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Trail: Learning the Java Language

## Lesson: Object-Oriented Programming Concepts

> 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.

### What Is an Object?

> 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.

### What Is a Message?

> Software objects interact and communicate with each other using messages.

### What Is a Class?

> A class is a blueprint or prototype that defines the variables and the methods common to all objects of a certain kind.

### What Is Inheritance?

> A class inherits state and behavior from its superclass. Inheritance provides a powerful and natural mechanism for organizing and structuring software programs.

### What Is an Interface?

> 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.

### How Do These Concepts Translate into Code?

> 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.

### Questions and Exercises: Object-Oriented Concepts

> Test your understanding of objects, classes, messages, and so on by doing some exercises and answering some questions.

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