null pointer exception at run time - exception

Question

This is my first post here. I am trying to create a singly link list. I am using AtEnd and AtStart methods to insert values at the end or in the beginning of the list and using display method to print all the values. The insertion methods seems to be working fine (at least I think so) but whenever I call display method it shows only the first value and then there is a null pointer exception. For example when I run this code I see only 9 and then there is the NPE despite the fact that I have put a check on the display method for "not null".
class node {
private int data;
private node next;
node() {
}
node(int data) {
this.data = data;
this.next = null;
}
public int getData() {
return data;
}
public void setData(int data) {
this.data=data;
}
public node getNext() {
return next;
}
public void setNext(node next) {
this.next = next;
}
}
public class list extends node {
node head;
list() {
}
public void AtStart(int val) {
node n = new node(val);
if (head == null) {
head=n;
} else {
n.setNext(head);
int temp = head.getData();
head.setData(val);
n.setData(temp);
//n = head;
}
}
public void AtEnd(int val) {
if (head == null) {
node n = new node(val);
head = n;
} else {
node t = head;
for(; t.getNext() != null; ) {
if(t.getNext() == null) {
t.setNext(new node (val));
}
t = t.getNext();
}
}
}
public void display() {
node t = head;
for(; t.getNext() == null;) {
if (t !=null) {
System.out.println(t.getData());
t = t.getNext();
}
}
}
}
public static void main(String args[]) {
list l = new list();
l.AtStart(16);
l.AtEnd(6);
l.AtEnd(36);
l.AtStart(9);
l.AtEnd(22);
l.display();
}

Answer 1

i dont get what your AtStart function does, it should be much simpler:
public void AtStart(int val){
if(head==null){
head=n;
}
else{
head.setnext(head);
head.setData(val);
}
}