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bject-oriented software is all about objects. An object is a "black box" which receives and sends messages. A black box actually contains code (sequences of computer instructions) and data (information which the instructions operates on). Traditionally, code and data have been kept apart. For example, in the C language, units of code are called functions, while units of data are called structures. Functions and structures are not formally connected in C. A C function can operate on more than one type of structure, and more than one function can operate on the same structure.Not so for object-oriented software! In o-o (object-oriented) programming, code and data are merged into a single indivisible thing -- an object. This has some big advantages, as you'll see in a moment. But first, here is why SDC developed the "black box" metaphor for an object. A primary rule of object-oriented programming is this: as the user of an object, you should never need to peek inside the box!
An object
Why shouldn't you need to look inside an object? For one thing, all communication to it is done via messages. The object which a message is sent to is called the receiver of the message. Messages define the interface to the object. Everything an object can do is represented by its message interface. So you shouldn't have to know anything about what is in the black box in order to use it.
And not looking inside the object's black box doesn't tempt you to directly modify that object. If you did, you would be tampering with the details of how the object works. Suppose the person who programmed the object in the first place decided later on to change some of these details? Then you would be in trouble. Your software would no longer work correctly! But so long as you just deal with objects as black boxes via their messages, the software is guaranteed to work. Providing access to an object only through its messages, while keeping the details private is called information hiding. An equivalent buzzword is encapsulation.
Why all this concern for being able to change software? Because experience has taught us that software changes. A popular adage is that "software is not written, it is re-written". And some of the costliest mistakes in computer history have come from software that breaks when someone tries to change it.
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ow are objects defined? An object is defined via its class, which determines everything about an object. Objects are individual instances of a class. For example, you may create an object call Spot from class Dog. The Dog class defines what it is to be a Dog object, and all the "dog-related" messages a Dog object can act upon. All object-oriented languages have some means, usually called a factory, to "manufacture" object instances from a class definition.
Spot
You can make more than one object of this class, and call them Spot, Fido, Rover, etc. The Dog class defines messages that the Dog objects understand, such as "bark", "fetch", and "roll-over".
You may also hear the term method used. A method is simply the action that an object carries out in response to a message. It is the code, which gets executed when the message is sent to a particular object.
Arguments are often supplied as part of a message. For example, the "fetch" message might contain an argument that says what to fetch, like "the-stick". Or the "roll-over" message could contain one argument to say how fast, and a second argument to say how many times.
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f there is already a class which can respond to a bunch of different messages, what if you wanted to make a new, similar class which adds just a couple of more messages? Why have to re-write the entire class?Of course, in any good object-oriented language, you don't. All you need to do is create a subclass of the original class. This new class inherits all the existing messages, and therefore, all the behavior of the original class. The original class is called the parent class, or superclass, of the new class. Some more jargon -- a subclass is said to be a specialization of its superclass, and the conversely a superclass a generalization of its subclasses.
Inheritance also promotes reuse. You don't have to start from scratch when you write a new program. You can simply reuse an existing repertoire of classes that have behaviors similar to what you need in the new program.
For example, after creating the class Dog, you might make a subclass called Wolf, which defines some wolf-specific messages, such as hunt. Or it might make more sense to define a common class called Canis, of which both Dog and Wolf are subclasses.
Much of the art of o-o programming is determining the best way to divide a program into an economical set of classes. In addition to speeding development time, proper class construction and reuse results in far fewer lines of code, which translates to less bugs and lower maintenance costs.
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bject-oriented programming offers a new and powerful model for writing computer software. Objects are "black boxes" which send and receive messages. This approach speeds the development of new programs, and, if properly used, improves the maintenance, reusability, and modifiability of software.