Newbie needing help with code numbers and if-else

Question

I'm 100% new to any kind of programming and in my 4th week of an Intro to Java class. It's also an on-line class so my helpful resources are quite limited. I have spent close to 10 hours on my class project working out P-code and the java code itself, but I'm having some difficulty because the project seems to be much more advanced that the examples in the book that appear to only be partly directly related to this assignment. I have finally come to a point where I am unable to fix the mistakes that still show up. I'm not trying to get anyone to do my assignment for me, I'm only trying to get some help on what I'm missing. I want to learn, not cheat.
Okay, I have an assignment that, in a nutshell, is a cash register. JOptionPane prompts the user to enter a product code that represents a product with a specific price. Another box asks for the quanity then displays the cost, tax and then the total amount plus tax, formatted in dollars and cents. It then repeats until a sentinel of "999" is entered, and then another box displays the total items sold for the day, amount of merchandise sold, tax charged, and the total amount acquired for the day. If a non-valid code is entered, I should prompt the user to try again.
I have this down to 6 errors, with one of the errors being the same error 5 times. Here are the errors:
C:\PROGRA~1\XINOXS~1\JCREAT~1\MyProjects\Sales.java:50: 'else' without 'if'
else //if invalid code entered, output message
^
C:\PROGRA~1\XINOXS~1\JCREAT~1\MyProjects\Sales.java:39: unexpected type
required: variable
found : value
100 = 2.98;
^
C:\PROGRA~1\XINOXS~1\JCREAT~1\MyProjects\Sales.java:41: unexpected type
required: variable
found : value
200 = 4.50;
^
C:\PROGRA~1\XINOXS~1\JCREAT~1\MyProjects\Sales.java:43: unexpected type
required: variable
found : value
300 = 6.79;
^
C:\PROGRA~1\XINOXS~1\JCREAT~1\MyProjects\Sales.java:45: unexpected type
required: variable
found : value
400 = 5.29;
^
C:\PROGRA~1\XINOXS~1\JCREAT~1\MyProjects\Sales.java:47: unexpected type
required: variable
found : value
500 = 7.20;
^
And finally, here is my code. Please be gentle with the criticism. I've really put a lot into it and would appreciate any help. Thanks in advance.
import java.text.NumberFormat; // class for numeric formating from page 178
import javax.swing.JOptionPane; // class for JOptionOPane
public class Sales {
//main method begins execution ofJava Application
public static void main( String args[] )
double quantity; // total of items purchased
double tax; // total of tax
double value; // total cost of all items before tax
double total; // total of items including tax
double totValue; // daily value counter
double totTax; // daily tax counter
double totTotal; // daily total amount collected (+tax) counter
double item; //
String input; // user-entered value
String output; // output string
String itemString; // item code entered by user
String quantityString; // quantity entered by user
// initialization phase
quantity = 0; // initialize counter for items purchased
// get first code from user
itemString = JOptionPane.showInputDialog(
"Enter item code:" );
// convert itemString to double
item = Double.parseDouble ( itemString );
// loop until sentinel value read from user
while ( item != 999 ) {
// converting code to amount using if statements
if ( item == 100 )
100 = 2.98;
if ( item == 200 )
200 = 4.50;
if ( item == 300 )
300 = 6.79;
if ( item == 400 )
400 = 5.29;
if ( item == 500 )
500 = 7.20;
else //if invalid code entered, output message
JOptionPane.showMessageDialog( null, "Invalid code entered, please try again!",
"Item Code", JOptionPane.INFORMATION_MESSAGE );
} // end if
} // end while
// get quantity of item user
itemString = JOptionPane.showInputDialog(
"Enter quantity:" );
// convert quantityString to int
quantity = Double.parseDouble ( quantityString );
// add quantity to quantity
quantity = quantity + quantity;
// calculation time! value
value = quantity * item;
// calc tax
tax = value * .07;
// calc total
total = tax + value;
//add totals to counter
totValue = totValue + value;
totTax = totTax + tax;
totTotal = totTotal + total;
// display the results of purchase
JOptionPane.showMessageDialog( null, "Amount: " + value +
"\nTax: " + tax + "\nTotal: " + total, "Sale", JOptionPane.INFORMATION_MESSAGE );
// get next code from user
itemString = JOptionPane.showInputDialog(
"Enter item code:" );
// If sentinel value reached
if ( item == 999 ) {
// display the daily totals
JOptionPane.showMessageDialog( null, "Total amount of items sold today: " + quantity +
"\nValue of ites sold today: " + totValue + "\nTotal tax collected today: " + totTax +
"\nTotal Amount collected today: " + totTotal, "Totals", JOptionPane.INFORMATION_MESSAGE );
System.exit( 0 ); // terminate application
} // end sentinel
} // end message
} // end class Sales

