An object is one of the first things that an infant learns to recognize. The multitude of 'things' that hover above its crib, that smile and talk to it, that pat its head and pinch its cheeks are people. Sometime later, it learns to recognize other such things - the circular thing with a dial on one wall of the house is a clock, the longish things neatly arranged on the shelves are books, and that weird contraption resting on the table and making a noise is the tape recorder. Mind you, the infant does not know that these things are so named, but it learns to recognize them much in the same way that it learns to recognize its mother and father.
As the infant grows older, he/she learns to recognize an object as not only having a definite, distinct boundary, but also as something that has a unique identity, quite apart from the other objects surrounding it.
A simple definition of an object would be a tangible entity that exhibits some well defined behavior'.
Take a tennis ball, for example:
It is a tangible, visible entity that has a visible boundary.
- It has a defined purpose.
- It can be acted upon (you can hit it with a racket and send it flying).
Let us not limit our definition of an object to merely something that can be seen, held and touched, such as a tennis ball, a light bulb, or an automobile. For the purpose of software development, the concept of an object needs refinement. Take the example of the 'Acme Nuts and Bolts Company'. An organization, you will argue, does not have a visible boundary, like a tennis ball or a tape recorder. True, but while it does not possess a physical boundary, it does have a conceptual boundary. Like all organizations, it has a specific defined purpose, and one can direct a specific action towards it (just ask the customers and shareholders). Thus, by our definition, the 'Acme Nuts and Bolts Company' is an object.
According to Grady Booch, a leading exponent of the object-oriented approach, an object has the following characteristics:
- It has a state.
- It may display a behavior.
- It has a unique identity.
The state of an object is indicated by a set of attributes and their values.
The behavior refers to the change of these attributes over a period of time.
What is identity! Each object has a unique identity, just as each person has a unique identity. Thus two tennis balls may have the same color, be made of the same material, have the same weight, the same circumference and display the same behavior, but will still have distinct identities.
Look at the world around you. It is full of objects. Objects of different shapes, sizes and colors; objects stationary and mobile; objects that exist in this world for different purposes. If we were to deal with this vast number and variety of objects simultaneously, we would be at a loss. We can only do what zoologists do. The earth is inhabited by millions of animals. A zoologist cannot do any meaningful study of these animals without organizing them in a logical manner. Thus, the zoologists have classified animals into their kingdom, genus, family and species. Similarly, the millions of objects that we see in this world can be classified based on common attributes that they possess.
Let us take a simple example from the animal world. The peacock, the sparrow, and the kingfisher are all called Birds. Why? All of them share some common characteristics. All three have properties that are common to the family of birds. For example, all of them lay eggs and hatch their young ones; are covered with feathers, have hollow bone structures and have the ability to fly. Thus, we may say that the peacock, the sparrow and the kingfisher all share structural and behavioral similarities and belong to the class called Birds.
From the above example, we may define a class as 'a set of objects that share common attributes and behaviors'.
We started by saying that the world around us is full of objects. Making a conscious attempt to identify the objects around you and classifying them into classes can be fun.
The object-oriented approach on the other hand, views a system as a collection of real world objects. An object represents an entity that has an identity, and displays a number of properties and behaviors. A description of common properties and behaviors for a similar set of objects constitutes a class. A system can contain any number of objects and classes. Object oriented systems can be upgraded easily to reflect the change and growth of the system.
Present day software development takes an object-oriented approach as against the procedural approach. The object-oriented approach has added value in building stable software systems as it can include problem domains of varying sizes and complexities.
Most of the modern languages and operating systems use and support object-orientation. For example, popular languages like C++, Visual C++ and Java support object-orientation.
