Java初学者分配：在文件中插入用户控制的随机数并将其求和
Java beginner assign.: insert user controlled random numbers in a file and sum it

我对您的代码进行了一些更改，现在它似乎可以按您希望的那样正常工作。 阅读我的评论以了解您做错了什么。

import javax.swing.JOptionPane;
import java.io.*;
import java.util.Random;
import java.util.Scanner;

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

        String fileName;
        int sum = 0; // this should be integer because you want to sum the numbers...

        String upperRangeLimit;
        String userRandomNumber;

        boolean done = false;
        int x = 1;
        // int n;

        while (!done) {
            fileName = JOptionPane.showInputDialog("Enter a file name or done to exit: ");

            if (!(fileName.equals("done")))
                fileName = fileName + "_" + x++; // you wrote "fileName" like that which means you dont actually
                                                    // reference the variable

            // I didnt understand this part , i mean you ask for the input correctly but the text you provide to the user isnt write
            upperRangeLimit = JOptionPane.showInputDialog("Enter the upper range limit:\n Maximum range is 100");
            userRandomNumber = JOptionPane.showInputDialog("Error; Maximum range is 100. Enter another number: ");

            File file = new File(fileName);

            // convert user input to integers
            int limit = Integer.parseInt(upperRangeLimit);
            int range = Integer.parseInt(userRandomNumber);

            // write to the file
            PrintWriter outputFile = new PrintWriter(fileName);

            for (int n = 1; n < limit; n++) {
                // (Math.random() * ((max - min) + 1)) + min => way to generate random number in
                // range
                int random = (int) ((Math.random() * ((range - 1) + 1)) + 1);
                outputFile.println(random);
            }
            outputFile.close();

            // read the file
            Scanner inputFile = new Scanner(file);

            while (inputFile.hasNext()) {
                // sum up all of the lines
                // the integers are written in the file as strings so you have to convert them
                // back to integers again
                int nextNum = Integer.parseInt(inputFile.nextLine());
                sum = sum + nextNum;
            }
            System.out.println(sum);
            inputFile.close();

            if (fileName.equals("done"))
                done = true;
        }

    }
}