Basic Concepts of Object Orientation

What is an Object?

An Object represents an entity either physical (box), conceptual (chemical process), or software (list).

An Object is a concept, an abstraction, a thing with sharp boundaries and meaning for an application. It has

An Object is represented as a rectangle with a underlined name in UML.

What is a Class?

A Class is a description of a group of objects with common properties (attributes), behavior (operations), relationships, and semantics

A class is an abstraction. An object is an instance of a class.

Example of a Class

A class is represented by a compartmentalized rectangle in UML. It has three sections - Name, Attributes, and Operations. You can show as many or as few of the Attributes and Operations in the diagram. Most of the times for the sake of clarity the Attribute and Operation lists are suppressed.

You start from real world objects - abstract out what you do not care and go through the process of classification of what you care. A Class is the result of this classification. Classes are then used as templates within a software system to create software objects.

What is an Attribute?

An Attribute is a named property of a class. It has a type. It describes the range of values that that property may hold.

What is an Operation (Method)?

An Operation is a service that can be requested from any object of the Class to affect behavior. An Operation can either be a command or a question. A question should never change the state of the object only a command can. The outcome of the Operation depends on the current state of the object.

What is an Interface?

An Interface is a collection of operations that are used to specify a service provided by a class or component. It represents a contract with the user. In UML an Interface is represented either by a "lollipop" or by a rectangle with the word "interface" above the name of the Interface.

What is Polymorphism?

The Greek term polymorphous means "having many forms". Interfaces allow us to define polymorphism in a declarative way. Two elements (classes) are polymorphic with respect to a set of behaviors if they realize the same Interface. Interfaces allow us to build a "plug-and-play" architecture. Classes that realize the same Interface may be substituted for one another in the system, thereby changing the implementation without affecting the User.

What is a Component?

A component is a physical and replaceable part of a system that conforms to and provides the realization of a set of interfaces. Software components include: Source code components (.java files, data files), Binary code components (Java Beans, COM objects, DLLS), Executable components (.exe's).

A Package is a general purpose mechanism for organizing elements into semantically related groups. A Package owns its elements. An element cannot be owned by more than one package. In UML a Package is represented as a tabbed folder.

A Subsystem is a combination of a package and a class. It realizes one or more interfaces which define its behavior.

What are the various Relationships?

Dependency is a semantic relationship between two things in which a change to one thing (independent thing) may affect the semantics of the other (dependent thing). This is a non-structural, "uses" type relationship as in Class Window uses Class Event. A change in Event (say a mouse click or key stroke) causes a change in Window but not vice versa. In UML Dependency relationship is denoted by a dashed line.

Association is a structural relationship between objects. The association may have a name. Professor "works for" University. Each end of the association has a role - Professor (employee) and University (employer). An Association is represented by a solid line in UML.

Multiplicity defines how many objects participate in a relationship. For each role you can specify the multiplicity of its class - how many objects of the class can be associated with an object of the other class.

Aggregation is a relationship between a whole and its parts. An object physically contains other objects - Car is physically composed of an Engine and four Wheels. A Family is a collection of Parents and Children. It is represented in UML as a solid line with an open diamond.

Composition is a form of aggregation with strong ownership and coincident lifetimes of the part with the aggregate. The whole "owns" the part and is responsible for the creation and destruction of the part. Student and schedule. It is represented as a solid line with a filled diamond in UML.

Generalization is a relationship among classes where one class shares the structure and behavior of one or more classes. Generalization defines a hierarchy of abstractions in which a subclass inherits from one or more superclass. Generalization is an "is-a-kind of" relationship.

A subclass inherits its parent's attributes, operations, and relationships. A subclass may add additional attributes, operations, relationships. It may also redefine inherited operations (use caution here!). This relationship is represented as an open arrow going from the subclass to its parent(s) in UML.

A subclass may be used anywhere the superclass is used, but not vice versa. Generalization is the name of the relationship. Inheritance is the mechanism that the generalization relationship represents.

Realization is the relationship between one class that serves as the contract that the other class agrees to carry out. (The interface and the class that implements the interface). It is represented as a dashed line and an arrow in UML.

Strengths of Object Orientation

What is UML?

Unified Modeling Language is a language for specifying, visualizing, constructing, and documenting a software intensive system.

What is a Software Process?

A software process defines WHO is doing WHAT, WHEN and HOW to reach a certain goal which could be to build a new software product or enhance an existing one. The Rational Unified Process (RUP) is a generic process that uses UML as a modeling language. An actor is someone or something outside the system that interacts with the system. A use-case is a sequence of actions a system performs that yields an observable result of value to a particular actor.

What are the different views of Software Architecture?

What are the Lifecycle Phase in the development of Software?

Within each phase there is a series of iterations. Each iteration results in a release encompassing larger and larger subsets of the final application

In our development process for this class we will take the following path

Who is an actor?

An actor is someone or something outside the system that interacts with the system. An actor can be a human being or another system or a device. An actor is represented by a stick figure in UML.

What is a Use-Case?

A Use-Case is a sequence of actions a system performs that yields an observable result of value to a particular actor. A Use-Case describes functionality of the system from the user's point of view. A Use-Case is represented by an oval in UML.