In Linked list how to produce value - oop

Question

I would like to repeatedly enter a number that is added to a linked list.
But there's an error in the code at line x = new Node():
No enclosing instance of type Main is accessible. Must qualify the allocation with an enclosing instance of type Main (e.g. x.new A() where x is an instance of Main).
Is there a way to fix my code?
static Node head;
static Node p;
static Node q;
static Node x;
class Node {
int data;
Node next;
public Node link;
// Constructor to create a new node
// Next is by default initialized
// as null
Node(int d) {
data = d;
next = null;
}
public Node() {
// TODO Auto-generated constructor stub
}
}

Answer 1

Two issues:
class Node should either be declared as static, or be moved to a separate file.
p = x should happen outside the else block, since it should get this value also when the if condition was true:
if(head == null) {
head = x;
} else {
p.link = x;
}
p = x;
Some remarks:
If you really want to use the Node constructor without arguments, then it is better to define explicitly what the new Node's properties should be:
public Node() {
data = 0;
next = null;
}
However, it would be better to not have this constructor signature at all, and only construct the object using the data as argument:
if(num != -999){
x = new Node(num);
...and now you don't need to do any of this any more:
x.data = num;
x.link = null;

Answer 2

The inner Node class is not static, meaning it belongs to a specific instance of the enclosing Main class. Since it doesn't refer to any instance methods this seems like it was not done intentionally. Make the class itself static (i.e., static class Node {) and you should be fine.

Answer 3

First, the class Node cannot be referenced from a static context. To fix this, make it static or move it to its own file. Second, the null pointer exception happens since you don't assign p in the special case where the list is empty.
Overall, I suggest that you clean up your class and use a more structured approach. Rename p to last to make it clear that this is a reference to the last element of the list. Move the functionality to add a node into its own method to make the code more readable. Use next in the Node class to point to the next node instead of link. Create an instance of the class where your head and last reference is defined and make them private. Use break inside the loop to only define the magic number (-999) once and exit the loop when it is entered.
The whole class could look like this:
public class CustomLinkedList {
private Node head = null;
private Node last = null;
static class Node {
int data;
Node next = null;
}
public void add(int num) {
Node x = new Node();
x.data = num;
if (this.head == null) {
this.head = x;
} else {
this.last.next = x;
}
this.last = x;
}
public static void main(String[] args) {
CustomLinkedList list = new CustomLinkedList();
int count = 0;
do {
try {
BufferedReader dataIn = new BufferedReader(new InputStreamReader(System.in));
System.out.print("Enter number " + (++count) + ": ");
String strNum = dataIn.readLine();
int num = Integer.parseInt(strNum);
if (num != -999) {
list.add(num);
} else {
break; // exit loop
}
} catch (IOException e) {
System.out.print(e.getMessage());
}
} while (true);
}
}