ceg860 (Prasad)L6MR1 Modularity Extendibility Reusability

ceg860 (Prasad)L6MR2 Criteria for a Design Method to be Modular Decomposability Helps in the task of obtaining independent sub- problems (division of labor). –E.g., Top-down design. Composability Favors development of well-defined software elements that can be freely combined in different contexts. –E.g., Subroutines for Numeric Computation ( FORTRAN ). –E.g., Functions for List processing (LISP). –In general, top-down design does not favor composability.

ceg860 (Prasad)L6MR3 Continuity Small changes in problem specification should trigger small changes in software modules. –E.g., Use of symbolic constants. –E.g., Pascal’s monolithic programs vs Ada’s packages. –E.g., C’s-Union type vs C++/Java’s Class hierarchy (Polymorphism/Dynamic binding). Understandability –E.g., Documentation Protection Containing the effects of run-time errors.

ceg860 (Prasad)L6MR4 Modular Software Direct Mapping Structure of the problem domain is reflected in the structure of the software solution. Strong ( Internal ) Cohesion ( intra-modular ) Degree of binding among elements in the module. Minimal ( External ) Coupling ( inter-modular ) Dependency between modules.

ceg860 (Prasad)L6MR5 Information Hiding Separation of behavior specification from implementation details. Client may rely only on server’s public interface. Open-Closed Principle Module for extension (by server) in future. Module for use (by client) now. Single Choice Principle Whenever a software system must support a set of alternatives, one and only one module in the system should know their exhaustive list.

ceg860 (Prasad)L6MR6 Benefits of Reusability Enhancing reusability enhances almost all the other software quality by promoting sharing of experts’ work. –Timeliness (Time to market) –Decreased maintenance effort –Reliability –Efficiency –Consistency –Economy

ceg860 (Prasad)L6MR7 The reuse-redo dilemma The realities of practical software development requires combining reuse and adaptation. The right notion of module must reconcile reusability and extendibility, closure and openness, satisfying today’s demands and trying to guess what future holds.

ceg860 (Prasad)L6MR8 Reusable Modules Reusable modules must be adaptable. Abstractions/Frameworks –“Lower-level” details changeable/pluggable. E.g., Java applets, Java threads, etc. Generalization through parameterization –Customizable by instantiation. E.g., Generic stack, Generic queue, etc. Software Components –Design Patterns

ceg860 (Prasad)L6MR9 Example : Table Data Structure Applications –Symbol table in Compiler –File System Directory –Database (Relations) Implementation based on: Array, Linked list, Binary search trees, Hash tables, B-trees, Heaps, etc

ceg860 (Prasad)L6MR10 Table Implementations HASH SEQUENTIAL TREE ARRAYLINKED FILE

ceg860 (Prasad)L6MR11 Searching a sequential table Has Has(t : seq_table; x : element) : bool is do from start until after or else found(x) loop forth end; return (not after) end;

ceg860 (Prasad)L6MR12

ceg860 (Prasad)L6MR13 Requirements on Reusable Modules Type variation E.g., Ada generics, C++ templates, etc Routine grouping (“quantum”) –As it is possible to apportion “work” among the routines in different ways and yet achieve the same overall behavior. (Mutual consistency) Set of charactersE.g., Set of characters abstraction can be implemented using a sorted list, a list without duplicates, a sorted list without duplicates, a packed boolean array, a string, etc.

ceg860 (Prasad)L6MR14 Factoring commonality Code sharing (server) –E.g., Inheritance, Abstract class, etc Organizing implementation variations Common behavior (server) –E.g., Polymorphism, Dynamic binding, etc Representation Independence (client) –E.g., Dynamic binding. Reusing existing modules (client) –E.g., Composition, Delegation.

ceg860 (Prasad)L6MR15 Example Module Structures –Routines : Functions and Procedures E.g., C, Pascal, etc –Packages E.g., Modula, CLU, etc –Generic Routines and Packages E.g., Ada, etc. –Class Hierarchy E.g., Java, etc. –Generic Class Hierarchy E.g., C++, Ada-95, etc.

ceg860 (Prasad)L6MR16 Overloading Ad hoc polymorphism Capturing Similarity Convenient for code integration (avoiding apparent name clashes). –Name Overloading Conserving names. –Operator Overloading Conforming to domain vocabulary.

ceg860 (Prasad)L6MR17 Object-Oriented Software Development Bases the architecture of a software system on modules deduced from the types of objects it manipulates (rather than the functions it is intended to ensure). IssuesIssues –How to find and use relevant object types? Object-Oriented Analysis and Design –How to describe and implement the object types and their relationships? Object-Oriented Programming (Language)