Learning Resources

Object Oriented Programming

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 circle with an inner circle filled with items, surrounded by gray wedges representing methods that allow access to the inner circle.


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 picture of an object, with bibycle methods and instance variables.


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:

  1. 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.
  3. 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.
  4. 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?


The following Bicycle class is one possible implementation of a bicycle:

class Bicycle {

    int cadence = 0;
    int speed = 0;
    int gear = 1;

    void changeCadence(int newValue) {
         cadence = newValue;

    void changeGear(int newValue) {
         gear = newValue;

    void speedUp(int increment) {
         speed = speed + increment;   

    void applyBrakes(int decrement) {
         speed = speed - decrement;

    void printStates() {
         System.out.println("cadence:" +
             cadence + " speed:" + 
             speed + " gear:" + gear);

The syntax of the Java programming language will look new to you, but the design of this class is based on the previous discussion of bicycle objects. The fields cadence, speed, and gearrepresent the object's state, and the methods (changeCadence, changeGear, speedUpetc.) define its interaction with the outside world.

You may have noticed that the Bicycleclass does not contain a mainmethod. That's because it's not a complete application; it's just the blueprint for bicycles that might be used in an application. The responsibility of creating and using new Bicycleobjects belongs to some other class in your application.

Here's a BicycleDemo class that creates two separate Bicycleobjects and invokes their methods:

class BicycleDemo {
    public static void main(String[] args) {

        // Create two different 
        // Bicycle objects
        Bicycle bike1 = new Bicycle();
        Bicycle bike2 = new Bicycle();

        // Invoke methods on 
        // those objects



cadence:50 speed:10 gear:2
cadence:40 speed:20 gear:3

What Is Inheritance?


Object-oriented programming allows classes to inherit commonly used state and behavior from other classes. In this example, Bicyclenow becomes the superclass of MountainBike, RoadBike, and TandemBike. In the Java programming language, each class is allowed to have one direct superclass, and each superclass has the potential for an unlimited number of subclasses:

A diagram of classes in a hierarchy.


A hierarchy of bicycle classes.

The syntax for creating a subclass is simple. At the beginning of your class declaration, use the extendskeyword, followed by the name of the class to inherit from:

class MountainBike extends Bicycle {

    // new fields and methods defining 
    // a mountain bike would go here



What Is an Interface?

As you've already learned, objects define their interaction with the outside world through the methods that they expose. Methods form the object's interface with the outside world; the buttons on the front of your television set, for example, are the interface between you and the electrical wiring on the other side of its plastic casing. You press the "power" button to turn the television on and off.

In its most common form, an interface is a group of related methods with empty bodies. A bicycle's behavior, if specified as an interface, might appear as follows:

interface Bicycle {

    //  wheel revolutions per minute
    void changeCadence(int newValue);

    void changeGear(int newValue);

    void speedUp(int increment);

    void applyBrakes(int decrement);

To implement this interface, the name of your class would change (to a particular brand of bicycle, for example, such as ACMEBicycle), and you'd use the implementskeyword in the class declaration:

class ACMEBicycle implements Bicycle {

    // remainder of this class 
    // implemented as before


What Is a Package?


The Java platform provides an enormous class library (a set of packages) suitable for use in your own applications. This library is known as the "Application Programming Interface", or "API" for short. Its packages represent the tasks most commonly associated with general-purpose programming. For example, a Stringobject contains state and behavior for character strings; a Fileobject allows a programmer to easily create, delete, inspect, compare, or modify a file on the filesystem; a Socketobject allows for the creation and use of network sockets; various GUI objects control buttons and checkboxes and anything else related to graphical user interfaces. There are literally thousands of classes to choose from. This allows you, the programmer, to focus on the design of your particular application, rather than the infrastructure required to make it work.