双链接列表中removeElement（）的指针存在java问题
关于如何使用类类型元素的对象的上一个和下一个实例变量修改指针，我遇到了一些问题。双链接列表中填充了元素对象，其中包含lastName、firstName、phoneNumber、previous和next实例变量。removeElement方法接受lastName作为参数，并查找具有该字符串的元素，然后将其从列表中删除。然而，当修改应该从列表中删除元素的指针时，我遇到了一个异常。具体而言，在这段代码中：
previousNode.nextElement = currentNode.nextElement;
currentNode = currentNode.nextElement;
currentNode.previousElement = previousNode;
它在删除中间的一个节点之后尝试删除一个节点，然后尝试删除列表末尾的节点。因此，我肯定没有正确地修改实例变量以链接已删除元素之前和之后的元素。在遵循程序约束的同时，如何设置上一个和下一个元素的指针
下面是代码，以防它有助于回答问题：
package linkedlist;
import java.util.Scanner;
class Element
{
String firstName;
String lastName;
long phoneNumber;
Element nextElement; // Pointer to next element in the list
Element previousElement; // Pointer to the last element in the list
// Default Constructor
public Element()
{
this.firstName = null;
this.lastName = null;
this.phoneNumber = 0;
this.nextElement = null;
this.previousElement = null;
}
// Constructor providing first and last name
public Element( String first, String last, long number )
{
this.firstName = first;
this.lastName = last;
this.phoneNumber = number;
this.nextElement = null;
this.previousElement = null;
}
public String toString()
{
return( this.firstName + " " + this.lastName + " Cell: " + this.phoneNumber);
}
}
class ElementList
{
Element firstNode;
Element lastNode;
public ElementList()
{
this.firstNode = null;
this.lastNode = null;
}
public void addElement( String first, String last, long number )
{
Element newNode;
newNode = new Element( first, last, number );
if ( this.firstNode == null)
{
this.firstNode = newNode;
this.lastNode = newNode;
}
else
{
this.lastNode.nextElement = newNode;
newNode.previousElement = this.lastNode;
this.lastNode = newNode;
}
}
public void deleteElement( String last )
{
Element currentNode, previousNode = null;
currentNode = this.firstNode; //Temporarily assigns it
while( currentNode != null ) //Checks if there's items.
{
if ( currentNode.lastName.equalsIgnoreCase( last ) == true ) //Checks the last name
{
// We want to delete this node
if ( this.firstNode == currentNode )
{
// Delete first Node, point first node to next element
this.firstNode = this.firstNode.nextElement;
}
else
{
/* Point the next element of the previous element to the next element
of the current element, thus deleting the current element
*/
previousNode.nextElement = currentNode.nextElement;
currentNode = currentNode.nextElement;
currentNode.previousElement = previousNode;
}
break;
}
else
{
// Move to next element
previousNode = currentNode; // Save current node to previous
currentNode = currentNode.nextElement; // Move to next node
}
}
}
public void printElements()
{
Element currentNode;
System.out.println( "\nList of Elements\n================");
if ( this.firstNode == null )
{
System.out.println( "No Elements in List\n" );
}
else
{
currentNode = this.firstNode; // Point to first element
while ( currentNode != null ) // Traverse entire list
{
System.out.println( currentNode ); // Print Element contents
currentNode = currentNode.nextElement; // Go to next node
}
}
}
public void printReverseElements()
{
Element currentNode;
System.out.println( "\nList of Elements in Reverse\n================");
if(this.lastNode == null)
{
System.out.println("No Elements in List\n");
}
else
{
currentNode = this.lastNode;
while (currentNode != null)
{
System.out.println(currentNode);
currentNode = currentNode.previousElement;
}
}
}
}
public class LinkedList
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
ElementList list;
String first, last;
double number;
list = new ElementList(); // Instantiate the ElementList
System.out.print( "Enter Last Name, <CR> to Exit : " );
last = keyboard.nextLine(); // Read last Name
while ( last.length() != 0 )
{
System.out.print( "Enter First Name : " );
first = keyboard.nextLine(); // Read first Name
System.out.print("Enter Phone Number : ");
number = keyboard.nextLong();
list.addElement(first, last, (long) number);
list.printElements();// Add Element to ElementList
if ( keyboard.hasNextLine())
{
keyboard.nextLine();
}
System.out.print( "Enter Last Name, <CR> to Exit : " );
last = keyboard.nextLine();
}
list.printElements();
list.printReverseElements();
System.out.print( "Enter Last Name to Delete or <CR> to Exit: " );
last = keyboard.nextLine();
while ( last.length() != 0 )
{
list.deleteElement( last );
list.printElements();
System.out.print( "Enter Last Name to Delete or <CR> to Exit: " );
last = keyboard.nextLine();
}
}
}
或者为了方便起见，仅使用removeElement方法：
public void deleteElement( String last )
{
Element currentNode, previousNode = null;
currentNode = this.firstNode; //Temporarily assigns it
while( currentNode != null ) //Checks if there's items.
{
if ( currentNode.lastName.equalsIgnoreCase( last ) == true ) //Checks the last name
{
// We want to delete this node
if ( this.firstNode == currentNode )
{
// Delete first Node, point first node to next element
this.firstNode = this.firstNode.nextElement;
}
else
{
/* Point the next element of the previous element to the next element
of the current element, thus deleting the current element
*/
previousNode.nextElement = currentNode.nextElement;
currentNode = currentNode.nextElement;
currentNode.previousElement = previousNode;
}
break;
}
else
{
// Move to next element
previousNode = currentNode; // Save current node to previous
currentNode = currentNode.nextElement; // Move to next node
}
}
}
作业（跳到任务5）：https://drive.google.com/file/d/1POEAsdNrB3wJPI0ddsbJp2HnUay5pgei/view?usp=sharing
任何答复都将不胜感激
# 1 楼答案
从列表中删除最后一个元素时会出现问题
按如下方式更新firstNode、lastNode对象：