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I'm wanting to write testing scripts for Java classes using Python. I am able to compile, run, provide input, and capture output using subprocess, java.util.Scanner, and other tools. The code runs, but I'm not capturing the run log I was hoping to.

The Issue

As of now the output is captured without including the input... I'm getting (where 1, 2, 3, 4 is the input from a file):

Number? Number? Number? Number? Sum: 10

I want to capture:

Number? 1
Number? 2
Number? 3
Number? 4
Sum: 10

Current Code
Python

import os.path, subprocess

def compile_java(java_file, inputFile):
    #setup input file
    stdin = get_expected(inputFile)

    #compile java
    subprocess.check_call(['javac', java_file])

    #prep for running
    java_class, ext = os.path.splitext(java_file)
    cmd = ['java', java_class]

    #run java with input, and capturing output
    proc = subprocess.run(cmd, input=stdin, check=True, capture_output=True, text=True)

    print(proc.stdout)

def get_expected(filename):
    content = ""
    with open(f'{filename}', 'r') as f:
        for line in f.readlines():
            content += line
    return content

compile_java('Example.java', 'example.in')

Java - Example.java

import java.util.*;

public class Example{
      public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        
        int sum = 0;
        for(int i =0; i < 4; i++){
           System.out.print("Number? ");
           sum = sum + scan.nextInt();
        }
        System.out.println("Sum: " + sum);
     }
}

Input File - example.in

1
2
3
4

Note: I'm wanting to test the Java file, so I don't want to edit the Java in any way. I'm hoping there is a way I can capture the output in this format through just changing the Python code.

user1319485
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  • I'm continuing to work on this... I found a [post](https://stackoverflow.com/questions/10363853/reading-writing-to-a-popen-subprocess) where the user used `p.stdin.write("...")`. I like this idea, but when I ran the code I got `Exception in thread "main" java.util.NoSuchElementException` I was hoping I could use this to provide the input line by line, but still struggling. – user1319485 Jul 28 '21 at 03:30

1 Answers1

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If you just want to capture your output in that format, why not do the following:

for(int i =0; i < 4; i++){
    int a = scan.nextInt();
    System.out.println("Number? " + a);
    sum = sum + a;
}
mtm
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  • Thanks for the idea/comment but this doesn't accomplish my goal. My end goal is to be able to black box test the Java file via running it with my Python file and providing an input file. I see how your solution could work, IF I edited the Java file, but that misses my goal. – user1319485 Jul 31 '21 at 15:36
  • In that case, yes, this wouldn't be a solution. But since whatever comes in the `proc.stdout` is what the Java program prints and since `System.out.println("Number? " + a);` doesn't change the logic of your Java program, you can try this, if nothing else works. – mtm Jul 31 '21 at 15:49