Try the following code:
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_launch);
context = getApplicationContext();
if (checkPlayServices())
{
gcm = GoogleCloudMessaging.getInstance(this);
regid = getRegistrationId(context);
if (regid.isEmpty())
{
registerInBackground();
}
else
{
Log.d(TAG, "No valid Google Play Services APK found.");
}
}
}
private String getRegistrationId(Context context)
{
final SharedPreferences prefs = getGCMPreferences(context);
String registrationId = prefs.getString(PROPERTY_REG_ID, "");
if (registrationId.isEmpty()) {
Log.d(TAG, "Registration ID not found.");
return "";
}
int registeredVersion = prefs.getInt(PROPERTY_APP_VERSION, Integer.MIN_VALUE);
int currentVersion = getAppVersion(context);
if (registeredVersion != currentVersion) {
Log.d(TAG, "App version changed.");
return "";
}
return registrationId;
}
private SharedPreferences getGCMPreferences(Context context)
{
return getSharedPreferences(LaunchActivity.class.getSimpleName(),
Context.MODE_PRIVATE);
}
private static int getAppVersion(Context context)
{
try
{
PackageInfo packageInfo = context.getPackageManager()
.getPackageInfo(context.getPackageName(), 0);
return packageInfo.versionCode;
}
catch (NameNotFoundException e)
{
throw new RuntimeException("Could not get package name: " + e);
}
}
private void registerInBackground()
{ new AsyncTask() {
Override
protected Object doInBackground(Object... params)
{
String msg = "";
try
{
if (gcm == null)
{
gcm = GoogleCloudMessaging.getInstance(context);
}
regid = gcm.register(SENDER_ID);
Log.d(TAG, "########################################");
Log.d(TAG, "Current Device's Registration ID is: "+msg);
}
catch (IOException ex)
{
msg = "Error :" + ex.getMessage();
}
return null;
} protected void onPostExecute(Object result)
{ //to do here };
}.execute(null, null, null);
}
You may want to make sure you added all the required lines in the AndroidManifest file, as well as declaring a GCM Service and a GCM Broadcast Receiver.
Link to the full answer