这给我带来了一些麻烦......我不太明白发生了什么。这是一项数据结构作业，我按照教师 UML 图的规定创建了每个课程。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，也不明白为什么没有指定计数器来跟踪列表中的项目。我也不明白我写的异常类，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：（名称+“为空”）；“ 我只是把那个空了。我不太明白这是如何工作的。我也完全不知道泛型是如何工作的。我做了一些阅读，但并没有弄清楚多少。我' 将发布我的代码，任何帮助将不胜感激。我请了一天假来做这件事，我觉得自己像个白痴，因为这似乎并不是特别困难。
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;
}
}
这是列表类。
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)
{
}
}
最后是main方法和测试类：
// 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 removed\n", removedItem );
list.print();
removedItem = list.removeFromFront();
System.out.printf( "\n%d removed\n", removedItem );
list.print();
removedItem = list.removeFromBack();
System.out.printf( "\n%d removed\n", removedItem );
list.print();
removedItem = list.removeFromBack();
System.out.printf( "\n%d removed\n", removedItem );
list.print();
} // end try
catch ( EmptyListException emptyListException )
{
emptyListException.printStackTrace();
} // end catch
} // end main
} // end class ListTest
以下应该是输出：
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)