Object – Oriented Programming Concepts
Table of Contents
1) 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 speed) and behavior (changing gear, applying brakes). Identifying the state and behavior for real-world objects is a great way to begin thinking in terms 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.
A bicycle modeled as a software object.
2) 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 is the blueprint from which individual objects are created.
The following Bicycle class is one possible implementation of a bicycle:
class Bicycle {
int speed = 0;
int gear = 1;
void speedUp(int increment) {
speed = speed + increment;
}
void applyBrakes(int decrement) {
speed = speed - decrement;
}
void printStates() {
System.out.println(" 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 field’s speed, and gear represent the object's state, and the methods (changeGear, speedUp etc.) define its interaction with the outside world.
You may have noticed that the Bicycle class does not contain a main method. 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 Bicycle objects belongs to some other class in your application.
Here's a BicycleDemo class that creates two separate Bicycle objects 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
bike1.speedUp(10);
bike1.changeGear(2);
bike1.printStates();
bike2.speedUp(10);
bike2.changeGear(2);
bike2.speedUp(10);
bike2.changeGear(3);
bike2.printStates();
}
}
The output of this test prints the ending pedal cadence, speed, and gear for the two bicycles:
speed:10 gear:2
speed:20 gear:3
3) What is Inheritance?
Different kinds of objects often have a certain amount in common with each other. Mountain bikes, road bikes, and tandem bikes, for example, all share the characteristics of bicycles (current speed, current gear). Yet each also defines additional features that make them different: tandem bicycles have two seats and two sets of handlebars; road bikes have drop handlebars; some mountain bikes have an additional chain ring, giving them a lower gear ratio.
Object-oriented programming allows classes to inherit commonly used state and behavior from other classes. In this example, Bicycle now becomes the superclass of MountainBike, RoadBike, andTandemBike. 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 hierarchy of bicycle classes.
The syntax for creating a subclass is simple. At the beginning of your class declaration, use the extends keyword, 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
}
This gives MountainBike all the same fields and methods as Bicycle, yet allows its code to focus exclusively on the features that make it unique. This makes code for your subclasses easy to read. However, you must take care to properly document the state and behavior that each superclass defines, since that code will not appear in the source file of each subclass.
4) 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 {
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 implements keyword in the class declaration:
class ACMEBicycle implements Bicycle {
int speed = 0;
int gear = 1;
// The compiler will now require that methods
// changeGear, speedUp, and applyBrakes
// all be implemented. Compilation will fail if those
// methods are missing from this class.
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);
}
}
Implementing an interface allows a class to become more formal about the behavior it promises to provide. Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.
5) What Is a Package?
A package is a namespace that organizes a set of related classes and interfaces. Conceptually you can think of packages as being similar to different folders on your computer. You might keep HTML pages in one folder, images in another, and scripts or applications in yet another. Because software written in the Java programming language can be composed of hundreds or thousands of individual classes, it makes sense to keep things organized by placing related classes and interfaces into packages.
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