java.lang.NullPointerException

java.lang.NullPointerException (aka NPE). Yep! You're using nothing thinking it is something, roughly speaking.

A simple example:


Here is your car:
public class MyCar{

public void startEngin(){
}
Here are some cases where  java.lang.NullPointerException will show up:
class Driver(){
  MyCar myCar;
public void drive(){
//trying to start the engine of a car not yet created.
//we should've created it like this:  myCar = new MyCar();
myCar.startEngine();
}
}



You're using an object referencing null.