1. http://www.slideshare.net/mbrambil
http://home.dei.polimi.it/mbrambil
http://twitter.com/MarcoBrambi
SOFTWARE REUSE
AND
DESIGN FOR REUSE
Marco Brambilla

2. Context of the Lesson
 Prerequisites
 Objectoriented programming
 Software engineering basics (UML, patterns, ...)

3. Agenda
 Introduction to reuse
 Benefits and issues for reuse
 Levels of reuse
 System
 Architecture
 Design
 Implementation
 Some details on design

4. Introduction
 Definition of reuse
 Design for reuse and reuse of design
 Purpose and state of the practice
 in other disciplines
 in software engineering

5. Benefits of software reuse
Benefit Explanation
Increased Reused software has been tried and tested
dependability in working systems
Reduced process The cost and risk of existing software is
risk already known
Effective use of reusable software encapsulates their
specialists knowledge
Accelerated both development and validation time may
development be reduced

6. Problems with reuse
Problem Explanation
Increased reused elements of the system may become
maintenance costs increasingly incompatible with system changes
Not-invented-here Companies rewrite components because they
syndrome believe they can improve on them or because they
feel they must own them.
Creating, Generality of components doesn’t come for free.
maintaining, and Development processes have to be adapted
using a component
library
Finding and Software components have to be discovered in a
understanding library, understood and, sometimes, adapted
components
Understanding Reused elements always come with prerequisites the
applicability application field must comply with

7. The 4 layers of reuse
4. Whole system Configuration
and BlackBox
reuse of Config.
applications
3. (Macro-, System-,
Enterprise-, Global-) Reuse of application
Architecture Frameworks
frameworks,
middleware, services
2. Design (micro architecture) Reuse of
designs and Patterns
object and function reuse components
1. Implementation Reuse of
classes and
methods Programming

8. 4. Approaches supporting reuse at
system level
 Software product lines
 COTS (Commercial, off-the-shelf) product reuse
 Configurable vertical applications
 ERP (Enterprise Resource Planning) systems

9. 3. Approaches supporting reuse at
architecture level
 Architectural patterns standard sw architectures
 Application frameworks classes at system level
 Legacy system wrapping interfaces to old code
 Service-oriented systems shared (third-party) services

10. 2. Reuse at design level
 Objectorientation object design and development
 Design patterns reusable software solutions
 Model-driven engineering models and transformations
 Aspect-oriented software development perspectives
 Component-based development cbse, component-model

11. 1. Approaches supporting reuse at
implementation level
 Program libraries, APIs set of reusable artefacts
 Program generators code generators

12. [D] Reuse at design level
 Mix of design best practices
 Not granted by the (design or coding) language
 ... but: some paradigms may help in the job
 Objectorientation object design and development
 Design patterns reusable software solutions
 Model-driven engineering models and transformations
 Aspect-oriented software development perspectives
 Component-based development cbse, component-model

13. [D1] OO Principles orient to reuse
 Open/close principle
 Software entities should be open for extension but closed for modifications.
 Dependency inversion
 High-level modules should not depend on low-level modules. Both should
depend on abstractions.
 Abstractions should not depend on details. Details should depend on
abstractions.
 Interface segregation principle
 Clients should not be forced to depend on/ implement interfaces that they
don't use.
 Single responsibility – separation of concerns
 A class should have only one reason to change
 Substitution (Liskov)
 If a program is using a Base class, then the reference to the Base class can
be replaced with a Derived class without affecting the overall functionality

14. [D1] Object orientation for reuse
Encapsulation, modularization, and inheritance :
the main reuse features of OO that support reuse
Component
Package

15. [D1] Encapsulation
Encapsulation: to expose only enough of a module to
allow other modules to make use of it.
You can syntactically seal off implementation details,
leading to more flexibility and maintainability in your
system.
Every time an item is designed as private (restricted), it
encompasses potential for reuse and redefinition.
Every item of the system can change independently,
no impact to the other modules.

