The Object Model

• Every system is made of smaller sub-systems or components

  • System working = components working with each other

• A component

  • Is a logical grouping of objects

  • Has a well-defined purpose and functionality

  • Completes a task by using its own functionality, or delegating to other components

• An object

  • Is the basic “functional unit” in the object model

  • Has a singular, well-defined purpose

  • Communicates with other objects by passing messages (i.e. calling methods)

The Object Model outside software

The Object Model outside software

How are objects defined?

• Each object has a specific purpose

  • Maintain customer records

  • Validate a credit card

  • “Behave like a stack”

• Each object is created so that it can be used by other objects in a specific way

  • Object A assumes object B can handle a particular task

  • Objects of the same “type” are from the same class

• Class=data type, Object=actual thing

The object model

• Each object has

  • State

  • Behavior

• State of an object

  • Each object has some data associated with it

    • Members or variables of the object

  • Values of that data together decide the “state” of the object

• Behavior of the object

  • Each object has capabilities to operate on its or other data

    • Operations are in the form of methods or functions

  • “Behavior” changes “state”

The object model

• Creating an object

  • Declare a variable of the class type

  • Instantiate the class, and store the result in that variable

  • Often the two steps are in one line...in C++, not in Java

• Every object gets its own copy of fields, and shares access to its class's methods

  • (Except static objects)

• Type of an object

  • The class whose constructor created this object

  • (inheritance makes this somewhat complicated)

Object-oriented Methodology:
Abstraction

• Objects expose only those details that are necessary for other objects to know

  • Make themselves appear simpler than they really are

• Why?

  • To adhere tightly to the expectations from an object

    • If A assumes B can perform a task, B exposes just enough functionality for A to use, no more

  • To hide its own complexity

• Interface of an object

  • Its “public” face

  • “Interface abstracts implementation”

Object-oriented Methodology:
Abstraction

Object-oriented Methodology:
Abstraction

Object-oriented Methodology:
Abstraction in Design

• Design the interface for each object first

• Design one object in detail using only interfaces of the others

  • Details of only one object, interfaces of others

• Separate the “what” from the “how”

  • “What”: what can an object do? (the interface)

  • “How”: how does the object do it? (details)

  • “What” is used in design: Just care about what other objects do, not how they do it

Object-oriented Methodology:
Standardizing an Abstraction

• Standardize method names, parameters and output

• Java: abstract class and interfaces

  • Abstract class: One or more methods are abstract

    • abstract class X

    • abstract public void methodname(…);

  • Interfaces: merely the method signatures

    • interface X

Object-oriented Methodology
Abstraction and Hierarchy

Object-oriented Methodology
Encapsulation

• Combining everything necessary for a cohesive entity (data+operations) into a class

• Hiding all the (bitter) details within a capsule

• Classes that represent self-complete entities = good encapsulation

Object-oriented Methodology
Information Hiding

• Hiding all the details that are not necessary to use the object

  • Other objects

• Good information hiding: expose only what others can use

  • Information hiding creates the abstraction

• Information hiding in Java

  • Access modifiers: private, protected, public, default

  • Rule of thumb: make all variables private, only some methods public

  • Accessors and modifiers

• Interface

  • The set of methods and variables of an object that another object can see and use

Object-oriented Methodology
Cohesion

• Objects that represent an entity should contain all the functionality within it for that entity (and nothing else)

  • Good encapsulation

• Bad/Low cohesion: classes that don't completely represent one entity, classes with multiple purposes

  • Classes with no self-sufficient role

    • E.g. classes with methods that change external data only

  • Classes with multiple purposes/roles

    • E.g. class that mixes operations and I/O

Object-oriented Methodology
Decoupling

• Objects should use each other only when needed

  • Bad/Low decoupling: State of one object being manipulated directly within several objects

• Effects of Low cohesion and decoupling

  • Compromises functional separation of various objects

  • Code and structure cluttered, complicates testing, maintenance

Characteristics of a Well-Designed Object-Oriented System

• Objects/components with clear, singular purpose

• Objects/components with well-defined interactions

• Good encapsulation

• Effective use of abstraction through information hiding

• High cohesion, high decoupling

How does all of this improve our lives?

Advantages of the Object Model

• Division of the system into well-defined components

• Well-defined interaction within the system

• Easier code reuse

• Easier system integration

• Applicable to all types of sub-systems

Advantages of the Object Model:
Division into components

• Semantic division into components (objects)

  • Division into procedures is more awkward

  • Good if cohesion/decoupling is high

• Independent development and testing

  • Each object is self-sufficient and purposeful

  • Each component can be independently stabilized and made robust

  • If object boundaries are well-defined

• Resilient to later changes

  • Future upgrades change only some classes

  • Changes are modular and isolated

  • If cohesion/decoupling is high

Advantages of the Object Model:
Well-defined interactions

• Objects interact with each other by message passing

  • Calling each other methods

  • Method names, signatures decided during design

    • Enforcing this in "structured" design is difficult

• Good design disciplines interactions

  • Well-defined message passing

  • Effect of change is deterministic and finite

    • If decoupling is high

• Interaction and cohesion: a balance

Advantages of the Object Model:
Code re-use

• Using readymade code is always preferable

  • Less time and effort than re-inventing the wheel

  • Less testing, higher reliability

    • Provided code is from a reliable source

• The basis for all readymade packages and libraries

• Object re-use more convenient than procedural re-use

  • Objects well-defined components

  • Pick-and-choose re-use possible because of overriding

  • Well-defined interface, more amenable for reuse

Advantages of the Object Model:
Easier System Integration

• Early dependency detection

  • During analysis and design

• Concurrency and version control easier

  • Versions of objects instead of files

• Integration testing is separable and delayed

  • Using “placeholders” is easier to delay integration testing

  • Known message passing interfaces from design

• Integration process is smoother

  • Well-fitting, “pre-conceptualized” parts

Advantages of the Object Model:
Applicable to all types of subsystems

• User interface design

• Data access design

• Processing workhorses