ---------------------------------------------------
    // Constructor -- initializes top
    // ---------------------------------------------------
      public LinkedStack() {
        top = new Node();
        size = 10;
      }

    // ---------------------------------------------------
    // Adds element to top of stack if it's not full,else
    // does nothing.
    // ---------------------------------------------------
      public void push(Object val) {
        counter++; 
        if(counter > 0 && counter <= size) {]
          top.setNext(top);
          top.setElement(val);
        }
        else {
          top.setElement(val);
        }
      }

    // ---------------------------------------------------
    // Removes and returns value at top of stack. If stack
    // is empty returns null.
    // ---------------------------------------------------
      public Object pop() {
        if (counter > 0) {
          Object val = top.getElement();
          top = top.getNext();
          counter--;
          return(val);
        }
        else {
          return(null);
        }
      }

    // ---------------------------------------------------
    // Returns true if stack is empty,false otherwise.
    // ---------------------------------------------------
      public boolean isEmpty() {
        if(counter == 0) {
          return(true);
        }
        else {
          return(false);
        }
      }

    // ---------------------------------------------------
    // Returns true if stack is full,false otherwise.
    // ---------------------------------------------------
      public boolean isFull() {
        if(counter == size) {
          return(true);
        }
        else {
          return(false);
        }
      }
    }

Node.java:

//***********************************************************
// Node.java
// A general node for a singly linked list of objects.
//***********************************************************

public class Node {

  private Node next;
  private Object element;

//----------------------------------------------------
// Creates an empty node
//----------------------------------------------------
  public Node() {
    next = null;
    element = null;
  }

//----------------------------------------------------
// Creates a node storing a specified element
//----------------------------------------------------
  public Node(Object element) {
    next = null;
    this.element = element;
  }

//----------------------------------------------------
// Returns the node that follows this one
//----------------------------------------------------
  public Node getNext() {
    return next;
  }

//----------------------------------------------------
// Sets the node that follows this one
//----------------------------------------------------
  public void setNext(Node node) {
    next = node;
  }

//----------------------------------------------------
// Returns the element stored in this node
//----------------------------------------------------
  public Object getElement() {
    return element;
  }

//----------------------------------------------------
// Sets the element stored in this node
//----------------------------------------------------
  public void setElement(Object element) {
    this.element = element;
  }
}

StackTest.java:

// *******************************************************
// StackTest.java
//
// A simple driver that exercises push,pop,isFull and isEmpty.
// Thanks to autoboxing,we can push integers onto a stack of Objects.
//
// *******************************************************

public class StackTest {

public static void main(String[] args) {
   StackADT stack = new LinkedStack ();

   //push some stuff on the stack
   for (int i=0; i<10; i++)
     stack.push(i*2);

   //pop and print
   //should print 18 16 14 12 10 8 6 4 2 0
   while (!stack.isEmpty())
     System.out.print(stack.pop() + " ");
     System.out.println();

   //push a few more things
   for (int i=1; i<=6; i++)
     stack.push(i);

   //should print 6 5 4 3 2 1
   while (!stack.isEmpty())
     System.out.print(stack.pop() + " ");
     System.out.println();
  }
}

public void push(Object val) {
    if(!isFull()) {
        counter++;
        Node lastTop = top;
        top = new Node(val);
        top.setNext(lastTop);
    }
}

public Object pop() {
    if(!isEmpty()) {
        counter--;
        Node lastTop = top;
        top = top.getNext();
        return lastTop.getElement();
    } else {
        return null;
    }
}