Linked List dont work completely...

Well, my problem is with the list... when i called number() to insert the number to the list, and print it out it works
but when i called display() i got NullPointerExcpetion error....can anyone see why my listed turn empty?

PS: dont worry about extra stuffs tho

Program is below

Sorry, i accidently took out row and col

public class Grid
{
private Node head;
private Node tail;
//private Node current;
int row = 10;
int col = 6;

public Grid()
{
head = null;
tail = null;
}

public Grid(Value n)
{
head = new Node(n);
tail = head;
//current = head;
}

private void insert(Value n)
{
Node cell = new Node(n); //create node

if(head == null)
{
head = cell;
tail = cell;
}
else
{
tail.setRight(cell);
//cell.setRight(current);
tail = cell;
}
}

public void display()
{
System.out.println(head.getValue()+", ");
Node temp = head;
while(temp!=null) //loops forward displaying node's data
{
System.out.print(temp.getValue()+", ");
temp = temp.getRight(); //move to the next node in the list
}
}

public void number()
{
for(double i = 0; i <= row*col - 1; i++)
{
insert(new Value(Double.toString(i)));
System.out.print(tail.getValue() + " ");
}

System.out.println("");

System.out.println(tail.getValue()+", ");
Node temp = head;
while(temp!=null) //loops forward displaying node's data
{
System.out.print(temp.getValue() + ", ");
temp = temp.getRight(); //move to the next node in the list
}

}

}

When posting code, use the &#91;CODE]...&#91;/CODE] tags to keep it formatted and readable. If you want help with errors, post the full error message text and stack trace if present. The error message should tell you on which line the error occurs.

As far as I can see, if Node and Value work correctly (I don't know, you didn't post that code), the Grid display method will only throw a NullPointerException if the Grid is empty (and, for some reason, when it isn't empty it always prints the first node twice).

Unless you post the full text of the exception, that's all I can say.

Experience is a poor teacher: it gives its tests before it teaches its lessons...
Anon.

Thanks dlord,
I found my problem, it was in my main class
At first i accidently put my Grid grid = new Grid(); inside the loop, so basicly i creted a new linked each time i loop. Well, if u got any suggestion please let me know.

Code:

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public static void main(String[] args) throws IOException
        {
                Grid grid = new Grid();
               
                int loop = 0;
                String line;
               
                BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
               
                do {
                        System.out.println("Operations");
                        System.out.println("  display                dis                        assign cell                as");
                        System.out.println("  fill                        f                        number                        n");
                        System.out.println("  add cells                a                        subtract cells                s");
                        System.out.println("  multiply cells        m                        divide cells                d");
                        System.out.println("  add rows                ar                        subtract rows                sr");
                        System.out.println("  multiply rows                mr                        divide rows                dr");
                        System.out.println("  add columns                ac                        subtract columns        sc");
                        System.out.println("  multiply columns        mc                        divide columns                dc");
                        System.out.println("  insert row                ir                        insert column                ic");
                        System.out.println("  delete row                delr                        delete column                delc");
                        System.out.println("  quit                        q");
                        System.out.print("-> ");       
                       
                       
                        try {
                        //Grid grid = new Grid();
                        line = in.readLine();
                       
                        if(line.equals("dis")) {
                                                grid.display();
                                        }
                                        else if(line.equals("n")) {
                                                grid.number();
                                        }
                                        else if(line.equals("q")){                                                //quit program
                                                System.exit (0);
                                        }
                        }
                       
                        catch (IOException ioe)
                        {
                                System.out.println("IO error trying to read your name!");
                                System.exit(1);
                        }
                } while(loop == 0);

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Quote:

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Originally Posted by Tekdg

Well, if u got any suggestion please let me know.

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Suggestion about what? If you have a question, ask it.

A prudent question is one-half of wisdom...
F. Bacon