Thread: 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);
 
	}
	//Second constructor
	public Cash(int total_units)
	{
		this(total_units,null);
	}
 
	//Create third constructor
	public Cash(Retail myRetail)
	{
		this(0,myRetail);
	}
 
	//Create a a constructor
	public Cash(int total_units,Retail myRetail)
	{
			this.total_units=total_units;
			this.myRetail=new Retail(myRetail);
	}

I'm seriously need help 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; however, even my teacher uses this so I'm confused here wahts going on.