Copying Objects Through Constructors?

I have a class named Cash shown below

public class Cash
{
//Quanity Field and RetailItem Object field
private int total_units;
private Retail myRetail;
 
//Create first constructors
public Cash()
{
this(0,null);
 
[Code] ....

I'm seriously need to understand the concept of making shallow copies and deep copies. My book has shown me that its better to perform deep copies and I have followed that method. if you guys look in my constructor of the cash class

//Create a a constructor
public Cash(int total_units,Retail myRetail)
{
this.total_units=total_units;
this.myRetail=new Retail(myRetail);

Apparently I have field in Cash Class named

Retail myRetail

When I pass in an argument from my demo, I'm making a copy of that object from the demo class. In my retail class, I have the following copy constructor .

//Make a copy constructor
public Retail(Retail Object1)
{
Item_Name=Object1.Item_Name;
Item_Number=Object1.Item_Number;
// if I use this then my program would work//this.cost=Object1.cost;
 
//if I use this part of code below, my program won't work at all and I would get an error saying Exception in thread "main" java.lang.NullPointerException
this.cost.Item_Cost=Object1.cost.Item_Cost;
this.cost.Wholesale_Cost=Object1.cost.Wholesale_Cost;
}

My question is why can't I perform a deep copy there. I know if I do

this.myRetail=myRetail

in my cash constructor it would work, but then the book says its not a good method;