About to fail Java..HELP

Question

Here's the story..I am horrable at Java. I took this class..I am having a lot of trouble and the teacher will not help me.. Here's my assignment..
Write a program to compute mileage statistics for an automobile. Mr. Magoo has kept track of several tankfuls of gasoline by recording miles driven and gallons used for each tankful. Develop a Java application that will let Mr. Magoo input miles driven and gallons used (both ints) for each tankful and then display the miles per gallon obtained (a double). When Mr. Magoo enters -1 for miles, the program will display the total miles, total gallons, (both ints) and overall miles per gallon (a double, display rounded to 2 decimal places).Use a priming read similar to that found on page 4 of the Chap 4 handout. Continue processing input values until Mr. Magoo enters the sentinel value of -1. Use a message dialog at the start of the program that tells him what the valid input values are and what sentinel value to use to end processing. When Mr. Magoo enters an invalid amount (<= 0) for either value, there should be an error dialog box. Mr. Magoo might possibly enter two (or more) invalid amounts in a row! Make sure the program catches all invalid data.After Mr. Magoo enters the sentinel value, print out the totals. Invalid values are not included in the totals. There should be 1 information message which says "Valid miles and gallons are >0 and on the next line.."Enter -1 for miles to end processing" and then 1 box to enter mmiles, 1 to enter gallons, and an information message to say "MPG this tankful is: " TEST DATA:
     MILES     GALLONS
     155     6
     -300
     375     11
     284     8
     145     -5
     309     10
     -1
heres my code which is very wrong, not complete and generated many errors:
import javax.swing.JOptionPane;
public class Mpg
public static void main(String args[])
          String getMpg;
          double Mpg, gallons, miles;
          JOptionPane.INFORMATION_MESSAGE (null, "Valid miles and gallons are >0
          "\nEnter -1 for miles to end processing.");
          Mpg = Double.parseDouble(getMpg);
          Mpg = miles / gallons;
          JOptionPane.showInputDialog (null, "Number of miles: " + miles +","Results", JOptionPane.QUESTION_MESSAGE );
          JOptionPane.showInputDialog (null, "Number of gallons: " + gallons +", "Results", JOptionPane.QUESTION_MESSAGE );
          JOptionPane.showInputDialog (null, "MPG this tankful: " + Mpg +", "Results", JOptionPane.INFORMATION_MESSAGE );
          System.exit(0);
I would be thankful for any help I can recieve. I have no idea how to make this program work. I am banging my head against walls!!!! Thanks Everyone!

Answer 1

Hi,
Whilst not agreeing with the tone of some of the messages the principle is correct up to a point.
Anyway a little help is needed here :)
Your program could be done in any language as its basically a procedural task. Lets look at that first.
Your input screen - be it the command line or a JOptionPane must have a "switch" behind it that filters out unwanted input and only calls the mpg formula if the input is correct.
To make the calculation you need to temporarily store two values "milestravelled" and "gallonsused"
The input reciever should look for "milestravelled" first and when that has been correctly input ( > 0 ) it will treat the next input as "gallonsused".
Once the two values are stores it should automatically - or by request - calculate the MPG figure and display it.
Secondly, your code had some syntax errors I've corrected - there was an extra +" after the miles, gallons and Mpg variables
JOptionPane.showInputDialog (null, "Number of miles: " + miles ,"Results", JOptionPane.QUESTION_MESSAGE );
JOptionPane.showInputDialog (null, "Number of gallons: " + gallons , "Results", JOptionPane.QUESTION_MESSAGE );
JOptionPane.showInputDialog (null, "MPG this tankful: " + Mpg , "Results", JOptionPane.INFORMATION_MESSAGE );
Your information message declaration is not correct either. I suggest you check your syntax using the JDK documentation or your book.
I hope that helps a little and will encourage you to work out the rest on your own.
PS: Why are you using swing? A simple command line application would do fine for this task. Is it a course requirement or are you using a java development program to learn java. If its the second reason I think I can see why you're a bit lost. The IDE programs stick in a load of code that may not be necessary at all. The best way to learn is using the trusty JDK and notepad :)
Bye