16. [D1] Modularization
Components, Interfaces, packages: basic mechanisms
that ONLY aim at modularization (and thus reuse)
 Components allow system to be assembled from binary
replaceable elements
 A component is physical – bits, not concepts (Iike classes)
 A component provides the realization of a set of interfaces.
 A component can be replaced by any other component(s)
that conforms to the interfaces.
 A component is part of a system.

17. Example
[D1] Modularization example
simulation.exe Render.java
IRender
LightModel
IModels ILighting
Environment

18. Example
[D1] Packaging example

19. [D1] Overriding and Overloading
Overriding, overloading, and polimorphism are the
concrete mechanisms for reuse based on inheritance
Overriding, or hiding, is when you have a method with
the same name and parameters as in a superclass
 the rest of the superclass is reused
Overloading is when you have multiple methods of the
same name, but with different parameter lists.
 the object is more likely to be reused

20. [D1] Polymorphism (Many Forms!)
Polymorphism is when objects of various types define a
common interface of operations for users.
 users can share usage, although at runtime instances
of different types can be bound
 Literally means many forms
 Can submit/use an instance of a subclass when a super
type is expected
 Reference and object can be different
 Arguments and return types can be polymorphic

21. [D1] What does OO bring you?
 Avoid duplicate code
 Define a common API (protocol or contract) for a group
of classes
 Change in one place
 Can override methods if more specific behavior is
needed
 Code doesn’t need changing when new sub
 You can extend and change behavior, even if you don't
have source code

22. Example
[D1] What’s going to get printed?
public class Animal {
public static void hide() {
System.out.format(“Hide animal."); } public static void main(…) {
public void override() { Cat myCat = new Cat();
System.out.format(“Override Animal."); } Animal myAnimal = myCat;
} //myAnimal.hide(); //Bad style!
Animal.hide(); //Better!
myAnimal.override();
}
public class Cat extends Animal { }
public static void hide() {
System.out.format(“Hide Cat."); }
public void override() {
System.out.format(“Override Cat."); }
}

23. Example
[D1] The answer
 The Cat class overrides the instance method in Animal called
override and hides the class method in Animal called hide
 For class methods, the runtime system invokes the method
defined in the compile-time type of the reference
 For instance methods, the runtime system invokes the method
defined in the runtime type of the reference
 The hide method in Animal.
 The override method in Cat.

24. [D2] Design patterns
 A design pattern is a reusable solution to a recurrent
problem
 Software design patterns are based (somehow) on work
by the architect Christopher Alexander
 A design pattern captures design expertise – not
created but abstracted from existing design examples
 Using design patterns is reuse of design expertise
 Design patterns provide a vocabulary for talking about
design

25. [D2] How patterns arise
Problem
Forces
Solution
Benefits Consequences
Related Patterns

26. [D2] Patterns vs. “design”
 Patterns are design
 But: patterns transcend the “identify classes
and associations” approach to design
 Instead: learn to recognize patterns in the
problem space and translate to the solution

27. Example
[D2] Composite pattern
 Construct part-whole hierarchy
 Simplify client interface to leaves/composites
 Easier to add new kinds of components
0..*
Client Component
Operation()
Add(Component)
Remove(Component)
children
Leaf Composite
Operation() Operation()
Add(Component)
Remove(Component)
For all c in children
c.Operation();

28. Example
[D2] Composite pattern
 Example: figures in a structured graphics toolkit
Controller
0..* 0..*
View Figure
children
paint()
translate()
getBounds()
LabelFigure BasicFigure CompositeFigure parent
paint() paint() paint()
addFigure(Figure)
removeFigure(Figure)
For all c in children
c.paint();

29. For your reference
Creational Design Patterns
Manage the way objects are created
 Singleton - Ensures that only one instance of a class is created and
Provides a global access point to the object.
 Factory(Simplified version of Factory Method) - Creates objects without
exposing the instantiation logic to the client and Refers to the newly
created object through a common interface.
 Factory Method - Defines an interface for creating objects, but let
subclasses to decide which class to instantiate and Refers to the newly
created object through a common interface.
 Abstract Factory - Offers the interface for creating a family of related
objects, without explicitly specifying their classes.
 Builder - Defines an instance for creating an object but letting
subclasses decide which class to instantiate and Allows a finer control
over the construction process.
 Prototype - Specify the kinds of objects to create using a prototypical
instance, and create new objects by copying this prototype.
 Object Pool - reuses and shares objects that are expensive to create..

