I've got an assignment that requires me to test if an integer entered by the user is prime. I've got a good base for a general test, but the assignment is combined with another problem that requires me to generate a list of all prime numbers from 1 to 100, then export that information as a .txt file. I'm a unsure of the formula I should use to make that list and how to print it to a file. Thank you for your time!

import java.io.*;
import java.util.Scanner;
public class PrimeNumberList
{
    public static void main(String[] args) throws IOException
    {
        System.out.println("This program will check if an integer value " +
        "entered by you use a prime number! \nIt will then generate a " +
        "list of prime numbers from 1 - 100 in a .txt file!");

        int userNumber;
        Scanner keyboard = new Scanner(System.in);

        System.out.print("Please enter a number or type 0 to generate " +
        "the list and close the program: ");
        userNumber = keyboard.nextInt();

        while(userNumber > 0)
        {
            isPrime(userNumber);

            if (isPrime(userNumber) == false)
                {
                    System.out.println("That is not a prime number!");
                }

            else
                System.out.println("That is indeed a prime number!");

            System.out.print("Please enter a number or type 0 to generate " +
            "the list and close the program: ");
            userNumber = keyboard.nextInt();
        }

        while(userNumber = 0);
        {
            String filename;
            System.out.print("Please enter a name for your file: ");
            filename = keyboard.nextLine();

            Printwriter outputFile = new PrintWriter(filename);

            for (int num = 0; num < 100; num++)
            {
                //Here is where I want to generate a list of all
                //prime numbers up to 100 and output that to a file.
            }
        }

    }

    public static boolean isPrime(int userNumber)
    {
        for(int i = 2; i < userNumber; i++)
        {
            if(userNumber % i == 0)
                {
                    return false;
                }
            }
            
            return true;
    }

}