Program is not running

public class CircularLinkedList {


//Both of this variable are pointing to null as default unless you asign anything to them
  //define the first variable
  private Node first;

  //def last variable
  private Node last;


  //create constructor that gives this variable values
  public CircularLinkedList(){
    first = null;
    last = null;

  }
  //this method retunrn true if its empty
  private boolean isEmpty(){
    return first == null;
  }

  //implement the insert first method
  public void insertFirst(int data){
    //iniitilize a new Node
    Node newNode = new Node();
    //give this node some data
    newNode.data = data;//that's gonna be the data that pass in parameter

    if(isEmpty()){
      last = newNode;//we assign the end of the list to be the the newNode we just created.
    }
    //this newNode next field should reference whatever the old first was referecing by assigning first to newNode.next
    newNode.next = first;//newNode is being assign to reference old first
    first = newNode;//first place, first node that's connected to the engine of train
  }
  //this methid insert at the last of node
  public void insertLast(int data){
    //create new node
    Node newNode = new Node();
    //assign data that pass in to the parameter
    newNode.data = data;

    if(isEmpty()){
      first = newNode; //if true then first newNode will be the first
    }else { //if not empty
      last.next = newNode;//next value of the last node will point to the new node.
      last = newNode; //make the new node we created be the last one in the list
    }

  }
  //delete first method
  public int deleteFirst(){ //delete something at the beginning of the list
   int temp = first.data;//whatever the data in the first node

   //this means when we only have a single node
   if(first.next == null){
     //since we're deleting that one and only node
     last = null;//then of course last should point to null
   }
   //first node always point to the next node
   first = first.next;//first will point to old's next value
   return temp;
  }

  //display lists method
  public void displayList(){
    System.out.println("List (first --> last)");
    Node current = first;
    while(current != null){
      current.displayNode();
      current = current.next;
    }
    System.out.println();
  }


}

public class App {

  public static void main(String[] args){
    CircularLinkedList myList = new CircularLinkedList();
    myList.insertFirst(1);
    myList.insertFirst(2);
    myList.insertFirst(3);
    myList.insertFirst(4);
    myList.insertLast(5);

    myList.displayList();
  }
}