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If you've never used an object-oriented language before, you need to understand the underlying concepts before beginning to write code. You need to know 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 chapter describe the concepts behind object-oriented programming. The last section shows how these concepts translate into code.
This chapter has been updated to reflect features and conventions of the latest release, JDK 5.0. If you notice any errors or omissions (or something you really like), please tell us.
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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