我有一个程序可以从 .txt 文件中读取杂货清单。我正在使用 Stack 类中的方法创建一个测试驱动程序，结果打印在 outputlist.txt 文件而不是控制台中。Stack 类包括 push() pop() 和 top() 方法。我的 push 和 pop 方法工作正常，但是当我调用我的 top 方法时，它不会打印到 outputlist.txt 文件。我已经测试过该方法是否会打印到控制台并且工作正常。这是我在尝试使用 top 方法写入文件时收到的错误消息：线程“main”java.lang.ClassCastException 中的异常：LLNode 无法转换为 GroceryItems。我已经包含了 TestDriver 类。任何意见，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;
}
}