Thread: Using this reference variable on constructors

I’m trying to simplify my constructor by using “this” reference variable. I’m aware how to use this reference when making constructors that involve fields of primitive data types; however, not so sure fields involving objects. I have made the following constructors and haver placed a question mark in the last parameter. Can anyone be willing to tell me and explain to me how I can use this reference that involves objects. Also, this chapter im learning related to inner classes.

 public class RetailItem
{
                //Create the fiels for item.
                //You need item name and item number. then you would need
                //cost
                private String description;
                private int Item_Number;
                private CostData cost;
 
                //Retail Item construcotr
                public RetailItem()
                {
                                this("",0,0,0,?);
                }
 
                //Constructor 2
                public RetailItem(String description)
                {
                                this(description,0,0,0,?);
                }
 
                //constructor 3
                public RetailItem(String description,int Item_Number)
                {
                                this(description,Item_Number,0,0,?);
                }
 
                //constructor 4
                public RetailItem(String description,int Item_number,double Retail,double wholesale, CostData object)
                {
                                this.description=description;
                                this.Item_Number=Item_Number;
                                cost=new CostData(Retail,wholesale);
                }

Here is partially done inner class

 private class CostData
                {
                                //Fields of this
                                private double wholesale_Price;
                                private double Retail_price;
 
                                //Create a constructor
                                public CostData(double wholesale_Price,double Retail_price)
                                {
                                                this.wholesale_Price=wholesale_Price;
                                                this.Retail_price=Retail_price;
                                }
 
                                public String toString()
                                {
                                                DecimalFormat formatter=new DecimalFormat("#0.00");
 
                                                String data="Price Information\n"+"WholeSale Price: "+formatter.format(wholesale_Price)+"\nRetail Price: " +formatter.format(Retail_price);
                                                return data;
                                }
                }

In my book, it had the code

public RetailItem(String desc,int itemNumber,
double wholesale, double retail)
{
 
description=desc;
ItemNumber=itemNumber;
cost=new CostData(wholesale, retail);
 
}

Being aware I could have use this method provided by the book, but for future reference, suppose theirs were objects declared in the fields. That's why I wanted to use this method and see how it works. Now, asking you this, what would be a conventional way to do this?

Originally Posted by fahman_khan75@yahoo.com

I’m trying to simplify my constructor by using “this” reference variable. I’m aware how to use this reference when making constructors that involve fields of primitive data types; however, not so sure fields involving objects.

I'm not entirely sure if I completely understand your post, but it appears there is some confusion with the "this" keyword. The "this" keyword has 2 uses: (1) to refer to a field in the class, and (2) to call another constructor in the same class. See Using the this Keyword (The Java™ Tutorials > Learning the Java Language > Classes and Objects).

In the code in post #1,

Originally Posted by fahman_khan75@yahoo.com

                public RetailItem()
                {
                                this("",0,0,0,?);
                }

"this" (not a reference variable) is used to call the 5-parameter version of the constructor, which is

public RetailItem(String description, int Item_number, double Retail, double wholesale, CostData object)

Since the 5th parameter is of type CostData, all you need to do is to create an instance of CostData, and pass the reference to the instance of CostData in place of '?'. However, since the "this" constructor call must be the first statement in a constructor, you'll need to do the CostData instantiation inline, e.g.,

this("", 0, 0, 0, new CostData(0, 0));

The above will not work with your current implementation of the CostData inner class because the inner class (CostData) has an implicit reference to an instance of the outer class (RetailItem). The inner class can be instantiated only after the outer class is instantiated unless the inner class is static. Therefore to do the above you'll need to change CostData so that it looks like the following:

private static class CostData

An alternative will be to move CostData out of RetailItem so that CostData is no longer an inner class, and instead becomes a "regular" class. This way you don't need to make CostData static.

Finally, in the 5-parameter constructor since an instance of CostData is being passed in via the "object" parameter, you only need to assign it to the cost field, i.e.,

	public RetailItem(String description, int Item_number, double Retail, double wholesale,
		CostData object) {
		this.description = description;
		this.Item_Number = Item_Number;
		this.cost = object;
	}

Btw, the above excerpt has a small error. "this.Item_Number = Item_Number;" has no effect because Item_Number (with uppercase 'N') is the field name, whereas the constructor's 2nd parameter name is Item_number (with a lowercase 'n'). I would highly recommend that you follow the Java code convention. See Code Conventions for the Java Programming Language.