I am getting this type of exception or run time error after inserting element in a Linked_List: NullPointerException: Cannot invoke “Node.getNext()” because “” is null.
I have made Setter & Getter method to put & get value & next node in the code. Please check the code once & debug it, Tell me where I am wrong.
import java.util.;
import java.io.;
class Node
{
private int data;
private Node next;
public Node() //default constructor
{
data=0;
next=null;
}
public Node(int d,Node n) //constructor
{
data=d;
next=n;
}
public void setData(int d)
{
data=d;
}
public void setNext(Node n)
{
next=n;
}
public int getData()
{
return data;
}
public Node getNext()
{
return next;
}
}
class LinkedList
{
private Node start;
private int size;
public LinkedList()
{
size=0;
start=null;
}
public boolean isEmpty()
{
if( start==null)
return (true);
else
return (false);
}
public int getListSize()
{
return size;
}
public void viewList()
{
Node t;
if(isEmpty())
{
System.out.println("List is empty");
}
else
{
t = start;
for( int i=1; i<=size; i++)
{
System.out.println(" "+ t.getData());
t = t.getNext();
}
}
}
public void insertAtFirst(int val)
{
Node n;
n = new Node();
n.setData(val);
n.setNext(start);
start=n;
size++;
}
public void insertAtLast(int val)
{
Node n,t;
n=new Node();
n.setData(val);
t=start;
if(t==null)
start=n;
else
{
while(t.getNext() != null)
{
t = t.getNext();
}
t.setNext(n);
}
size++;
}
//Insertion of node at a given position
public void insertAtPos(int val, int pos)
{
if(pos==1)
insertAtFirst(val);
else if (pos==size+1)
{
insertAtLast(val);
}
else if (pos>1 && pos<=size)
{
Node n,t;
n = new Node(val,null);
t=start; //Temporary Node
for(int i=1; i<(pos-1); i++)
{
t=t.getNext();
}
n.setNext(t.getNext());
t.setNext(n);
size++;
}
}
//deletion operations
public void deleteFirst()
{
if(start == null)
System.out.println("List is already empty");
else
{
start=start.getNext();
size--;
}
}
public void deleteLast()
{
if(start==null)
{
System.out.println("List is already empty");
}
else if(size==1)
{
start=null;
size--;
}
else
{
Node t;
t = start;
for(int i=1; i<size-1; i++)
t = t.getNext();
t.setNext(null);
size--;
}
}
public void deleteAtPos(int pos)
{
if(start==null)
System.out.println("List is already empty");
else if(pos<1 || pos>size)
System.out.println("Invalid position");
else if(pos==1)
deleteFirst();
else if( pos==size)
deleteLast();
else
{
Node t,t1;
t= start;
for(int i=1; i<pos-1; i++)
t = t.getNext();
t1 = t.getNext();
t.setNext(t1.getNext());
size--;
}
}
public int midNode()
{
Node slow = start;
Node fast = start;
Node fast1 = fast.getNext();
while( fast.getNext()!=null && fast1.getNext()!=null )
{
slow = slow.getNext();
fast = fast.getNext().getNext();
}
return slow.getData();
}
}
public class MidNode
{
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
LinkedList list = new LinkedList();
boolean flag = true;
int position, val;
while(flag)
{
System.out.println(" ");
System.out.println("1. Add item to the list at the start");
System.out.println("2. Add item to the list at the end");
System.out.println("3. Add item to the list at the given position");
System.out.println("4. Delete first item from the list");
System.out.println("5. Delete last item from the list");
System.out.println("6. Delete item from the list at given position");
System.out.println("7. view list");
System.out.println("8. Exit");
System.out.println("Enter your choice: ");
int choice = sc.nextInt();
switch(choice)
{
case 1:
System.out.println("Enter value of list item");
val = sc.nextInt();
list.insertAtFirst(val);
break;
case 2:
System.out.println("Enter value of list item");
val = sc.nextInt();
list.insertAtLast(val);
break;
case 3:
System.out.println("Enter position: ");
position = sc.nextInt();
System.out.println("Enter value of list item");
val = sc.nextInt();
list.insertAtPos(val, position);
break;
case 4:
list.deleteFirst();
break;
case 5:
list.deleteLast();
break;
case 6:
System.out.println("Enter position: ");
position = sc.nextInt();
list.deleteAtPos(position);
break;
case 7:
list.viewList();
break;
case 8:
flag = false;
break;
default:
System.out.println("Invalid choice");
}
}
System.out.println(list.midNode());
}
}
Hey @rohandhakad20 You have instantiated the LinkedList using line :
LinkedList list = new LinkedList();
But The class where you have made all the functions of linkedlist is named = Node by you
So either make your class name LinkedList or Instantiate in main method like this.
Node list = new Node();
No, I have made all working functions in LinkedList class itself, that’s why I have made object of it in my main method as
LinkedList list = new LinkedList() ;
If I will comment out midNode method, then my Code will work finely, Run my Code without midNode method once please.
@rohandhakad20 Yeah you have made all the function in linkedlist but your are using Node class as a name.
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