Q: related to threads of services in android the service is started by a single activity 4 times, how many threads are created by a service? And if service is started by 4 different activities then how many threads are created? Which function must be implemented inside a service inherited from intentService? how many threads are required to create inside this function?
public class HelloService extends Service {
  private Looper mServiceLooper;
  private ServiceHandler mServiceHandler;

  // Handler that receives messages from the thread
  private final class ServiceHandler extends Handler {
      public ServiceHandler(Looper looper) {
          super(looper);
      }
      @Override
      public void handleMessage(Message msg) {
         long endTime = System.currentTimeMillis() + 5*1000;
          while (System.currentTimeMillis() < endTime) {
              synchronized (this) {
                  try {
                      wait(endTime - System.currentTimeMillis());
                  } catch (Exception e) {
                  }
              }
          }
          // Stop the service using the startId, so that we don't stop
          // the service in the middle of handling another job
          stopSelf(msg.arg1);
      }
  }

  @Override
  public void onCreate() {

    HandlerThread thread = new HandlerThread("ServiceStartArguments",
            Process.THREAD_PRIORITY_BACKGROUND);
    thread.start();

     mServiceLooper = thread.getLooper();
    mServiceHandler = new ServiceHandler(mServiceLooper);
  }

  @Override
  public int onStartCommand(Intent intent, int flags, int startId) {
      Toast.makeText(this, "service starting", Toast.LENGTH_SHORT).show();

      Message msg = mServiceHandler.obtainMessage();
      msg.arg1 = startId;
      mServiceHandler.sendMessage(msg);

      // If we get killed, after returning from here, restart
      return START_STICKY;
  }

  @Override
  public IBinder onBind(Intent intent) {
      return null;
  }

  @Override
  public void onDestroy() {
    Toast.makeText(this, "service done", Toast.LENGTH_SHORT).show(); 
  }
}


A: 
the service is started by a single activity 4 times, how many threads are created by a service?

Depends. Your Service creates a new thread in onCreate, so if it's never killed then you'll only have 1 thread created. If somehow it did get killed, you would end up with one for every time you started your Service.

And if service is started by 4 different activities then how many threads are created?

Same as above.

Which function must be implemented inside a service inherited from intentService?

You might try reading the docs for IntentService, you'd find you want to implement onHandleIntent.

how many threads are required to create inside this function?

Zero.
