This work investigates the question of modular specification of generic Java classes and methods. We propose extensions to the Krakatoa Modeling Language, a part of the Why platform for proving that a Java or C program is a correct implementation of some specification. The new constructs we propose for the specification of generic Java programs are presented through two significant examples: the specification of the generic method for sorting arrays which comes from the java.util.Arrays class in the Java API, and the specification of the java.util.HashMap class defining a generic hash map and its use for memoization. The main ingredient is the notion of theories and the instantiation relation between them. We discuss soundness conditions and their verification.