Thread: need help implementing LinkedList

The Node class has a value and a pointer to the next Node.
Have you tried creating an instance of the Node class? You should be compiling and testing the code after each small step to check if there are any problems.

The LinkedList class should control a list of Nodes. It would have methods to change the list of Nodes it controls.
What is your problem with the constructor? What is it supposed to do? Are there overloads to it(different versions)?

What type of LinkedList? Singly or Doubly?

Singly just has a next variable but Doubly has both a next and a previous variable and so is a bit harder to do, though perhaps more efficient in some ways.

Let me help you a bit.

public class Node {
 
    //construct student object
    public Student value;
 
    public Node next;
 
 
    // default constructor
    public Node(){}
 
 
    // non-default constructor
 
 
    public Node(Student value)
    {
      //   this.next = new Node(s);  Probably not what you want.  Perhaps you should make Node an inner class, if you haven't already, stupid thing won't let me see your post and type a reply at the // same time
 
setValue(value);
    }     
 
public void setValue(Student value)
{
this.value = value;
}
 
public Student getValue()
{
return value;
}
 
public void setNext(Node next)
{
this.next = next;
}
 
public Node getNext()
{
return next;
}
}

You need to create a main to test it. And, I would put some second thought into that constructor if I were you. I can see an infinitely growing linked list in your future when you try to create a Node object.

Test with code to create an instance of the Node class, compile and execute it.

To visualize a linked list, take a piece of paper, draw some Nodes with data and next and chain the Nodes together. Have a first Node (will be in the linked list class) that points to the first Node in the list.

Does it compile and execute?

For debugging I'd add a toString() method to the Node class that would return the value and next variables. Then you can print a Node and see it all.

Originally Posted by Norm

Does it compile and execute?

For debugging I'd add a toString() method to the Node class that would return the value and next variables. Then you can print a Node and see it all.

ok, I changed it around to fit the exercise, but still I'm not displaying anything to screen. I presume that my problem is either in the "add" or "display" method of my LinkedList class

public class Node {
 
    //construct student object
    public Student value;
 
    public Node next;
 
    //constructor
    public Node(Student s)
    {
        this.value = s;
    }
 
}

public class LinkedList {
 
 
    private Node first;
 
    public LinkedList()
    {
        first.value = null;
    }
 
    //add students to the list
    public void add(Student s)
    {
        Node newNode = new Node(s);
        newNode.next = first;
        first = newNode;    
 
    }
 
    //display list of student
    public void display()
    {
        if(first == null)
            System.out.println("List is empty");
        else
            while(first != null)
            {
                System.out.println(first.value);
                first = first.next;
            }
    }
}

public class Tester {
 
 
    public static void main(String[] args) {
 
 
        LinkedList students = new LinkedList();
 
        UnderGrad john = new UnderGrad("john", "doe", 2.7, "computer Science", "phisics");
        UnderGrad jorge = new UnderGrad("jorge", "vazquez", 3.8, "computer Science", "programming");
 
        students.add(john);
        students.add(jorge);
 
        students.display();       
 
    }
}

this is the error I'm getting

run:
Exception in thread "main" java.lang.NullPointerException
	at LinkedList.<init>(LinkedList.java:13)
	at Tester.main(Tester.java:12)
Java Result: 1

Did you try adding a Public String toString() {} method to the Node class that returns a String with the value and next values. It allows you to print a node easily for debugging. For example if you printed first, it would print the list of nodes.

run:
Exception in thread "main" java.lang.NullPointerException
at LinkedList.<init>(LinkedList.java:13)

Look at line 13 in the LinkedList class and see what variable on that line has a null value. Then backtrack in the code to see why it does not have a valid non-null value.

I would add ID Strings to make it clearer:

return "Node: " +value + " next=" + next;

The this & toString() are redundant