OOPS

What is the concept of abstract classes?

raj — Tue, 20 Mar 2007 08:23:11 +0530

When one is building a class hierarchy, it is common to design some classes that will never have any instances, and are intended to be used onlyby subclasses. For example, the Animal class in the animal hierarchy is such a class. There will never be an instance of an Animal. Instead, there would be more specialized subclasses of Animal such as Horse or Snake that would have instances. Classes that have no instances are called abstract classes.

What is object lifetime?

raj — Tue, 20 Mar 2007 05:38:06 +0530

Objects are dynamic entities. They are created as required during program execution. When an object is created, it is instantiated, or bound to the class definition. An instantiated member of a class is called an object or an instance. When a new object first comes into existence, a special method called the constructor is automatically invoked by the run-time system. The constructor is responsible for giving an instance its initial state. Once an object has been created, it can receive and send messages.

Explain communication via messages in object oriented world?

raj — Tue, 20 Mar 2007 05:32:44 +0530

Messages are how objects communicate with one another. Any object may send a message to other objects, and it may receive messages from other objects. In practical programming terms, sending a message is accomplished by invoking or calling some class method, and receiving a message is accomplished by having a class method called by a different object.

What is abstraction of objects in object oriented terminology?

raj — Tue, 20 Mar 2007 05:26:59 +0530

Abstraction is one of the important aspect of all programming, not just object oriented programming. Abstraction serves as a way model the program objects with the real world problem. For example, if you were trying to write an address book program, you would use abstractions such as names, addresses, phone numbers, alphabetical order, and other information associated with an address book. You would also define operations for manipulating the attributes such as adding a new name or changing an address. Abstraction is thus modeling the real world in terms that can be implemented as a computer program and can be represented as a class and objects.

What is polymorphism?

raj — Tue, 20 Mar 2007 05:19:25 +0530

Polymorphism is the fundamental characteristic of object oriented 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. The specialized class will often implement a behavior that is somewhat different than the generalized class, but the name used to define the behavior will be the same. It is important that a given instance of an object use the correct behavior, and the property of polymorphism allows this to happen automatically and seamlessly.

What are class hierarchies?

raj — Tue, 20 Mar 2007 04:09:33 +0530

In an object orienetd design, classes of objects are arranged into hierarchies that model and describe relationships among the classes. The simplest relationship is an association. For example, there could be an employment association between a student and a school.

What are encapsulated classes?

raj — Tue, 20 Mar 2007 04:03:51 +0530

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

What is an Object Oriented System?

raj — Tue, 20 Mar 2007 03:57:12 +0530

A way to define an object oriented system is to use a list of properties that characterize object oriented systems in the form of a class. A class is a description of a set of objects. The set of objects share common attributes and common behavior. 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.