Modify your stack implementation in such a way that a client application using your stack is unaware of the underlying array-based implementation. For instance, in a C or C++ implementation, you would define function prototypes in .h files and implementation details in .c files. In Java and C#, you use interfaces. The application uses the interfaces (or .h files) with no knowledge of how the functions are implemented. That way the array-based implementation could be replaced with a linked list-based one without any change to the client application