I'm having issues as to how to modify the pointers, using the previous and next instance variables of an object of class type Element. The doubly linked list is filled with Element objects with a lastName, firstName, phoneNumber, previous, and next instance variables. The removeElement method accepts the lastName as a parameter and finds an element with that exact String and then deletes it from the list. However, when modifying the pointers that are supposed to delete the Element from the list I came across an exception. Specifically, at this block of code:
previousNode.nextElement = currentNode.nextElement;
currentNode = currentNode.nextElement;
currentNode.previousElement = previousNode;
It throws an exception after deleting a node in the middle and then trying to delete the node at the end of the list. So I'm definitely not properly modifying the instance variables to link the elements previous and next to the deleted element. How could I set the pointers of both the previous and next elements towards each other, while following the constraints of the program?
Here's the code in case it will contribute to an answer:
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();
}
}
}
or just the removeElement method for convenience:
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
}
}
}
The assignment as well (skip to task 5): https://drive.google.com/file/d/1POEAsdNrB3wJPI0ddsbJp2HnUay5pgei/view?usp=sharing
Any answer will be appreciated!
