Abstract data types Internal implementation of an abstract data type should never be changed Abstract data types is one of the basic ideas behind object orientation Abstract data types should have a well defined AP Transparency means that all internal details of the implementation of an abstract data type is externally visible APls should not be changed if possible Abstract data types should always allow client code to directly access data for the purpose of debugging Abstract data types can only be implemented in an object oriented language Transparency means that the internal implementation of an abstract data type is externally invisible Abstract data types facilitates reuse of code