Future get() gets nullpointer exeception in java - swing

Question

I'm implementing a function that detects if there is a webcam. This piece of code works fine in windows and I had no problem with it in linux centos OS. Now I'm trying to run the same code in Ubuntu, here an exception is thrown.
Exception in thread "main" java.lang.NullPointerException
at CameraProperties.CheckForCameraPlugin.check(CheckForCameraPlugin.java:51)
at Main.Main.main(Main.java:39)
The code is given below.
public boolean check()
{
boolean b = true;
service = Executors.newFixedThreadPool(1);
task = service.submit(new InitialCameraChecker());
try
{
final String str;
// waits the 10 seconds for the Callable.call to finish.
str = task.get();
if (str.matches("nodevice"))
{
b = false;//Return false if no camera device found
}
else
{
b = true;
}
}
catch (InterruptedException | ExecutionException ex)
{
msgbox.showJoptionPane(15);
}
service.shutdownNow();
return b;
}
The callable class is given below
class InitialCameraChecker implements Callable<String>
{
private List<Integer> devices = new ArrayList<Integer>();
private final static String VERSION_ID = "1.0.0";
private String res;
//Checking for the Camera
public String call()
{
try
{
loadWebcam();
discoverDevices();
if (devices.isEmpty())
{
res = "nodevice";//No we cam device found
}
else
{
res = "founddevice";//Found Web Cam Device
}
}
catch (Exception ex)
{
System.out.println("Exception_logout" + ex.toString());
}
return res;
}
//Discovering the camera device
private void discoverDevices()
{
for (int i = 0; i < 10; i++)
{
CvCapture cap = null;
try
{
cap = cvCreateCameraCapture(i);
int res = cvGrabFrame(cap);
if (res > 0)
{
devices.add(i);
break;
}
}
catch (Exception e)
{
System.out.println("Exception in camaracheck Thread1");
}
finally
{
if (cap != null)
{
try
{
cvReleaseCapture(cap.pointerByReference());
}
catch (Exception e)
{
e.printStackTrace();
}
}
}
}
}
//Loading the dlls for starting the camera
private void loadWebcam()
{
String tmpDir = System.getProperty("java.io.tmpdir");
File faPath = new File(tmpDir + File.separator + "WebcamApplet_" + VERSION_ID.replaceAll("\\.", "-"));
System.setProperty("jna.library.path", faPath.getAbsolutePath());
}
}
Please tell me what is the problem. This works fine in windows.