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Object-Oriented Programming Concepts: A Primer |
As the name implies, objects are the key concept 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 in programs 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 "objectify" abstract concepts. For example, "event" is a common object used in GUI window systems to represent the event when a user presses a mouse button or types 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 (which are described later in ).
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 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.
As you can see from the diagrams, the object's variables make up the center or nucleus of the object and the 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. Thus, the implementation can change at any time without changing other parts of the program.
The Benefit 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:
- Modularity--the source code for an object can be written and maintained independently of the source code for other objects. Also, an object can be easily passed around in the system. You can give your bicycle to someone else and it will still work.
- Information hiding--an object has a public interface which other objects can use to communicate with it. But the object can maintain private information and methods that can be changed at any time without affecting the other objects that depend on it. You don't need to understand the gear mechanism on your bike in order to use it.
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