Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 1 Object-Oriented Analysis and Design - CDT309 Period 4, Spring 2008 Object-oriented concepts

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 2 OOP basic concepts With abstraction, we identify essential features of a system Encapsulation is the packing of information with objects Objects are entities containing data and procedures The methods describe the dynamic behaviour of objects, representing the service they provide Message: a request to execute a method A class is a template for creating objects An instance is an object that belongs to a class With inheritance, we can reuse class specifications A class hierarchy is tree structure showing inheritance relations With polymorphism, we can hide different implementations of a method Independence: an object should be responsible for itself Interfaces describe how users interact with a class

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 3 Abstraction Ability to view ”real world” problems with varying degrees of detail

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 4 Encapsulation The packaging of several items into one single unit + =

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 5 Encapsulation Objects carry with themselves everything they need to perform their function (data and methods in the same entity) Makes it possible to hide information from the outside world

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 6 What is an object? Our commonsense concept of an entity with meaning. For example: –Physical entity: a car –Abstract entity: an angel –Conceptual entity: blueprint for an airplane –Software entity: an array

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 7 Common properties in objects Boundaries: the object is well defined State: how is it now Behaviour: the object can change state Identity: it has a name

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 8 Objects and classes We group objects into classes For example, the classes of airplanes, dogs, people, cars, busses … The objects of a class have common: –properties: all planes have wings, windows, shape… –behaviour: airplanes can fly, turn, land,… –semantics: press the right button to takeoff The buss (object) you took yesterday is an instance of the class of busses

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 9 Class: example The class of all courses offered by IDE Attritubes or properties (what kind of information has the object?): name, code, period, shedule,… Operations or behaviour (what can we do with a course?): (un)register a student, assign a lecturer, check if it is full,… What is the semantics?

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 10 Sections of a class A class has 3 sections: Course Name Code … Register() Is_full() Lecturer() … First section: the class name Second session: the class attributes Third section: the behaviour of the class (operations or metods)

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 11 Relationships between objects Association (aggregation and composition) Dependency Generalization (inheritance)

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 12 Association Connection among classes with structural dependence Course Name Code … … University Name Code … … Is given by Association Association name

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 13 Association and multiplicity Course Name Code … … University Name Code … … is 1 given by multiplicity Student Name Code … … 1 is 1..5 reading

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 14 Association and multiplicity Unspecified Exactly one * 0..* 1 * 2, 4..6 One or more Zero or one Specified range Multiple, disjoint ranges Zero or more (unlimited) }

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 15 Association types Aggregation Composition

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 16 Dependency relation Connection among classes without structural dependence A client depends on the delevery boy to receive the pizza: but the delivery boy can be changed, and this should not make any difference for the client.

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 17 Generalization Hierarchy of abstractions (single and multiple inheritance) ”is-a” relationship A penguin is a bird

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 18 Single inheritance Inheritance from one class Superclass Subclasses Generalization relationship

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 19 Multiple inheritance Inheritance from more than one class Superclasses Subclass Generalization relationship

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 20 What is inherited? A subclass inherits the attibutes, methods and relationships from the superclass And can also –Add additional attibutes, operations, relationships –Redefine inherited methods

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 21 Cohesion Tightly coupled systems –Missuse or orveruse of composition –Difficult to change

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 22 Cohesion Loosely coupled systems –Missuse or orveruse of inheritance –Difficult to use

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 23 Problem You are requested to build a system to help bird researchers to keep track of their birds and observations. What is the most general class?

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 24 Inheritance Factor out commonality go to from the general to the specific

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 25 Inheritance Generalization-specialization concept: penguins don’t fly! Possible solution: specilize the class into two

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 26 Inheritance and encapsulation Inheritance can weaken encapsulation: if you change the superclass, the change is reflected in the hierarchy. What’s the problem here?

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 27 Inheritance and encapsulation Subtleties in the ”is-a” relationship: a window in a GUI is much more than a rectangle

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 28 Polymorphism Different behaviour for the same message Overloading

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 29 Composition Objects can include other objects: a car has an engine, and the engine is itself a separate entity or object

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 30 Aggregation and association Aggregation: a car has an engine Association: a computer has a keyboard Can you explain the difference in the relationships in the following diagram?

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 31 Dependencies How can we avoid making objects highly dependent when using composition? Mixing domains increase dependency, but can be useful: for example, VCR and DVD player in the same unit

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 32 Cardinality of composition The number of objetcs that participate in a composition: –Which objects collaborate with which objects? –How many objects participate in each collaboration? –Is the collaboration optional or mandatory?

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 33 More relationships Constructed-with: a stack is constructed with a list Knows-of: a browser knows of which platform it is executing

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 34 Cardinality of composition Example: the department offers several courses. Each course must have at least 1 lecturer, and a lecturer can have zero or more children. A course have at most 30 students and at least 10. Draw a class hierarchy for this example How can you represent the cardinality in an implementation language?

Marcelo Santos – OOAD-CDT309, Spring 2008, IDE-MdH 35 Someone was developing an system and came up with the diagram bellow. Explain its meaning based on your knowledge: