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Exception in thread "main" java.util.IllegalFormatConversionException: f != java.lang.Integer

I do not know how to fix my format conversion. Help would be appreciated.

import javax.swing.JOptionPane;
public class SoftwareSales {
public static void main(String[] args) {
// This program displays the amount of discount (if any) and the total amount of the purchase after the discount.
    String userInput;
    String userOutput;
    double packagePrice = 99;
    int userNumberOfPackages;
    double discount = 0;
    double subTotal = 0;
    double total = 0;
    userInput = JOptionPane.showInputDialog( "Enter how many packages you are buying" );
    userNumberOfPackages = Integer.parseInt( userInput );
    if ( userNumberOfPackages < 10){
        subTotal = packagePrice * userNumberOfPackages;
        discount = 0;
        total = subTotal - discount;
    } else if ( userNumberOfPackages < 20 ){
        subTotal = packagePrice * userNumberOfPackages;
        discount = ( (double)20/100) * subTotal;
        total = subTotal - discount;
    } else if ( userNumberOfPackages < 50 ){
        subTotal = packagePrice * userNumberOfPackages;
        discount = ( (double)30/100) * subTotal;
        total = subTotal - discount;
    } else if ( userNumberOfPackages < 100 ){
        subTotal = packagePrice * userNumberOfPackages;
        discount = ( (double)40/100) * subTotal;
        total = subTotal - discount;
    } else{
        subTotal = packagePrice * userNumberOfPackages;
        discount = ( (double)50/100) * subTotal;
        total = subTotal - discount;
    userOutput = String.format( "Packages purchased: \nSubtotal: $\nDiscount: $\nTotal: $", userNumberOfPackages, subTotal,
            discount, total);
    JOptionPane.showMessageDialog( null, userOutput);
    System.exit(0);
                a) Usually there's a line number associated with the error message. Please mark the line that is mentioned. b) Your code can be reduced until you get a minimal reproducible example. Do that for us. We don't like reading dozens of lines of code if the problem can be expressed in 5 lines.
– Thomas Weller
                Jul 18, 2018 at 14:02
                at java.util.Formatter$FormatSpecifier.failConversion(Formatter.java:4302)         at java.util.Formatter$FormatSpecifier.printFloat(Formatter.java:2806)         at java.util.Formatter$FormatSpecifier.print(Formatter.java:2753)         at java.util.Formatter.format(Formatter.java:2520)         at java.util.Formatter.format(Formatter.java:2455)         at java.lang.String.format(String.java:2940)         at SoftwareSales.main(SoftwareSales.java:45)
– user10100043
                Jul 18, 2018 at 14:07
Let's sum up the comments, since they will not only fix your problem but describe the procedure for any problem.
Get all the exception information. That is type, message and callstack.
Look at the call stack and identify the topmost class you have written yourself.
Get the line number.
Look at that line and think of potential issues.
Applied to your case:
at java.util.Formatter$FormatSpecifier.failConversion(Formatter.java:4302) at
java.util.Formatter$FormatSpecifier.printFloat(Formatter.java:2806) at
java.util.Formatter$FormatSpecifier.print(Formatter.java:2753) at
java.util.Formatter.format(Formatter.java:2520) at
java.util.Formatter.format(Formatter.java:2455) at
java.lang.String.format(String.java:2940) at 
SoftwareSales.main(SoftwareSales.java:45) 
Only SoftwareSales.java was written by yourself. So the problem is in SoftwareSales.java, line 45.
It's this line:
userOutput = String.format( "Packages purchased: \nSubtotal: $\nDiscount: $\nTotal: $", userNumberOfPackages, subTotal,
            discount, total);
With String.format() you want to insert 4 numbers into a string. But where should they be inserted? There's no placehoder.
Looking at some documentation:
  
format: a format string
And a format string needs something with a %. Likely %ffor floating point and %d for decimal.
Finally (untested):
userOutput = String.format( "Packages purchased: %d\n
       Subtotal: $%f\n
       Discount: $%f\n
       Total: $%f", 
       userNumberOfPackages, subTotal, discount, total);
        
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