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Principles of object oriented

systems

Software Engineering Department

8/8/2019 Lecturer 2


Software Engineering and Databases

y Software engineering is a profession and field of study

dedicated to designing, implementing, and modifying

software

y The term software engineering first appeared in the 1968

NATO Software Engineering Conference, and was meant

to provoke thought regarding the perceived "software

crisis" at the time.

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y Since the field is still relatively young compared to its

sister fields of engineering, there is still much debate

around what software engineering actually is, and if it

conforms to the classical definition of engineering.

y The IEEE Computer Society's Software Engineering Body

of Knowledge defines "software engineering" as the

application of a systematic, disciplined, quantifiable

approach to the development, operation, andmaintenance of software, and the study of these

approaches; that is, the application of engineering to

software.

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y One of the most important techniques reducing complexity

of software syetms is abstraction.

y An abstraction denotes the essential characteristics of an

object that distinguish it from all other kinds of objects andthus provide crisply defined conceptual boundaries, relative to

the perspective of the viewer.

y Exist in any design proceses.

y Allows developers to modularize program easily andnaturally, speeding up designing large software projects.

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y Database systems are often large software systems

with the following features:

y A large data stored in the exteral memory and need to be organized in a way that gurrantee

updating effectivelly

y Data entities have complex inter-relationships

y Data needs to be shared between users and the

integrity must be maintained

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Software Engineering and

Databases

y Providing appropriate tools for processing effectively

data, especially with large databases, is a difficult task

of conventional programming languages.

y However, representing complex data objects can be

done with some modern languages.

y This allows us defining abstract data objects and

equivalent methods => important for database

applications.

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y Commercial database systems usually provide

simple data types

y => affects designer and programmers.

y This requires new capabilities for creating complex

data types

y => forming object-oriented databases.

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Databases

y The difference between OOB and conventional

databases is that

y it focuses on defining complex data types and data

abstraction.

y Certainly, it can be benefited from results of research in

OOP.

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Databases

y Objects in OOB and OOP are both used for data

encasulation

y

Differences:y Objects in OOB are persistent (even the loop of creating it

is completed ) as oppose to objects in OOP.

y OOB supports versioning. This allows accessing different

versions of the same objects, i.e databases recording

yOOB can be interleaved with conditions from theapplications.

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Cycle of databases

y Requirement analysis

y Data modelling and software design

y Execution

y Testing

y Maintenance

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Notes on software design

y Three aspects of software design:

y Reliability : The software follows the all

specifications and operate well in both normallas well as abnormal conditions.

y Ex tendability : Can be compatible with some

changes of specifications and database.

yReusability : Modules can be reused in otherapplications.

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Testing

y Quality of a database system is measured based on

corectness and reliablity.

y

Goals of testing:y Detect errors in periods of modelling and execution.

y Determine if the system satisfies the user needs?.

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Maintenance

y Practically it is not possible to carry a complete testing

for a large database system. Further, some errors might

happen during later use.

y Tasks:

y Repair corruptions during the usage.

y Implement change sof systems responding to the new

requirements.

y Implement schames to improve effectiveness and user interface.

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Object Oriented Systems

y The term object-oriented seems to be the buzzword of

the 1980s and 1990s.

y

Everybody is using it, but with such a wide range ofdifferent meanings that no one seems to know exactly

what the other is saying.

y It appears in several disciplines, including programming

languages, databases and knowledge bases, artificial

intelligence and computer systems in general.

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Object-Orientation - What Does

it Mean?

y Object-orientation can be seen as a kind of technique of

organizing a system in terms of objects.

y The foundation of the whole approach is encapsulation. This

means that the properties of data and the procedures for themanipulation of this data are combined into an object.

y Encapsulation restricts the effects of change by placing a wall

of code around pieces of data.

yThe objects communicate through messages which invoke theobjects procedures, usually called methods.

y Hence, manipulation of the data is provided by those

methods only

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Features of OO Systems

y object,

y type,

y inheritance,

y abstraction,

y concurrency, and

y persistence

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Object and Abstraction

y The dimensions object and abstraction reflect the basic

principle of encapsulation, which can be found in

conventional languages like Modula-2 or Ada.

y An abstraction denotes the essential characteristics of an

object that distinguish it from all other kinds of objects

and thus provide crisply defined conceptual boundaries,

relative to the perspective of the viewer.

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Object and Abstraction

y Entity abstraction An object that represents a usefulmodel of a problem domain or solution domain entity

y Action abstraction An object that provides a generalized

set of operations, all of which perform the same kind offunction

y Virtual machine abstraction An object that groupsoperations that are all used by some superior level ofcontrol, or operations that all use some junior-level setof operations

y Coincidental abstraction An object that packages a setof operations that have no relation to each other

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Example: defining a class

Class Fruit{

int grams;

int cals_per_gram;

int total _calories() {

return( grams*cals_per_gram);

}

}

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Inheritance

y Inheritance is the more innovative part of the approach,

because it is not provided by these conventional

programming languages.

y It is a tool for (automatically) broadcasting definitions

of attributes and methods from already defined pieces

of data and operations to pieces developed separately

and thus supports reusability.

