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Object-oriented programming (OOP) is an approach to designing modular, reusable software

systems. Although discussions of object-oriented technology often get mired in the details of one language vs. the other, the real key to the object-oriented approach is that it is a modelling approach first.Although often hyped as a revolutionary way to develop software by zealous proponents, the object-oriented approach is in reality a logical extension of good design practices that go back to the very beginning of computer programming. Object-orientation is simply the logical extension of older techniques such as structured programming and abstract data types. An object is an abstract data type with the addition of polymorphism and inheritance.
Rather than structure programs as code and data, an object-oriented system integrates the two using the concept of an "object". An object has state (data) and behavior (code). Objects correspond to things found in the real world. So for example, a graphics program will have objects such as circle, square, menu. An online shopping system will have objects such as shopping cart, customer,product. The shopping system will support behaviors such as place order, make payment, and offer discount. The objects are designed as class hierarchies. So, for example, with the shopping system there might be high level classes such as electronics product, kitchen product, and book. There may be further refinements for example under electronic products: CD Player, DVD player, etc. These classes and subclasses correspond to sets and subsets in mathematical logic.
The goals of object-oriented programming are:
• Increased understanding.
• Ease of maintenance.
• Ease of evolution.