interface PersonInterface
{
  public int getId() ;

}

class Person implements PersonInterface
{
	int id ;
	String firstName ;
	String laststName ;

	//a static data member does not need
	//an instance of an object. It is not
	//associated with an object.
    static int NoPersonObjects = 0 ;

    //Constructor called when creating the object
    Person()
    {
      NoPersonObjects++ ;
	}

    public int getId()
     {
        return id ;
	 }
	//A static method is like a global method
    static int getNoPersonObjects()
     {
		//Cannot access class data members
		//id firstName
        return NoPersonObjects ;
	 }

}


public class Reference
{
   public static void incrementPersonId( Person personObject )
   {
	  //Any changes made here are made in the object in the heap
	  //and are reflected in the original object
      personObject.id++ ;
   }

   public static void main( String args[] )
     {
	   //Compiler will not allow
       //Person personObject ;
       //null means there does not exist a value
       //yet for the personObject1
       Person personObject1 = null ;
       System.out.println( personObject1 ) ;

       // Can't call a method without an object
       // getId()

       //We get a handle/reference but it's not the
       //raw memory address to the person object.
       //Person object is created on the heap
       personObject1 = new Person() ;
       personObject1.id = 1 ;

       System.out.println( "personObject.getId(): " +
       personObject1.getId()  ) ;

       Person personObject2 = new Person() ;
       personObject2.id = 2 ;

       incrementPersonId( personObject2  ) ;

       System.out.println( "personObject2.id: " +
           personObject2.id ) ;

       //Using the static method getNoPersonObjects()
       System.out.println( "static method Person.getNoPersonObjects():" +
          Person.getNoPersonObjects() ) ;
       System.out.println( "static method personObject1.getNoPersonObjects():" +
          personObject1.getNoPersonObjects() ) ;

      //Using the static data member NoPersonObjects
       System.out.println( "static data member Person.NoPersonObjects:" +
          Person.NoPersonObjects ) ;
       System.out.println( "static data member personObject1.NoPersonObjects:" +
          personObject1.NoPersonObjects ) ;

       PersonInterface personInterfaceObject = personObject2 ;
       //Compiler error no access to id
       //System.out.println( personInterfaceObject.id ) ;
       //Can call methods defined in the interface
       System.out.println( personInterfaceObject.getId() ) ;
	 }

}