What is meaning If Process return zero or One

Question

I have written Java code to take POSTGRESQL, But Program executing fine without error and p.exitValue() return Zero.But backup file is not creating.Below is code

Runtime rt = Runtime.getRuntime();
Process p;
ProcessBuilder pb;
rt = Runtime.getRuntime();

pb = new ProcessBuilder(
    "/home/Java Softwares/postgres_setup/pgsql/bin/pg_dump",
    "--version", "9.2.18",
    "--host", "2BrigHQ",
    "--port", "5432",
    "--username", "postgres",
    "--no-password",
    "--dbname", "postfixdb",
    "--format", "plain",
    "--blobs",
    "--verbose",
    "--file",
    "/usr/backup/" + backupdate + "_pg_backup.sql");
try {
    final Map < String, String > env = pb.environment();
    env.put("PGPASSWORD", "admin");
    p = pb.start();

    final BufferedReader r = new BufferedReader(
        new InputStreamReader(p.getErrorStream()));
    String line = r.readLine();

    while (line != null) {
        System.err.println(line);
        line = r.readLine();

    }

    r.close();
    p.waitFor();
    System.out.println(p.exitValue());
    System.out.println("backup successfull");
} catch (IOException | InterruptedException e) {
    System.out.println(e.getMessage());
}