Object-oriented programming is a programming language paradigm or model. In this model, programs are organized around data, also known as objects. Java is unlike C, a Procedural Oriented Language (POL), in which the programs are organized around functions and logic.
An object is simply data that has attributes and behavior.
An example of an object is a human being. A human being has properties like name, date of birth, and gender.
Another example of an object is a button that you see on your computer. It has properties such width, height, background color, foreground color, and text.
In other words, objects — both in the real world and in the world of programming — are entities that have certain basic characteristics.
The basic characteristics of objects are as follows.
We will discuss each of them briefly in this section.
Every object has an identity, both in the real world and the programming world.
Imagine you own a Lamborghini Aventador and you parked it before you went shopping. When you returned after shopping, you find two Lamborghini Aventadors parked next to each other. It so happens both the cars have the same color and are identical. Obviously, you own one of them and somebody else owns the other Aventador.
How do you recognize your car? Of book, there are a number of ways you can identify your car. For one, you could take a peek at your number plate. Since a number is unique to every instance of car in your region, you can easily identify your car.
Similarly, in the programming world each instance of an object can be identified by its reference. A reference is usually an address where the instance resides in memory.
We mentioned the term instance a couple of times. So what exactly does it mean?
An instance is simply an existence or occurrence of an object.
The process where you create an instance of an object is known as instantiation.
For example, the Lamborghini you own is an instance of Lamborghini Aventador, the object. Similarly, the Lamborghini parked next to yours is yet another instance, where Lamborghini Aventador is again the object.
Think of it this way, an object is simply an idea, an instance is the idea brought to life, and the process of bringing the idea to life is instantiation.
The state determines the attributes of an object.
For example, a car can have the attributes such as speed, RPM, gear, fuel level, and engine temperature.
The type of an object determines what attributes the object has. Thus, all instances of a particular object have the same attributes. But there is no guarantee that all the instances have the same attributes.
When we say type, we actually mean a class. We will learn more about classes in the next chapter.
From the parking lot example, your Lamborghini may have a different fuel level than the Lamborghini parked next to yours.
The combination of the values assigned to all the attributes of an object is called the object’s state. Unlike an objects identity, the state can change over its lifetime.
For example, the engine temperature of your Lamborghini may heat up and cool down during its lifetime. Similarly, the fuel level can increase and decrease.
The behavior of an object is simply the capabilities of an object. Like state, the behavior of an object depends on its type.
Unlike state, behavior is not different for each instance of an object.
For example, a car is capable of moving, which is a behavior. All cars are capable of moving. Which goes to say, behavior is not different from instance to instance.