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If you've never used an object-oriented language before, you need to understand the underlying concepts before you begin writing code. You need to understand what an object is, what a class is, how objects and classes are related, and how objects communicate by using messages. The first few sections of this trail describe the concepts behind object-oriented programming. The last section shows how these concepts translate into code.
An object is a software bundle of related variables and methods. Software objects are often used to model real-world objects you find in everyday life.
Software objects interact and communicate with each other using messages.
A class is a blueprint or prototype that defines the variables and the methods common to all objects of a certain kind.
A class inherits state and behavior from its superclass. Inheritance provides a powerful and natural mechanism for organizing and structuring software programs.
An interface is a contract in the form of a collection of method and constant declarations. When a class implements an interface, it promises to implement all of the methods declared in that interface.
This section looks at a small applet, and shows you the code that creates objects, implements classes, sends messages, establishes a superclass, and implements an interface.
Test your understanding of objects, classes, messages, and so on by doing some exercises and answering some questions.
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