Have you looked at what P contains? Its possibly just the PID of the process,
but you never know.

Yeah, it just gives the process  ID... i think i might save the
result of the c++ program  (for example a string) to a text file and
see if java  can read the text file, all in one process :) :)

I found this code  on the web. I believe it is what you are looking for?

import java.io.*;

public class  RunCommand {

public static  void main(String args[]) {

String s = null;

try {

// run  the Unix "ps -ef" command

//Process p = Runtime.getRuntime().exec("ps -ef");
Process p = Runtime.getRuntime().exec("date");

BufferedReader stdInput = new BufferedReader(new
InputStreamReader(p.getInputStream()));

BufferedReader stdError = new BufferedReader(new
InputStreamReader(p.getErrorStream()));

// read the output from the command

System.out.println("Here is the standard output of the command:\n");
while ((s = stdInput.readLine()) != null) {
System.out.println(s);
}

// read any errors from the attempted command

System.out.println("Here is the standard error of the command (if
any):\n");
while ((s = stdError.readLine()) != null) {
System.out.println(s);
}

System.exit(0);
}
catch (IOException e) {
System.out.println("exception happened - here's what I know: ");
e.printStackTrace();
System.exit(-1);
}
}
}



// Copyright 1998 DevDaily Interactive, Inc. All Rights Reserved.

import java.io.*;

public class RunCommand {

public static void  main(String args[]) {

String s = null;

try {

// run the Unix "ps -ef" command

//Process p = Runtime.getRuntime().exec("ps -ef");
Process p = Runtime.getRuntime().exec("date");

BufferedReader stdInput = new BufferedReader(new
InputStreamReader(p.getInputStream()));

BufferedReader stdError = new BufferedReader(new
InputStreamReader(p.getErrorStream()));

// read the output from the command

System.out.println("Here is the standard output of the command:\n");
while ((s = stdInput.readLine()) != null) {
System.out.println(s);
}

// read any errors from the attempted command

System.out.println("Here is the standard error of the command (if
any):\n");
while ((s = stdError.readLine()) != null) {
System.out.println(s);
}

System.exit(0);
}
catch (IOException e) {
System.out.println("exception happened - here's what I know: ");
e.printStackTrace();
System.exit(-1);
}
}
}

try {
// Execute command
String command = "ls";
Process child = Runtime.getRuntime().exec(command);

// Get input stream to read from it
InputStream in = child.getInputStream();
int c;
while ((c = in.read()) != -1) {
process((char)c);
}
in.close();
} catch  (IOException e) {
}


sending input

try {
// Execute command
String command = "cat";
Process child = Runtime.getRuntime().exec(command);

// Get output stream to write from it
OutputStream out = child.getOutputStream();

out.write("some text".getBytes());
out.close();
} catch (IOException e) {
}