new ActivityTask(_context, dateStr, holder).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, null);
null of the last argument to method executeOnExecutor(Executor, Void...) doesn't exactly match the vararg parameter type. Cast to Void[] to confirm the non-varargs invocation, or pass individual arguments of type Void for a varargs invocation
new ActivityTask(_context, dateStr, holder).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, (Void[])null);