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What Is an Object?

As the name object-oriented implies, objects are key to understanding object-oriented technology. You can look around you now and see many examples of real-world objects: your dog, your desk, your television set, your bicycle.

These real-world objects share two characteristics: they all have state and they all have behavior. For example, dogs have state (name, color, breed, hungry) and dogs have behavior (barking, fetching, and slobbering on your newly cleaned slacks). Bicycles have state (current gear, current pedal cadence, two wheels, number of gears) and behavior (braking, accelerating, slowing down, changing gears).

Software objects are modeled after real-world objects in that they, too, have state and behavior. A software object maintains its state in variables and implements its behavior with methods.

Definition: An object is a software bundle of variables and related methods.
You can represent real-world objects using software objects. You might want to represent real-world dogs as software objects in an animation program or a real-world bicycle as a software object within an electronic exercise bike. However, you can also use software objects to model abstract concepts. For example, an event is a common object used in GUI window systems to represent the action of a user pressing a mouse button or a key on the keyboard.

The following illustration is a common visual representation of a software object:

Everything that the software object knows (state) and can do (behavior) is expressed by the variables and methods within that object. A software object that modelled your real-world bicycle would have variables that indicated the bicycle's current state: its speed is 10 mph, its pedal cadence is 90 rpm, and its current gear is the 5th gear. These variables and methods are formally known as instance variables and instance methods to distinguish them from class variables and class methods (described later in What Are Classes?). The following figure illustrates a bicycle modeled as a software object.

The software bicycle would also have methods to brake, change the pedal cadence, and change gears. (The bike would not have a method for changing the speed of the bicycle, as the bike's speed is really just a side-effect of what gear it's in, how fast the rider is pedaling, whether the brakes are on, and how steep the hill is.)

Anything that an object does not know or cannot do is excluded from the object. For example, your bicycle (probably) doesn't have a name, and it can't run, bark, or fetch. Thus there are no variables or methods for those states and behaviors in the bicycle class.

As you can see from the diagrams, the object's variables make up the center or nucleus of the object. Methods surround and hide the object's nucleus from other objects in the program. Packaging an object's variables within the protective custody of its methods is called encapsulation. Typically, encapsulation is used to hide unimportant implementation details from other objects. When you want to change gears on your bicycle, you don't need to know how the gear mechanism works, you just need to know which lever to move. Similarly in software programs, you don't need to know how a class is implemented, you just need to know which methods to invoke. Thus, the implementation details can change at any time without affecting other parts of the program.

This conceptual picture of an object--a nucleus of variables packaged within a protective membrane of methods--is an ideal representation of an object and is the ideal that designers of object-oriented systems strive for. However, it's not the whole story. Often, for implementation or efficiency reasons, an object may wish to expose some of its variables or hide some of its methods.

In many languages, including Java, an object can choose to expose its variables to other objects allowing those other objects to inspect and even modify the variables. Also, an object can choose to hide methods from other objects forbidding those objects from invoking the methods. An object has complete control over whether other objects can access its variables and methods and in fact, can specify which other objects have access. Variable and method access in Java is covered in Controlling Access to Members of a Class(in the Writing Java Programs trail).

The Benefits of Encapsulation

Encapsulating related variables and methods into a neat software bundle is a simple yet powerful idea that provides two primary benefits to software developers:

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