30. For your reference
Structural Design Patterns
Define structures of objects and classes that can work together and
define how the relations can be defined between entities.
 Adapter - Convert the interface of a class into another interface clients
expect. Adapter lets classes work together, that could not otherwise
because of incompatible interfaces.
 Bridge - Compose objects into tree structures to represent part-whole
hierarchies.
 Composite - Compose objects into tree structures to represent part-
whole hierarchies. / Composite lets clients treat individual objects and
compositions of objects uniformly.
 Decorator - add additional responsibilities dynamically to an object.
 Flyweight - use sharing to support a large number of objects that have
part of their internal state in common where the other part of state can
vary.
 Memento - capture the internal state of an object without violating
encapsulation and thus providing a mean for restoring the object into
initial state when needed.
 Proxy - provide a “Placeholder” for an object to control references to it.
 Facade - unified interface to a complex system.

31. For your reference
Behavioural Design Patterns
Define the interactions and behaviours of classes
 Chain of Responsibiliy - It avoids attaching the sender of a request to
its receiver, giving this way other objects the possibility of handling the
request too. The objects become parts of a chain and the request is sent
from one object to another across the chain until one of the objects will
handle it.
 Command - Encapsulate a request in an object, Allows the
parameterization of clients with different requests and Allows saving the
requests in a queue.
 Interpreter - Given a language, define a representation for its grammar
along with an interpreter that uses the representation to interpret
sentences in the language / Map a domain to a language, the language
to a grammar, and the grammar to a hierarchical object-oriented design
 Iterator - Provide a way to access the elements of an aggregate object
sequentially without exposing its underlying representation.
 Mediator - Define an object that encapsulates how a set of objects
interact. Mediator promotes loose coupling by keeping objects from
referring to each other explicitly, and it lets you vary their interaction
independently.

32. For your reference
Behavioural Design Patterns
Define the interactions and behaviours of classes
 Observer - Define a one-to-many dependency between objects so that
when one object changes state, all its dependents are notified and
updated automatically.
 Strategy - Define a family of algorithms, encapsulate each one, and
make them interchangeable. Strategy lets the algorithm vary
independently from clients that use it.
 Template Method - Define the skeleton of an algorithm in an operation,
deferring some steps to subclasses / Template Method lets subclasses
redefine certain steps of an algorithm without letting them to change the
algorithm's structure.
 Visitor - Represents an operation to be performed on the elements of an
object structure / Visitor lets you define a new operation without changing
the classes of the elements on which it operates.
 Null Object - Provide an object as a surrogate for the lack of an object of
a given type. / The Null Object Pattern provides intelligent do nothing
behavior, hiding the details from its collaborators.

33. References
 Ian Sommerville. Software Engineering,
Addison Wesley
 Martin Fowler et al. Refactoring:
Improving the Design of Existing Code,
Addison Wesley
 Ivar Jacobson et al. Software Reuse:
Architecture, Process and Organization
for Business Success, Addison Wesley
 E. Gamma, R. Helm, R. Johnson, H.
Vlissides (“the gang of four”), Design
Patterns, Addison-Wesley

34. Further readings
 Diomidis Spinellis, Cracking Software
Reuse, IEEE Software, 2007
 David A. Wheeler, Free-Libre / Open
Source Software (FLOSS) is Commercial
Software, web, 2009
 Frakes, W.B. and Kyo Kang. Software
Reuse Research: Status and Future, IEEE
TSE, 2005
 ...