Get the result of the method and save it to a variable

There is such code, I pass two parameters to the input and get the result of this method in the console, I need to save the result in a variable and pass it to another method how to do it right? Please do not rush tomatoes with a beginner in programming, I will be glad to any help. The result of the screen method.

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  public static String activation(String serialNumber, String keyName) throws IOException, InterruptedException, SQLException {
    LocalDate futureDate = LocalDate.now().plusMonths(12);
    String formattedDate = futureDate.format(DateTimeFormatter.ofPattern("yyyyMMdd"));
    String[] command =
            {
                    "cmd",
            };
    Process p = Runtime.getRuntime().exec(command);
    //new Thread(new SyncPipe(p.getErrorStream(), System.err)).start();
    new Thread(new SyncPipe(p.getInputStream(), System.out)).start();
    PrintWriter stdin = new PrintWriter(p.getOutputStream());
    stdin.println("C:\\tdes_ecb.exe " + serialNumber + " " + keyName + " " + formattedDate);

    stdin.close();

    int returnCode = p.waitFor();
    String code = Integer.toString(returnCode);

    return code;
}

static class SyncPipe implements Runnable {
    public SyncPipe(InputStream istrm, OutputStream ostrm) {
        inputStream = istrm;
        outputStream = ostrm;

    }

    public void run() {
        try {


            final byte[] buffer = new byte[1024];
            for (int length = 0; (length = inputStream.read(buffer)) != -1; ) {
                outputStream.write(buffer, 0, length);


                StringBuilder sb = new StringBuilder();
                for (int i = 0; i < buffer.length; i++) {
                    char c = (char) buffer[i];
                    sb.append(c);
                }
                String convertedString = sb.toString();

                key(convertedString);


            }


        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private final OutputStream outputStream;
    private final InputStream inputStream;
}

public void SyncPipe(InputStream inputStream, OutputStream outputStream) {
    this.inputStream = inputStream;
    this.outputStream = outputStream;
}