Linked List - Problems displaying list and deleting something that isn't there - singly-linked-list

Question

so I have an assignment that creates an alphabetically ordered linked list. I'm having trouble with the following:
Displaying the full list after every insertion and deletion. It only seems to print apple in every step.
Deleting tangerine, something that doesn't exist in the list anymore. I'm not very familiar with try and catch exceptions and I feel like I'm doing it wrong.
Thanks in advance, here is my code:
Node class:
class Node
{
String item; // data item
Node next; // next Node in list
public Node(String i)
{
item = i;
}
public void displayLink() // displays this link
{
System.out.println (item + " ");
}
}
SortedList Class:
class SortedList
{
Node start; // reference to the first item in the list
public SortedList()
{
start = null;
}
public boolean isEmpty() // true if no links
{
return (start == null);
}
public void insert(String key)
{
Node newNode = new Node(key); // creates new Node
Node previous = null; // start at the beginning
Node current = start;
// until end of list,
while (current != null && (current.item.compareTo(newNode.item)<0))
{ // while current isn't null and current.item is equal to newNode.item
previous = current;
current = current.next; // go to the next item
}
if (previous == null) // at beginning of list
start = newNode; // start --> newNode
else // not at the beginning
previous.next = current; // old previous --> newNode
newNode.next = current; // newNode --> old current
}
public Node remove(String key) // return and delete first Node
{
Node temp = start; // saves start
start = start.next; // deletes start
return temp; // returns String
}
public void displayList()
{
System.out.println("List (first to last): ");
Node current = start; // starts at the beginning of the list
while (current != null) // until end of the list
{
current.displayLink(); // prints data
current = current.next; // moves to next Node
}
System.out.println(" ");
}
}
And finally my demo class:
class SortedListDemo
{
public static void main (String [] args)
{
SortedList theSortedList = new SortedList(); // creates new list
theSortedList.insert("apple"); // inserts apple
theSortedList.displayList(); // displays list
theSortedList.insert("orange"); // inserts orange
theSortedList.displayList(); // displays list
theSortedList.insert("kiwi"); // inserts kiwi
theSortedList.displayList(); // displays list
theSortedList.insert("tangerine"); // inserts tangerine
theSortedList.displayList(); // displays list
theSortedList.insert("strawberry"); // inserts strawberry
theSortedList.displayList(); // displays list
theSortedList.remove("apple"); // deletes apple
theSortedList.displayList(); // displays list
theSortedList.remove("strawberry"); // deletes strawberry
theSortedList.displayList(); // displays list
theSortedList.remove("tangerine"); // deletes tangerine
theSortedList.displayList(); // displays list
theSortedList.insert("apple"); // inserts apple
theSortedList.displayList(); // displays list
try
{
theSortedList.remove("tangerine"); // deletes tangerine
theSortedList.displayList(); // displays list
}
catch (NullPointerException tangerine)
{
System.out.println ("Tangerine doesn't exist, so can't be deleted");
}
theSortedList.remove("apple"); // deletes apple
theSortedList.displayList(); // displays list
try
{
theSortedList.remove("tangerine"); // deletes tangerine
theSortedList.displayList(); // displays list
}
catch (NullPointerException e)
{
System.out.println ("Tangerine doesn't exist, so can't be deleted");
}
try
{
theSortedList.remove("apple"); // deletes tangerine
theSortedList.displayList(); // displays list
}
catch (NullPointerException e)
{
System.out.println ("Apple doesn't exist, so can't be deleted");
}
theSortedList.remove("kiwi"); // deletes kiwi
theSortedList.displayList(); // displays list
theSortedList.remove("orange"); // deletes orange
theSortedList.displayList(); // displays list
theSortedList.remove("strawberry"); // deletes apple
theSortedList.displayList(); // displays list
theSortedList.insert("job-well-done"); // inserts job-well-done
theSortedList.displayList(); // displays list
}
}
Here is the output I get:
List (first to last):
apple
List (first to last):
apple
List (first to last):
apple
List (first to last):
apple
List (first to last):
apple
List (first to last):
java.lang.NullPointerException
...
Thanks for all the help, much appreciated.

Answer 1

Well, I seem to have noticed two problems with your program, which can explain your troubles
You're not using the key given in the remove method when searching for the element to delete in the class. You're just deleting de first item (start).
Also, apart from when adding the first element in the list, your insert() method doesn't seem to add any new nodes to it. You can see in the program that you're referencing the "next" node of your newNode to an element in your list (the line "newNode.next = current;" ) but you're not referencing this newNode in any element in your list. That way it can never be iterated. Your list should have at least one element whose "next" node is referencing this newNode.
As a suggestion, you should use your isEmpty() method when displayng and removing items in your list, so you can avoid that exception when you ran your program, which was probalby caused for trying to reference a null node.
This link can help you with using and implementing data types with linked list: http://algs4.cs.princeton.edu/13stacks/
There is also a very interesting course on Coursera about Algorithms which help you further in this matter. It is called Algorithm Part I.
I hope I have managed to help you. Feel free to ask more question and I'll try to answer them.