I make a calculator in which we enter an expression and it shows us the result.

import java.io.File;
import java.io.PrintWriter;
import java.io.BufferedReader;
import java.io.IOException;
import java.util.Scanner;
import java.io.InputStreamReader;

public class Main {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        System.out.print("Enter expression: ");
        String expression = scan.nextLine();

        try {
            File file = new File("expression.txt");

            if(!file.exists()) {
                file.createNewFile();
            }

            PrintWriter pw = new PrintWriter(file);
            pw.print(expression);
            pw.close();

            String pythonFile = "calculate.py";
            Process process = Runtime.getRuntime().exec("chmod -R 777 python " + pythonFile);
            BufferedReader br = new BufferedReader(new InputStreamReader(process.getInputStream()));
            System.out.println(br.readLine());
        } catch(IOException e) {
            e.printStackTrace();
        }
    }
}

I work on the phone and use Replit for work. The problem is that it shows us not the result, but null.
Also my python code.

expressionFile = open('expression.txt', 'a+'))

expression = str(expressionFile.read())

result = eval(expression)

expressionFile.wtite("=" + str(result))

print(expressionFile.read())

expressionFile.close()

I tried using ProcessBuilder and change the path calculate.py, but it didn’t help.