The null Reference

• null refers to no object
• Can assign null to object variable:
  

  worldGreeter = null;
  

• Can test whether reference is null
  

  if (worldGreeter == null) . . .
  

• Dereferencing null causes NullPointerException
  Example:
  

  ...
  worldGreeter = null;
  ...
  worldGreeter.sayHello();     // this statement causes the exception