activityManager.getRunningTasks(1).get(0).topActivity.getPackageName();
activityManager.getRunningTasks(1).get(0).topActivity.getClassName();
no more available of android version 20 or abouve so you need to change this using below code
if (Build.VERSION.SDK_INT > 20) {
mPackageName = activityManager.getRunningAppProcesses().get(0).processName;
mClassName = activityManager.getRunningAppProcesses().get(0).getClass().getName();
} else {
mPackageName = activityManager.getRunningTasks(1).get(0).topActivity.getPackageName();
mClassName = activityManager.getRunningTasks(1).get(0).topActivity.getClassName();
}