import java.util.Scanner;

public class Driver
{
    public static void main(String [] args)
    {
	Scanner in = new Scanner(System.in);
	int index;
	int age;

	// 1) Reference to array created
	// Reference is null
	Person [] aList;

	// 2) One-dimensional array with 3 elements created
	// Each array element is a reference to a Person object
	// Currently each array element is null
	aList = new Person[3];

	// Creates a Person object, array element one now contains
	// the address of that object, the other elements in the array
	// are still null
	aList[1] = new Person();


	// If the user tries to access element 0 or 2 a null pointer 
	// exception resulting in a runtime error crashing the program will
	// be the result.
	System.out.print("Enter array element to access (hint only enter 1): ");
	index = in.nextInt();
	System.out.print("Enter new age value for the person: ");
	age = in.nextInt();

	// JT: Normally if accessing invalid indices is an issue then a check should
	// really be done: if (index != 1) then error
	// This was excluded for illustration purposes, there is only one object, one
	// array element (index 0) refers to it. The other array indices are still null!
	aList[index].setAge(age);

	System.out.println("Person #" + index + " is age: " + aList[index].getAge());

	// To prove that the array elements are references to objects and not actual objects
	// here is a shallow copy (both assignments only copy the address of the object)
	aList[0] = aList[1];    
	aList[2] = aList[1];    

	// Not null anymore
	System.out.println("Person #0 is age: " + aList[0].getAge());
	System.out.println("Person #2 is age: " + aList[2].getAge());

	// Proof: 3 refences to the same object
	aList[0].setAge(-1999);
	
	for (index = 0; index < 3; index++)
	    	System.out.println("Person #" + index + " is age: " + 
				   aList[index].getAge());

    }
}