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Inheritance Example

y Class C is a subclass of class B (itssuperclass) if its declaration has the formclass C extends B {

«

}

y The subclass is a specialization of thesuperclass

y The superclass is a generalization of thesubclass

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Concurrency

y The dimension of concurrency introduces the term

process.

y A process-based language extends the autonomy of objects

to the autonomy in time,y that is, operations may be executed concurrently or

separately depending on the systems need or even started

by these needs (triggers, daemons)

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Concurrency

y Concurrency focuses on process abstraction andsynchronization. The object is a concept that unifies thesetwo different viewpoints:

y Each object (drawn from an abstraction of the real world)

may represent a separate thread of control (a processabstraction). Such objects are called active.

y In a system based on an object-oriented design, we canconceptualize the world as consisting of a set of cooperativeobjects, some of which are active and thus serve as centers

of independent activity. Given this conception, we defineconcurrency as follows:

y Concurrency is the property that distinguishes an activeobject from one that is not active.

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Persistency

y Adding the dimension of persistency reflects the fact,

that there is a trend toward extending object-oriented

programming languages in the direction of databases,

y This occurs in parallel with the attempt of extending

database systems with object-oriented ideas.

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Persistency

y Object oriented languages and object-oriented

databases are natural complements of one another.

y The first one emphasizes processing, complex structuring

and local data,y the second one focuses on a more declarative approach,

consistency, persistency, sharing, and support for very large

amounts of data.

y The merger of these activities could be an object-

oriented database programming language which

combines the aspects of a programming language and a

database model.

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Polymophism

y Further essential object-oriented concepts are object-identity

and polymorphism.

y The first one means that an object exists independent of the

value it represents. This allows objects to share subobjects

and to be cyclic. Furthermore, such objects can be updated

without loosing their identity.

y The latter one provides the overloading of methods such that

the same message elicits (semantically similar) responses

depending on the class of the receiver object.

y Note, that this is facilitated by the message-passing paradigm

as a message specifies only a method identifier for an object

and not the code to be executed.

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Principles

y Encapsulation of data and methods:

y Encapsulation provides data abstraction.

y The state of an object is accessible only through its methods. Thestate of the object is generally represented by variables which arehidden from the clients.

y The implementation of the methods are not visible outside of anobject. A method is a piece of code that implements an operation onan object state. The set of methods defined for an object determinethe behavior of the object.

y Objects communicate through message passing.

y Methods and message passing:

y A message consists of an addressee, a method selector, and possiblyseveral actual parameters. In response to a message, the receiverinvokes the method identified by the selector specified within themessage.

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Principles

y Classification and instantiation:

y Objects are collected into classes. A class determines thestructural and behavioral properties of its instances. Thestructural properties are specified by a set of variables, the

behavioral properties by a set of methods.

y Class taxonomy and inheritance:

y A class is interpreted as a template for the creation of theinstances of the class, i.e., it defines both the type of theseinstances, and the way these instances are created as

reflections of real world 'things'. Classes are organized in alattice. A class may inherit operations from "superclasses" andmay have its operations inherited by "subclasses". Inheritanceis defined narrowly as a mechanism for resource sharing inclass hierarchies.

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Advantages

y E xtensibility, f lexibility, and changeability:

y Most DBMSs provide a fixed set of data types (e.g., relationalDBMSs provide relations and field types).

y An object-oriented system provides (1) predefined classes/types, (2) mechanisms to define new classes/types, and (3) auniform way of treating system and user definedclasses/types.

y Hence, new object classes/types including their methods canbe incorporated into the system.y Changes of object types/ classes and their methods are localized

to the relevant types/classes and are much easier controlled thanin conventional record-based systems, where many record typesmay be affected by a change.

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Advantages

y Consistent interpretation of data:

y Because of encapsulation, the behavior associated with an object type ispredetermined by a fixed set of operations.

y

Therefore, the database operations are constrained to be within thespecifications of these operations. This induces a more consistent interpretationof data by application programs.

y Modeling power:

y Inheritance of both variables and methods is a very powerful tool for datamodeling.

y It is a kind of modularization and supports the specification of reusable typedefinitions.

y In general, the modeling principles of generalization/specialization,identification, and aggregation are well supported in some object-oriented datamodels.

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The motivation for object-

oriented data models

y Impedance mismatch:

y Applications using conventional database technology suffer

under the need of two different languages, a database

language (e.g., SQL) and a host language (e.g., COBOL, PL/1).

y Since the type systems of these languages, different data

must be converted from one language environment to the

other one, and often type checking is lost.

y Different processing strategies require concepts like a cursor

to scan relations. In contrast to the host language the

database language is incomplete with respect to computationand recursion.

y Furthermore, the database language usually is a declarative

one, whereas the host language is procedural.

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The motivation for object-

oriented data models

y New applications require non-standard and complex

objects and operations:

y The conventional database technology does not satisfy

these special requirements e.g., in the field of AI,CAD/CAM, Engineering, CASE, or (multimedia) office

automation, although database technology plays a crucial

role in all these fields.

y New modelling techniques, the handling of complex

structures and multimedia data are required as well as

extensibility and flexibility, next to a high degree of

modularity and reusability for building these complex

applications.

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Extended concepts

y Object-oriented programming is a method of implementationin which programs are organized as cooperative collections ofobjects, each of which represents an instance of some class,and whose classes are all members of a hierarchy of classes

united via inheritance relationships.

y Object-oriented design is a method of design encompassingthe process of object oriented decomposition and a notationfor depicting both logical and physical as well as static anddynamic models of the system under design.

y Object-oriented design is a method of design encompassingthe process of objectoriented decomposition and a notationfor depicting both logical and physical as well as static anddynamic models of the system under design.

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The End

lecturer 2

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