Code returns "null" and "0"s in main module - why?

Question

I am coding a program that is in two modules. The main module serves it's standard function. The inventory class/module has three functions - 1) request stock input on 4 data points (1 String and 3 integers, one of which is a double) - 2) calculate the value of the inventory, and - 3) return string which reads out the 4 data points and the calculation showing the total value of the inventory. I've created the 3 functions in the inventory class/module, but obviously don't have them referring to each other correctly because when we come to the end of the program the output is null for the String and "0"s for the int/doubles. The code is below - any ideas about how to overcome the empty output? The code compiles fine and acts like it is going to work, but when it comes to the final command line it outputs
"null, which is item number 0, has 0 currently in stock at a price of 0.00 each. The total value of inventory in stock is 0.00"
Main module:
public class Main {
    @SuppressWarnings("static-access")
    public static void main(String[] args) {
        Inventory inventory = new Inventory(); //call Inventory class
        inventory.inventoryInput();
        Inventory results = new Inventory();
        results.inventoryResults(); //call for inventoryResults in Inventory class
}Inventory module:
import java.util.Scanner;
import static java.lang.System.out;
public class Inventory
  //declare and initialize variables
   int itemNumber = 0;
   String productName;
   int stockAmount = 0;
   double productCost = 0;
   double totalValue = 0;
   String inventoryResults;
   //initialize scanner
   Scanner input = new Scanner(System.in);
   public void inventoryInput ()
   out.println("Please enter item number: "); //prompt for item number
      itemNumber = input.nextInt();
   out.println( "Enter product name/description: "); //prompt for product name
      productName = input.next();
   out.println("Quantity in stock: ");
      stockAmount = input.nextInt(); // prompt for stock quantity
   out.println("What is the product cost for each unit? ");
      productCost = input.nextDouble(); // prompt for product cost
    } // end inventoryInput
    public double totalValue( double stockAmount, double productCost )
      totalValue = stockAmount * productCost;
      return totalValue; // return stock value
    } // end totalValue
    public void inventoryResults()
    out.printf("%s, which is item number %d, has %d currently in stock at a " +
     "price of %.2f each. The total value of inventory in stock is " +
     "%.2f\n.", productName, itemNumber, stockAmount, productCost, totalValue);
    } // end inventoryResult
}// end method

Answer 1

justStartingOut wrote:
Actually my final solution was quite simple - I moved the calculation and final formated print text statements into the body of Inventory's class code. It now works. "Works" scares me a bit.
Someone cooking dinner might rummage about in the fridge and the cupboards to see what's there. Do imaginative things with what they come up with. And, with a bit of luck come up with something tasty or, at any rate edible.
A physician deciding on a medical treatment would do well to try a more cautious approach. A specific aim would be a good thing. And a calculated appreciation of the documented effects of each medicine. And how they interact.
It's up to you to determine which approach your coding should resemble. But it seems to me that your original Main class had a perfectly good reason to exist. It was a driver class whose purpose seemed to be to create and use an Inventory. (And using an Inventory is a very different thing from being an Inventory, so it made perfect sense to have two different classes.) To me it works or not, depending on whether it fufills that purpose. And what you have done is not so much solve the problem of it not working, as avoid that problem.
(If by "moved" you mean that the outputting now occurs as part of - or is called from - inventoryInput() then that is not a good thing. The input method should input: just input.)
I think that is because once the original input was loaded into the program (when I entered product number, name, price and value), the entries were dropped when the code switched to the next step. I think your intuition is entirely correct. In particular look at what your original main() method does (my comments replacing yours):
Inventory inventory = new Inventory(); // (A) Create an inventory...
inventory.inventoryInput(); // ... and put stuff into it
    // (B) Create some new entirely different (and empty) inventory...
Inventory results = new Inventory();
results.inventoryResults(); // ... and print its contentsInstead of creating a second inventory, try printing the results of the first one.
Edited by: pbrockway2 on Apr 22, 2008 12:37 PM
Whoops ... just read reply 2.
It might sense, though to call totalValue() at the end your input method (because at that point the total value has changed). Or do away with that method - which you worked on getting right in your other thread ;)