How to differentiate between date formats?

I have two date formats :

"yyyy-MM-dd", "dd-MM-yyyy"

But when I am trying to parse the date :"11-05-2021" , it is not throwing exception for "yyyy-MM-dd" format. Below is the thing which I am trying to do :

SimpleDateFormat sdf = new SimpleDateFormat(format);
try {
    sdf.parse(d);
} catch (Exception e) {
    e.printStackTrace();
}

Why it is not giving the exception for "yyyy-MM-dd" format . How it can be differentiated between the two formats ?

Thanks