Write a program that use a loop to display the integers entered by the user from smallest to larger

Where the user enter a series of integers in a loop. The user enter -99 to signal the end of the series. Where all of the integer will be save in a file. After all the numbers have been entered, the program should read all of the integers save in the file and display all of numbers from smallest to largest.

I been trying but I just can't figure out, how to display the integers in order. Now I'm stuck. As you are able to see, I did not use any fancy programming, such as array or list, because I'm still learning about classes and had not learn any of that yet.

package chapter4;

import java.util.Scanner;

import java.io.*;

import java.util.Random;

public class ProChallenge10 {
    
    public static void main(String[]args) throws IOException{
        
        int integer =0; //Integer enter by the user.
                    
        boolean signal = false; // To end the loop.
        
        Scanner keyboard = new Scanner(System.in);
        
        //Created the file for the integers entered. 
        PrintWriter outputFile = new PrintWriter("ProChallenge10.txt"); 
        
        //Let the user enter a series of numbers.
        while(signal == false){
            
            System.out.println("Enter an integer");
            integer = keyboard.nextInt();
            
            outputFile.println(integer);
        
        //To end the program.
        if(integer == -99){
            signal = true;
            //Close the outputFile.
            outputFile.close();
            } 
        }
        
         //Open the file and read input from the file.
         File file = new File("ProChallenge10.txt");
         Scanner inputFile = new Scanner(file);
         
         //Read all of the values from the file and display their numbers. 
         while(inputFile.hasNext()){
            
            int number = inputFile.nextInt();
            
           }
         //Close the InputFile.
         inputFile.close();
    }
}

Add them to the List then sort() before printing it to the text file.

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.io.*;
import java.util.Random;

public class ProChallenge10 {

public static void main(String[]args) throws IOException{

    int integer =0; //Integer enter by the user.

    boolean signal = false; // To end the loop.

    Scanner keyboard = new Scanner(System.in);

    List<Integer> listofInt = new ArrayList<Integer>();// Array List for your integers

    //Created the file for the integers entered. 
    PrintWriter outputFile = new PrintWriter("ProChallenge10.txt"); 

    //Let the user enter a series of numbers.
    while(signal == false){

        System.out.println("Enter an integer");
        integer = keyboard.nextInt();

        listofInt.add(integer);// adding of int to list


    //To end the program.
    if(integer == -99){
        listofInt.sort(null);// sort the list
        for(int x=0;x<listofInt.size();x++){
            outputFile.println(listofInt.get(x));//Printing of list to text file
        }
        signal = true;
        //Close the outputFile.
        outputFile.close();
        } 
    }

     //Open the file and read input from the file.
     File file = new File("ProChallenge10.txt");
     Scanner inputFile = new Scanner(file);

     //Read all of the values from the file and display their numbers. 
     while(inputFile.hasNext()){

        int number = inputFile.nextInt();

       }
     //Close the InputFile.
     inputFile.close();
}

}