import java.util.*;
import java.io.*;

public class LinkedList{
  private Node first = null;
  private class Node{
  int data;
  Node next;
  }
  public void insert(int x){
    if(first==null){
      Node first = new Node();
      first.data = x;
      first.next = null;
    }
    else{
    Node old = first;
    first = new Node();
    first.data = x;
    first.next = old;
    }
  }

  public int delete(){
 //   if(first==null){ return null; }
  int item = first.data;
  if(first.next != null){
  first = first.next;
  }
  else { first = null;}
  return item;
  }
  public void size(){}
  public static void main(String args[]){
    LinkedList n = new LinkedList();
    n.insert(1);
    n.insert(2);
    n.insert(3);
    n.insert(4);
    n.insert(5);
   // while(n.next!=null){
      int x;
      x = n.delete();

  //  }
  }
}

Welcome to DrJava.  Working directory is C:UsersmohitDr Java
> run LinkedList
java.lang.NullPointerException
    at LinkedList.delete(LinkedList.java:26)
    at LinkedList.main(LinkedList.java:43)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)
>