Execute commands to running process

I'm currently trying to use processes in java to run a jar file. I'm able to run and read the content printed by the process. What I'm trying to achieve, is to write a command to the process. The jar file I'm running asks for user input and I am trying to allow the user to enter that input. This is my current code which isn't working:

public class Main {

public static void main(String[] args) {

    String command = "java -jar game.jar";

    Process process = executeCommand(command);

    CompletableFuture.runAsync(() -> {

        Scanner scanner = new Scanner(System.in);

        while (true) {

            String input = scanner.nextLine();

            if (input == null) {
                continue;
            }

            executeCommand(process, input);

        }

    });

    readOutput(process);

}

public static Process executeCommand(String command) {

    try {
        Process process = Runtime.getRuntime().exec(command);

        return process;
    } catch (IOException ex) {
        ex.printStackTrace();
        return null;
    }

}

public static List<String> readOutput(Process process) {

    List<String> output = new ArrayList<>();

    try {

        BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));

        String line;
        while((line = reader.readLine()) != null) {
            System.out.print(line + "\n");
            output.add(line);
        }

        process.waitFor();

        return output;

    } catch (IOException ex) {
        ex.printStackTrace();
        return output;
    } catch (InterruptedException ex) {
        ex.printStackTrace();
        return output;
    }

}

public static void executeCommand(Process process, String command) {

    try {

        OutputStream out = process.getOutputStream();

        out.write(command.getBytes());

    } catch (IOException ex) {
        ex.printStackTrace();
    }

}

}