The Java Tutorials

Lesson: Object-Oriented Programming Concepts
If you've never used an object-oriented programming language before, you'll need to learn a few basic concepts before you can

begin writing any code. This lesson will introduce you to objects, classes, inheritance, interfaces, and packages. Each discussion

focuses on how these concepts relate to the real world, while simultaneously providing an introduction to the syntax of the Java

programming language.

What Is an Object?

Objects are key to understanding object-oriented technology. Look around right now and you'll find many examples of real-world

objects: your dog, your desk, your television set, your bicycle.
Real-world objects share two characteristics: They all have state and behavior. Dogs have state (name, color, breed, hungry) and

behavior (barking, fetching, wagging tail). Bicycles also have state (current gear, current pedal cadence, current speed) and

behavior (changing gear, changing pedal cadence, applying brakes). Identifying the state and behavior for real-world objects is a

great way to begin thinking in terms of object-oriented programming.

Take a minute right now to observe the real-world objects that are in your immediate area. For each object that you see, ask

yourself two questions: "What possible states can this object be in?" and "What possible behavior can this object perform?". Make

sure to write down your observations. As you do, you'll notice that real-world objects vary in complexity; your desktop lamp may

have only two possible states (on and off) and two possible behaviors (turn on, turn off), but your desktop radio might have

additional states (on, off, current volume, current station) and behavior (turn on, turn off, increase volume, decrease volume,

seek, scan, and tune). You may also notice that some objects, in turn, will also contain other objects. These real-world

observations all translate into the world of object-oriented programming.



A software object.

Software objects are conceptually similar to real-world objects: they too consist of state and related behavior. An object stores

its state in fields (variables in some programming languages) and exposes its behavior through methods (functions in some

programming languages). Methods operate on an object's internal state and serve as the primary mechanism for object-to-object

communication. Hiding internal state and requiring all interaction to be performed through an object's methods is known as data

encapsulation - a fundamental principle of object-oriented programming.

Consider a bicycle, for example:



A bicycle modeled as a software object.

By attributing state (current speed, current pedal cadence, and current gear) and providing methods for changing that state, the

object remains in control of how the outside world is allowed to use it. For example, if the bicycle only has 6 gears, a method to

change gears could reject any value that is less than 1 or greater than 6.
Bundling code into individual software objects provides a number of benefits, including:

Modularity: The source code for an object can be written and maintained independently of the source code for other objects. Once

created, an object can be easily passed around inside the system.
Information-hiding: By interacting only with an object's methods, the details of its internal implementation remain hidden from

the outside world.
Code re-use: If an object already exists (perhaps written by another software developer), you can use that object in your program.

This allows specialists to implement/test/debug complex, task-specific objects, which you can then trust to run in your own code.
Pluggability and debugging ease: If a particular object turns out to be problematic, you can simply remove it from your

application and plug in a different object as its replacement. This is analogous to fixing mechanical problems in the real world.

If a bolt breaks, you replace it, not the entire machine.

What Is a Class?

In the real world, you'll often find many individual objects all of the same kind. There may be thousands of other bicycles in

existence, all of the same make and model. Each bicycle was built from the same set of blueprints and therefore contains the same

components. In object-oriented terms, we say that your bicycle is an instance of the class of objects known as bicycles. A class