My loop isn't working

[Tiffany Walker]

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I'm making an inventory program for class. It has a loop that should cycle through the array and print the information. Part of the loop is supposed to add the value from each inventory item together to get the total value of the inventory. However, instead the program simply adds the last value entered to itself repeatedly. There is no compiler error, but the program isn't doing what it should.

Here is my code for the Product class.

public class Product
{
			//DECLARE VARIABLES
	private String number;     //Item Number of product( used String as item 
				   // numbers sometimes contain alphabet characters)
	private	String artist;      //Name of the band or artist
	private	int units;         //How many of the product are in stock
	private	double price;      //How much one item costs
	private double value;      //How much the stock is worth


			//CONSTRUCTOR
	public Product( String itemNumber, String artistName, 
	   int instockUnits, double costPerUnit, double costOfStock )
	{
		number = itemNumber;
		artist = artistName;
		units = instockUnits;
		price = costPerUnit;
		value = 0.00;
	}

			
			//METHODS OF THE CLASS
	//Set the item number
	public void setNumber( String itemNumber )
	{
		number = itemNumber;
	}
	//End set item number

	
	//Get item number
	public String getNumber()
	{
		return number;
	}
	//End get item number


	//Set artist
	public void setArtist( String artistName )
	{
		artist = artistName;
	}
	//End set artist

	//Get artist
	public String getArtist()
	{
		return artist;
	}
	//End get artist

	//set units
	public void setUnits( int instockUnits )
	{
		units = instockUnits;
	}
	//end set units

	
	//Get units
	public int getUnits()
	{
		return units;
	} 
	//End get units

	
	//Set price
	public void setPrice( double costPerUnit )
	{
		price = costPerUnit;
	}
	//End set price


	//Get price
	public double getPrice()
	{
		return price;
	}
	//End get price

	

	//Calculate Value
	public double value( double price, int units )
	{
		return price * units;
	}
	//End calculate value

	//String output of Product
	public String toString()
	{
		return "Product " + number + "  " + artist + "'s CD.  Cost..." + 	

	   price + ".   Units..." + units + ".   Value..." + (price * units) + ".";
	}
	
}

And my loop

import java.util.Scanner;  //allows user input
import java.util.ArrayList;  //allows us to add to the array

public class InventoryDisplay
{
	//Main Method
	public static void main( String args[] )
	{
		//Create a product
		Product CD = new Product( "0000", "Band", 0, 0.00, 0.00 );

		//Create a Scanner
		Scanner input = new Scanner( System.in );

		double total = 0.00;

		//Greeting
		System.out.print( "Welcome to Inventory Control\n" );
		System.out.print( 
		   "This program displays inventory stock and value\n\n" );
		
		//Get the length of the array
		System.out.printf( "%s%s",
		   "Before we begin,", 
		   "how many CD's titles do you want to inventory? " );
		int count = input.nextInt();

		System.out.printf( "%s%s", "Thank you.", 
		   " Please fill in product information.\n\n" );
		
		//Create an array
		Product inventory[] = new Product[ count ];
				
		//Using inventory.length as a counter, loop input and output
		for ( int counter = 0; counter < inventory.length; counter++ )
		{
			System.out.print( "What is the product number of the CD? ");
			String theNumber = input.next();
			CD.setNumber( theNumber );
			System.out.println();

			System.out.print( "What is the name of the Artist? ");
			String theArtist = input.next();
			CD.setArtist( theArtist );
			System.out.println();

			System.out.print( "How many of this CD are in stock? ");
			int theUnits = input.nextInt();
			CD.setUnits( theUnits );
			System.out.println();

			System.out.print( "How much does this CD cost? ");
			double thePrice = input.nextDouble();
			CD.setPrice( thePrice );
			System.out.println();

			
			inventory[ counter ] = new Product( 
				theNumber, theArtist, theUnits, thePrice, 		

				CD.value( CD.getPrice(), CD.getUnits() ) );

			
		}
		
		for ( int counter = 0; counter < inventory.length; counter++ )
		{	
			total =+ CD.value( CD.getPrice(), CD.getUnits() );
			System.out.println( inventory[ counter ] );
		}	
				
		System.out.printf( "The total value of the inventory is %.2f\n\n", 	

	   total );

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[fred rosenberger]

assuming the loop you are talking about is the one on lines 66-72 of the InventoryDisplay class...it makes no reference to any array. I am GUESSING you want to loop over your "inventory" array, but nowhere in that block do you make reference to it. At least, not on line 68, where you are making your sum.

