Note to self: Java Generics - The Get and Put principle

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 <T> 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.