具有泛型类型的链表问题

Question

这给我带来了一些麻烦。。。我不太明白发生了什么。这是一个数据结构作业，我已经按照讲师UML图的规定创建了每个类。这似乎是一个奇怪的链表实现。我不太理解ListNode类中调用另一个构造函数的构造函数，也不理解为什么没有指定计数器来跟踪列表中的项。我也不理解我写的异常类，因为讲师指定"第二个构造函数调用超类构造函数，并发送以下字符串作为其参数：（name+"为空"）；"我只是把它留空。我不太明白这是怎么回事。我也完全不知道泛型是如何工作的。我读了一些书，但没有；我没怎么清理。我会发布我的代码，任何帮助都将不胜感激。我请了一天假来做这件事，我觉得自己像个十足的白痴，因为这似乎并不特别困难。

ListNode类：

 package linkedLists;
 public class ListNode<T> {
T data;
ListNode<T> nextNode; 
ListNode(T object)
{ 
    this(object, null); 
}
ListNode(T object, ListNode<T> node) 
{ 
    data = object; 
    nextNode = node; 
} 
T getData()
{ 
    return data; 
} 
ListNode<T> getNext() 
{
    return nextNode; 
}
}

这是List类。

  package linkedLists;
  public class List<T>{
      ListNode<T> firstNode;
ListNode<T> lastNode;
String name;
public List()
{
    this("list");
}
public List(String listName)
{
    name = listName;
    firstNode = null;
    lastNode = null;
}
public void insertAtFront(T insertItem)
{
    if(isEmpty())
    {
        ListNode<T> node = new ListNode<T>(insertItem);
        firstNode = node;
        lastNode = node;
    }
    else
    {
        ListNode<T> tempNode = firstNode;
        ListNode node = new ListNode(insertItem, tempNode);
        firstNode = node;
    }
}
public void insertAtBack(T insertItem)
{
    if(isEmpty())
    {
        ListNode<T> node = new ListNode<T>(insertItem);
        firstNode = node;
        lastNode = node;
    }
    else
    {
        ListNode<T> tempNode = lastNode;
        ListNode node = new ListNode(insertItem, tempNode);
        lastNode = node;
    }
}
public T removeFromFront() throws EmptyListException
{
    if(isEmpty())
    {
        throw new EmptyListException("gfy");
    }
    else
    {
        ListNode<T> nr;
        nr = firstNode;
        if(firstNode == lastNode)
        {
            firstNode = null;
            lastNode = null;
        }
        else
        {
            firstNode = firstNode.getNext();
        }
        return nr.getData();
    }
}
public T removeFromBack() throws EmptyListException
{
    if(isEmpty())
    {
        throw new EmptyListException("gfy");
    }
    else
    {
        ListNode<T> nr;
        nr = lastNode;
        if(firstNode == lastNode)
        {
            firstNode = null;
            lastNode = null;
        }
        else
        {
            ListNode<T> current = firstNode;
            ListNode<T> secondToLast = null;
            int numOfNodes = 0;
            while(current != lastNode)
            {
                secondToLast = current;
                current = current.getNext();
            }
            lastNode = secondToLast;
            lastNode.nextNode = null;
        }
        return nr.getData();
    }
}
  public boolean isEmpty( )
   {
    if(firstNode == null)
    return true;
    else
        return false;
}
public void print()
 {
    if(isEmpty())
    {
        System.out.println("List is empty!");
        return;
    }
    else
    {
        System.out.println(firstNode.getData());
        ListNode<T> printNode = firstNode;
        while(printNode != lastNode)
        {
            printNode = printNode.getNext();
            System.out.println(printNode.getData());                
        }
    }
 }

  }

异常类

package linkedLists;
       public class EmptyListException extends RuntimeException
     {
    public EmptyListException()
    {
        this("list");
    }
    public EmptyListException(String name)
    {
    }
      }

最后介绍了主要的方法和测试类别：

// ListTest.java
// ListTest class to demonstrate List capabilities.
import quiz1cs304summer2013kensotak.List;
import quiz1cs304summer2013kensotak.EmptyListException;
public class ListTest 
{
   public static void main( String[] args )
   {
      List< Integer > list = new List< Integer >(); // create a List
      // insert integers in list
      list.insertAtFront( -1 );
      list.print();
      list.insertAtFront( 0 );
      list.print();
      list.insertAtBack( 1 );
      list.print();
      list.insertAtBack( 5 );
      list.print();
      // remove objects from list; print after each removal
      try 
      { 
         int removedItem = list.removeFromFront();
         System.out.printf( "n%d removedn", removedItem );
         list.print();
         removedItem = list.removeFromFront();
         System.out.printf( "n%d removedn", removedItem );
         list.print();
         removedItem = list.removeFromBack();
         System.out.printf( "n%d removedn", removedItem );
         list.print();
         removedItem = list.removeFromBack();
         System.out.printf( "n%d removedn", removedItem );
         list.print();
      } // end try
      catch ( EmptyListException emptyListException ) 
      {
         emptyListException.printStackTrace();
      } // end catch
     } // end main
     } // end class ListTest

应输出以下内容：

The list is: -1 
The list is: 0 -1 
The list is: 0 -1 1 
The list is: 0 -1 1 5 
0 removed
The list is: -1 1 5 
-1 removed
The list is: 1 5 
5 removed
The list is: 1 
1 removed
Empty list

我的输出是：

-1 
0 
-1 
0 
-1 
Exception in thread "main" java.lang.NullPointerException
    at linkedLists.List.print(List.java:146)
    at ListTest.main(ListTest.java:18)

Answer 1

首先，您的打印方法甚至不尝试打印前导文本（"the list is："），而是在自己的行上打印每个项目。

考虑到这一点，看起来问题出在insertAtBack上：制作新ListNode的参数是向后的（新节点应该在最后一个节点temp节点之后）。