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.
