Summary Bab II

The Essence of Objects

Object orientation is the culmination of years of experience in finding an effective way to develop software, and is certainly the dominant method used to develop major software systems today. Object orientation is a technique of modeling a real-world system in software based on objects. The object is the core concept.

The attributes, or data, used inside an object are really only a tiny part of what an object is and does. An object also has a set of responsibilities that it carries out by providing services to other objects. It is often more useful to think of an object in terms of its responsibilities rather than its attributes. For example, it is the responsibility of a sensor object to keep track of the state of the sensor. A sensor object might respond to requests from other objects that use sensors to check the status of a sensor, to turn a sensor on or off, or to report on the sensor's values.

While a program is running, individual objects usually don't stand alone. They belong to a collection of other similar objects that all are members of the same group, or class. A program will be made up of many different classes, each class made up of similar objects.

·        class A class is a description of a set of objects. The set of objects share common attributes and common behavior. Class is similar in concept to abstract data types found in non- OO programming languages, but is more comprehensive in that it includes both structure and behavior. A class definition describes all the attributes of member objects of that class, as well as the class methods that implement the behavior of member objects.

·        object The basic unit of object orientation. An object is an entity that has attributes, behavior, and identity. Objects are members of a class, and the attributes and behavior of an object are defined by the class definition.

What Is an Object-Oriented System?

Any object-oriented software system will have the following properties:

1. ·Abstraction with objects
2.  Encapsulated classes
3.  Communication via messages
4.  Object lifetime
5.  Class hierarchies
6.  Polymorphism

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·        object orientation A method of developing software that uses abstraction with objects, encapsulated classes, communication via messages, object lifetime, class hierarchies, and polymorphism.

Fundamental Properties of an Object-Oriented System

Abstraction with objects

An abstraction is a mechanism that allows a complex, real world situation to be represented using a simplified model.abstraction A model of a real-world object or concept.

Encapsulated classes

encapsulation The process of hiding all the internal details of an object from the outside world. In Java, encapsulation is enforced by having the definitions for attributes and methods inside a class definition.

Interaction via messages

In order to accomplish useful tasks, objects need to interact with other objects. The interaction can be between objects of the same class, or objects of different classes. This interaction is handled by sending messages (in Java, this is done by calling methods) to other objects to pass information or request action.

Object lifetime

All objects have a lifetime. Many objects that are instances of the same class can exist at any given time. Each object has a unique identity, and will have attributes that are different from other instances of objects in the same class.

Class hierarchies

An ordering of classes. The most common OO hierarchies are inheritance and aggregation.In an OO design, classes of objects are arranged into hierarchies that model and describe relationships among the classes. Hierarchies can also be used to build the definitions of individual classes. This kind of hierarchy is called inheritance.

Polymorphism

Polymorphism is the final fundamental characteristic of objectoriented systems. When inheritance is used to extend a generalized class to a more specialized class, it will usually include extending some of the behaviors of the generalized class.

Abstraction with Objects

Abstraction is one of the basic tools of all programming, not just OO

programming. When trying to write a program to solve some real world problem, abstraction serves as a way model the real world problem. It can be a real-world thing or concept, or an abstraction of a thing or concept, expressed as

a software representation.

Encapsulated Classes

Encapsulation is one of the most important aspects of OO. It is what allows each object to be independent. The class definition is the main programming tool that is used to implement encapsulation. A class is a description of a collection of with common attributes, behavior, and responsibilities.

·        attribute Used to hold state information of an object. An attribute might be as simple as an on or off boolean variable, or it might be a complex structure such as another object. A class definition describes the attributes and operations (methods) of a class.

·        behavior The activity of an object that is visible to the outside world. Includes how an object responds to messages by changing its internal state or returning state information to other objects.

·        method An operation or service performed upon an object, defined as part of the declaration of a class. Methods are used to implement object behavior. Synonyms for method include member function, operation, and service.

·        state State reflects the current values of all the attributes of a given object, and is the result of the behavior of an object over time.