Execution of Program from Program with Time Constraints (Java)

I'm trying to make program which runs some executable program(call it p), given time limit t ms. It does following tasks:

1. If program p has executed normally, print it's output to console.
2. If program p couldn't execute completely within time limit, print "Sorry, needs more time!" and then terminate execution of p.
3. If program p has terminated abnormally (e.g. RuntimeError), print "Can I've some debugger?"

I'm using ProcessResultReader class in the following program from here. My program is working as long as p finishes it's execution normally or terminate abnormally. But, it doesn't terminate if p itself doesn't terminate after timeout.(Try p with simple while(true) loop with no exit condition). It seems that thread stdout is alive even after execution of stdout.stop(). What am I doing wrong in this code?

Thanks.

import java.util.concurrent.TimeUnit;
import java.io.*;

class ProcessResultReader extends Thread
{

    final InputStream is;
    final StringBuilder sb;

    ProcessResultReader(final InputStream is)
    {
        this.is = is;
        this.sb = new StringBuilder();
    }
    public void run()
    {
        try
        {
            final InputStreamReader isr = new InputStreamReader(is);
            final BufferedReader br = new BufferedReader(isr);
            String line = null;
            while ((line = br.readLine()) != null)
            {
                this.sb.append(line).append("\n");
            }
        }
        catch (final IOException ioe)
        {
            System.err.println(ioe.getMessage());
            throw new RuntimeException(ioe);
        }
    }

    @Override
    public String toString()
    {
        return this.sb.toString();
    }
    public static void main(String[] args) throws Exception
    {
        int t = 1000; 
        Process p = Runtime.getRuntime().exec(cmd); //cmd is command to execute program p 
        ProcessResultReader stdout = new ProcessResultReader(p.getInputStream());
        stdout.start();
        if(!p.waitFor(t, TimeUnit.MILLISECONDS))
        {
            stdout.stop();
            p.destroy();
            System.out.println("Sorry, needs more time!");
        }
        else
        {
            if(p.exitValue()==0) System.out.println(stdout.toString());
            else System.out.println("Can I've some debugger?");
        }
    }
}