<uses-permission android:name="android.permission.GET_TASKS"/>

ActivityManager manager = (ActivityManager)   getSystemService(Context.ACTIVITY_SERVICE);   
List<RunningTaskInfo> runningTasks = manager .getRunningTasks(1);   
RunningTaskInfo cinfo = runningTasks.get(0);   
ComponentName component = cinfo.topActivity;   
Log.e("current activity is ", component.getClassName());