Question

我有一个从.txt文件中读取购物清单的程序。我正在使用我的Stack类中的方法创建一个测试驱动程序，结果打印在outputlist.txt文件而不是控制台。 Stack类包括push（）pop（）和top（）方法。我的push和pop方法工作正常，但是当我调用top方法时，它不会打印到outputlist.txt文件。我已经测试过检查方法是否会打印到控制台并且工作正常。这是我尝试使用top方法写入文件时得到的错误消息：线程“main”中的异常java.lang.ClassCastException：LLNode无法强制转换为GroceryItems。我已经包含了TestDriver类。任何建议将不胜感激。

    import java.io.FileWriter;
    import java.io.File;
    import java.util.Scanner;
    import java.io.FileNotFoundException;
    import java.io.IOException;
    import java.io.PrintWriter;

    public class TestDriver{

       public static void main(String[] args)throws Exception{    
         FileWriter out = null;
         Stack<GroceryItems> item = new Stack<GroceryItems>("Grocery List");

          try{
                out = new FileWriter("outputlist.txt", true);                        
                File listFile = new File("grocerylist.txt");
                Scanner scan = new Scanner(listFile);      
                String name;
                int quantity;
                double price;



                while(scan.hasNextLine()){
                   name = scan.next();  
                   quantity = Integer.parseInt(scan.next());
                   price = Double.parseDouble(scan.next());
                   item.push(new GroceryItems(name, quantity, price));                      
                }
                out.write(item.toString());

                   for(int i = 0; i < 5; i++){
                      item.pop(); 
                   }
                   out.write("New List after 5 pops:\n");
                   out.write("^^^^^^^^^^^^^^^^^^^^\n");
                   out.write(item.toString() + "\n");
                   out.write("Top test:\n");
                   out.write("^^^^^^^^^^^^^^^^^^^^\n");
                   out.write(item.top().toString()); //This is where I am getting the error message.              



          }catch (IOException ioe){
             System.out.println(ioe.toString());
          }
          catch (StackOverflowException soe){
             out.write("Push attempted on a full stack.");
          }
          catch (StackUnderflowException sue){
             out.write("Pop attempted on an empty Stack.");

          }
          out.close(); 
           System.out.println(item.top()); //This prints to console fine.       

       }
    public class Stack<T> implements UnboundedStackInterface<T>{

       private LinkedList<T> list;  //top of the stack.


       public Stack(String name){
          list = new LinkedList<T>(name);
       }
       public void push(T element){
            list.insert(element);

       }
       public void pop(){            
         list.remove();        
       }    
       public T top()throws StackUnderflowException{
           if(isEmpty())
             throw new StackUnderflowException("Top attempted on empty stack.");
           else        
          return (T)list.getLast();                            
       }
       public boolean isEmpty(){
          return (list == null);   
       }
        public boolean isFull(){
            return false;
        }
       public String toString(){            
            return list.toString();
       }   
    }


    public class LinkedList<T>{


    private String name;
    private LLNode<T> first;
    private LLNode<T> last;
    private int size = 0;


    public LinkedList(String name){
        first = null;
        last = null;        
        this.name = name;
    }
    public void insert(T element){
        LLNode<T> newNode = new LLNode<T>(element);
        newNode.setLink(null);
      if(first == null)
         first = newNode;         
      if (last != null)
         last.setLink(newNode);      
        last = newNode;
        size++;
    }    
    public int size(){
        return size;            
    }
    public void remove()throws StackUnderflowException{
        LLNode<T> current;      
        current = first;    
        if (size == 0)
            throw new StackUnderflowException("Pop attempted on an empty stack.");
        else if (size == 1){
            last = null;
            first = null;
            size = 0;
        }
        else{
            while(current.getLink() != last){                              
                current = current.getLink();                

          }
            current.setLink(null);         
            last = current;     
            size--;
        }                   


    }
   public LLNode<T> getLast(){
      return last;   

   }        
    public String getName(){
        return name;
    }
    public String toString(){
        String listString = "List: " + name + "\n\n";
        LLNode<T> node;
        node = first;

        while(node != null){
            listString = listString + node.getInfo() + "\n";
            node = node.getLink();
        }
        return listString;

    }
}

Answer 1

问题在于Stack<T>您将list定义为LinkedList<T>，其中LinkedList<T>是您已实施的类。您的Stack#top正在LinkedList#getLast和getLast返回last LLNode<T>类型，在您的情况下为LLNode<GroceryItems>。由于LLNode不属于GroceryItems类型，因此您将获得该异常。

您尚未包含LLNode的实现，但如果LLNode<T>类中的方法返回T - 那么您需要调用它。

因此需要进行更改：out.write(item.top().toString()); //This is where I am getting the error message.

Answer 2

您的getLast()会返回LLNode<T>所以当您这样做时

return (T)list.getLast();

您尝试将T转换为LLNode<T>, which is wrong because T is not a subtype nor supertype of LLNode . This is why you are getting a ClassCastException`。

在top()方法中试试这个。

return (NodeLL<T>)list.getLast();

Stack Top Method不写入.txt文件

2 个答案: