Objects Classes and Instances

An object is a code-based abstraction of a real-world entity or relationship. For instance, you might have a Customer object that represents a real-world customer, such as customer number 123, or you might have a File object that represents C:\config.sys on your computer's hard drive.

A closely related term is class. A class is the code that defines an object, and all objects are created based on a class. A class is an abstraction of a real-world concept, and it provides the basis from which you create instances of specific objects. For example, in order to have a Customer object representing customer number 123, you must first have a Customer class that contains all of the code (methods, properties, events, variables, and so on) necessary to create Customer objects. Based on that class, you can create any number of objects, each one an instance of the class. Each object is identical to the others, except that it may contain different data.

You can create many instances of Customer objects based on the same Customer class. All of the Customer objects are identical in terms of what they can do and the code they contain, but each one contains its own unique data. This means that each object represents a different physical customer.

composition of an object

You use an interface to get access to an object's data and behaviors. This defines a contract for the object to follow. This is much like a real world legal contract that binds the object to a standard definition of data and behaviors, where in your interface you can define a what is needed to fulfill a contract. The object's data and behaviors are contained within the object, so a client application can treat the object like a black box, accessible only through its interface. This is a key object-oriented concept called encapsulation. The idea is that any program that makes use of this object will not have direct access to the behaviors or data; rather, those programs must make use of your object's interface.

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