With Java Generics, it can be useful to insert wildcards wherever possible, but how do you decide which wildcard is correct? When do you use ? super T, ? extends T and when should you not use a wildcard at all.
The Get and Put principle: Use an extends wildcard when you only get values from the structure. Use a super wildcard when you only put values into the structure. Don’t use a wildcard when you both get and put values from/into the structure.
The best example of this principle is the following copy method signature:
public static void copy(
Collection<? super T> destination, Collection<? extends T> source);
The method gets values out of the source, so source uses extends, and it puts values into the destination, so that is declared with the super wildcard.