 Adding to such a list requires either elements of type codice_30, any subtype of codice_30 or null (which is a member of every type).The mnemonic PECS (Producer Extends, Consumer Super) from the book Effective Java by Joshua Bloch gives an easy way to remember when to use wildcards (corresponding to covariance and contravariance) in Java.Here is an example of a generic Java class, which can be used to represent individual entries (key to value mappings) in a map