How to destroy process when it is passed as constructor parameter

Question

I have implemented a way to timeout my process running iperf server side based on this question's top answear and it works as intended but I am not sure how to destroy a process object that I pass inside Task's class constructor.
This is my modified code:

public class Main {

    public static void main(String[] args) throws IOException {
        Process p=null;
        ExecutorService executor = Executors.newSingleThreadExecutor();
        Future<String> future = executor.submit(new Task(p));
        try {
            System.out.println("Started..");
            System.out.println(future.get(5, TimeUnit.SECONDS));
            System.out.println("Finished!");
        } catch (TimeoutException | InterruptedException | ExecutionException e) {
            future.cancel(true);
            //gives NullPointerException as expected after thread times out
            p.destroyForcibly();
            System.out.println("Terminated!");
        }

        executor.shutdownNow();
    }
}

class Task implements Callable<String> {
    Process p;

    public Task(Process p) {
        this.p = p;
    }
    @Override
    public String call() throws Exception {
        String s;
        String toDisplay = "";
        this.p = Runtime.getRuntime().exec("iperf3.exe -s -1");
        BufferedReader br = new BufferedReader(new InputStreamReader(this.p.getInputStream()));
        while ((s = br.readLine()) != null) {
            toDisplay += s + "\n";
        }
        p.destroyForcibly();

        return toDisplay;
    }
}

I am guessing I should somehow set main's Process but I have no idea how to aproach this problem