Answer 1

Here you go. I haven't tested this but it does compile. I've put in a 'few helpful hints'.
import java.text.NumberFormat; // class for numeric formating from page 178
import javax.swing.JOptionPane; // class for JOptionOPane
public class TestTextFind {
//main method begins execution ofJava Application
public static void main( String args[] )
     double quantity; // total of items purchased
     double tax; // total of tax
     double value; // total cost of all items before tax
     double total; // total of items including tax
//     double totValue; // daily value counter
//     double totTax; // daily tax counter
//     double totTotal; // daily total amount collected (+tax) counter
// Always initialise your numbers unless you have a good reason not too
     double totValue = 0; // daily value counter
     double totTax = 0; // daily tax counter
     double totTotal = 0; // daily total amount collected (+tax) counter
     double itemCode;
     double item = 0;
     String itemCodeString; // item code entered by user
     String quantityString; // quantity entered by user
     // initialization phase
     quantity = 0; // initialize counter for items purchased
     // get first code from user
     itemCodeString = JOptionPane.showInputDialog("Enter item code:" );
     // convert itemString to double
     itemCode = Double.parseDouble ( itemCodeString );
     // loop until sentinel value read from user
     while ( itemCode != 999 ) {
* 1. variable item mightnot have been initialised
* You had item and itemCode the wrong way round.
* You are supposed to be checking itemCode but setting the value
* for item
          // converting code to amount using if statements
          if ( item == 100 )
          {itemCode = 2.98;}
          else if ( item == 200 )
          {itemCode = 4.50;}
          else if ( item == 300 )
          {itemCode = 6.79;}
          else if ( item == 400 )
          {itemCode = 5.29;}
          else if ( item == 500 )
          {itemCode = 7.20;}
          else {//if invalid code entered, output message
               JOptionPane.showMessageDialog( null, "Invalid code entered, please try again!",
               "Item Code", JOptionPane.INFORMATION_MESSAGE );
          } // end if
     } // end while
     // get quantity of item user
     itemCodeString = JOptionPane.showInputDialog("Enter quantity:" );
* 2.
* You have declared quantityString here but you never give it a value.
* I think this should be itemCodeString shouldnt it???
* Or should you change itemCodeString above to quantityString?
     // convert quantityString to int
//     quantity = Double.parseDouble ( quantityString );  // old code
     quantity = Double.parseDouble ( itemCodeString );
     // add quantity to quantity
     quantity = quantity + quantity;
     // calculation time! value
     value = quantity * itemCode;
     // calc tax
     tax = value * .07;
     // calc total
     total = tax + value;
     //add totals to counter
* 3. 4. and 5.
* With the following you have not assigned the 'total' variables a value
* so in effect you are saying eg. "total = null + 10". Thats why an error is
* raised.  If you look at your declaration i have assigned them an initial
* value of 0.
     totValue = totValue + value;
     totTax = totTax + tax;
     totTotal = totTotal + total;
     // display the results of purchase
     JOptionPane.showMessageDialog( null, "Amount: " + value +
     "\nTax: " + tax + "\nTotal: " + total, "Sale", JOptionPane.INFORMATION_MESSAGE );
     // get next code from user
     itemCodeString = JOptionPane.showInputDialog("Enter item code:" );
     // If sentinel value reached
     if ( itemCode == 999 ) {
          // display the daily totals
          JOptionPane.showMessageDialog( null, "Total amount of items sold today: " + quantity +
          "\nValue of ites sold today: " + totValue + "\nTotal tax collected today: " + totTax +
          "\nTotal Amount collected today: " + totTotal, "Totals",           JOptionPane.INFORMATION_MESSAGE );
          System.exit( 0 ); // terminate application
     } // end sentinel
} // end message
} // end class SalesRob.