[Winston Gutkowski]

Tiffany Walker wrote:I'm making an inventory program for class. It has a loop that should cycle through the array and print the information. Part of the loop is supposed to add the value from each inventory item together to get the total value of the inventory. However, instead the program simply adds the last value entered to itself repeatedly. There is no compiler error, but the program isn't doing what it should.

My reply isn't directed at your code, but more to inform you of a design pattern called "cooperative classes".

Since your Inventory system is likely to need to keep track of loads of "products", what you do is to split your class into Product (stuff that has to do with a specific product) and Products (stuff that has to do with many Products).

The only rule you need to generally follow, Java-wise, is that both classes should be in the same package; and you may need to make some private methods package-private.

One of the uses for a class like Products is for holding summary information (eg, total cost), and it works like this:
Instead of running through your entire list, just have your Product object talk to the Products object every time a change is made. That way, you only need to add/subtract the value from whatever running total you have.

Inventory maths is pretty simple; it's the model that gets complicated (often unnecessarily, even by experts; and believe me, I know (30 years, mostly in inventory systems of some description)). The trick is to keep it simple.

Winston

Oh, and while I'm on my hobby-horse: you should be very careful about using words like "value" with a Warehouse manager:

Value - Hunh? Whassat??

Cost (the amount you paid) - Might mean something, but usually filled in on a form by Head Office.

RetailPrice - Again, may have some meaning, though probably only grudgingly, because it usually means changing price tags.

RecoverableValue - Absolutely. Read: "if this warehouse burns down, what's it worth?"

(the above from "the Yorkshire guide to Inventory Management")

[Tiffany Walker]

Thank you for replying. So really I should redesign the classes, not tweak the code?

[Winston Gutkowski]

Tiffany Walker wrote:Thank you for replying. So really I should redesign the classes, not tweak the code?

A toughie. My reply was intended to address the problem in general; not yours.
It's quite likely that you'll get a solution to your code problem (ie, getting it to compile and run); the question for you then is: is that enough?

For now, my advice would be: get your current code running with the advice you get here; then read my post and see if it makes sense. If so, bang out a new question.

Winston

[Tiffany Walker]

Thanks for the help Winston. I got the code working.
total += inventory[counter].value(CD.getPrice(), CD.getUnits());
fixes my problem. I do have another class I didn't post here called
InventoryOfCD's that shows how the products occupy the inventory.
I made it honestly mimicking the textbook on a similar issue. Is that what
you referred to?

[Winston Gutkowski]

Tiffany Walker wrote:I do have another class I didn't post here called
InventoryOfCD's that shows how the products occupy the inventory.
I made it honestly mimicking the textbook on a similar issue. Is that what
you referred to?

Sounds like it. If you put your 'total' field in there, you could just add or subtract from it whenever you put CDs in or take them out of inventory rather than having to recalculate it every time.

Not that there's anything wrong with what you're doing now; it's just a different approach. Your way has the advantage of being dead simple, so it's unlikely to go wrong. Keeping a running total is a bit trickier. For a home CD collection or a small shop, your way is probably best; but for a Virgin distribution centre, the time you save from running through millions of CDs every time you want a total may make it worthwhile.

Either way, you might want to think about adding a 'getTotalValue()' method to your inventory class and putting the calculation code in there. That way you can change the way you do it later on without affecting any other code.

As with most things in programming, there's usually more than one way to do something and often no "right" answer. The trick is to know what the alternatives are.

Winston