Object Oriented Programming Concepts

Object Oriented
Programming
An introduction to Object Oriented
Programming concepts in Java for
absolute beginners.

Pre-requisites
• These slides aim at giving an introduction to the core
concepts of object oriented programming.
• You will need to have Java JDK installed.
• You will need to know how to compile and run Java
programs.
• The examples in these slides were compiled and
executed on a Mac running Java JDK 1.6.0_65
2

Concept 1: Classes
• A class is a blueprint of an object.
• It defines the behaviours and attributes of an object.
• Attributes, refer to the variables that can be of different
datatypes like Integer, Double, Long, String, etc.
• Behaviours, refer to the methods that are exposed by a
class.
3

Exercise 1: Create a class named “Ferrari” with
attributes Integer numberOf Wheels and behaviour
drive
• Create a directory named “oops”. We will be creating, compiling
and executing all files for the exercises within this folder.
• Create a file named Ferrari.java with the following contents.
Please do not copy/paste. Type in the code. Take a look at the
comments.
//Class name and file name should match. If you are writing a class called “Wrestler”, file should be Wrestler.java
public class Ferrari {
//Attribute
Integer numberOfWheels;
//Behaviour
public void drive(){
System.out.println("I am driving");
}
//Getter and setter
public void setNumberOfWheels(Integer numberOfWheels){this.numberOfWheels = numberOfWheels;}
public Integer getNumberOfWheels(){return this.numberOfWheels;}
}
• Compile the file using the command “javac Ferrari.java” and
make sure there are no errors.
4

Create a class “Audi” with the same attributes and behaviours as your
Ferrari class
Exercise 2
5

Concept 2: Objects
• An Object is an instantiation of a class.
• By instantiating an object, you are using the blueprint
provided by it’s class and making it usable.
• In the next example, you will be creating a class with a
main method where you will instantiate objects. The
main method is the entry point for all Java programs.
Every usable Java program, must have a main method
of the signature “public static void main(String[] args)”.
• Read more about the main method here.
6

Exercise 3: Objects
• Create a file named Main.java with the following
contents.
public class Main {
//main method
public static void main(String[] args){
//Create a new ferrari object of type Ferrari
Ferrari ferrari = new Ferrari();
//Set the attribute numberOfWheels by calling the setNumberOfWheels method
ferrari.setNumberOfWheels(4);
//Print the number of wheels by calling the getNumberOfWheels method
System.out.println(ferrari.getNumberOfWheels());
//Execute the behaviour drive by calling the drive() method
ferrari.drive();
}
}
• Compile and run the program using the commands
“javac Main.java” followed by “java Main”.
• Verify that you see the number of wheels and the text “I
am driving” printed out.
7

In the same main method as Exercise 3, instantiate an audi object of
type Audi that you created in Exercise 2.
Exercise 4
8

Concept 3: Abstraction and Inheritance
• Read the code in Ferrari.java and Audi.java. You will notice that both the
classes have the same attributes and behaviours. We are simply
rewriting the same code in both the classes.
• An Abstract class in Java, is where we declare and/or define all the
common attributes and behaviours, that can be inherited by classes that
extend the Abstract class.
• For example, both Ferrari and Audi are vehicles. All vehicles have
wheels. So, we can create an Abstract class Vehicle with attribute
numberOfWheels from which Ferrari and Audi extend and inherit the
properties.
• Confused? Take a look at the next exercise. Type in the code and
execute it instead of copy/pasting.

Exercise 5: Abstraction and Inheritance
• Create a file named Vehicle.java with the following content.
public abstract class Vehicle{
private Integer numberOfWheels;
public void setNumberOfWheels(Integer numberOfWheels){
this.numberOfWheels = numberOfWheels;
}
public Integer getNumberOfWheels(){
return this.numberOfWheels;
}
}
• Remove the attribute numberOfWheels and the getNumberOfWheels and setNumberOfWheels
methods from Ferrari class and Audi class. Update the class to extend Vehicle class.
public class Ferrari extends Vehicle{
//Behaviour
}
}
• Compile the new classes using the command “javac Vehicle.java Ferrari.java Audi.java Main.java”
• Run the program using the command “java Main”. You will see that even though Ferrari and Audi
do not have the attribute numberOfWheels, it is inheriting it from the Vehicle abstract class. This
resulted in lesser lines of code. And in large Java programs, helps in easier maintainability of the
application, and enables easier extension of the applications. For example, if I want all vehicles to
have a new property numberOfSeats, instead of adding the attribute in each class, I can add the
attribute to the abstract class and all the classes extending this class inherit the new attribute.

1. Move the behaviour drive() to the abstract class and remove the
behaviour from Ferrari and Audi classes.
2. Create an abstract class LivingBeing with attribute numberOfLegs
and it’s getter and setter methods. Create two classes Dog and
Man. Create a Main class with a main method. Instantiate a new
Dog and Man object. Set the number of legs and print the number
of legs.
Exercise 6

Concept 4: Interfaces
• An interface can be thought of a class that declares a
contract. All classes that implement an interface, must and
should provide a definition for the methods declared in the
interface.
• Let us take an example. A bird, and an aircraft, the entities
are not related to one another. But both have a common
behaviour. Both birds and aircrafts, fly. Birds and aircrafts are
flyable.
• Thinking in terms of interfaces, if we want to create a bird or
an aircraft, both should be flyable.
• Take a look at the next exercise.

Exercise 7: Inheritance
• Create a class Aircraft and another class Bird. We have already
identified that all birds and aircrafts, must be flyable. Therefore, we
can create an interface Flyable which both Aircraft and Bird can
implement. By implementing an interface, we are setting a contract
saying all birds and aircrafts must fly by providing a definition for
the fly method declared in the interface Flyable.
public interface Flyable{
public void fly(Integer altitude);
}
public class Aircraft implements Flyable{
public void fly(Integer altitude){
System.out.println("Aircraft in flight. Current altitude " + altitude);
}
}
public class Bird implements Flyable{
public void fly(Integer altitude){
System.out.println("Bird in flight. Current altitude " + altitude);
}
}
public class Main {
Bird bird = new Bird();
bird.fly(400);
Aircraft aircraft = new Aircraft();
aircraft.fly(5000);
}
}
Try: What happens when you remove the fly method from Bird class?

Create an interface Printable with print() method. Create two classes
DesktopPrinter() and LaserPrinter() that implement Printable method.
Exercise 8

Concept 5: Polymorphism and why use
interfaces?
• Looking at the examples, the first question we ask is why do we even need an interface?
• One of the main uses of interfaces, is to exercise the concept of polymorphism. Polymorphism,
simply means many forms.
• Take a look at the example below. We have used an ArrayList to add two completely unrelated
objects. Bird and Aircraft have nothing in common but both implement Printable.
public interface Printable{
public void print();
}
public class DesktopPrinter implements Printable{
public void print(){
System.out.println("Printing from a desktop printer");
}
}
public class LaserPrinter implements Printable{
public void print(){
System.out.println("Printing from a laser printer");
}
}
import java.util.ArrayList;
public class Main {
DesktopPrinter desktopPrinter = new DesktopPrinter();
LaserPrinter laserPrinter = new LaserPrinter();
ArrayList<Printable> printableObjects = new ArrayList<Printable>();
printableObjects.add(desktopPrinter);
printableObjects.add(laserPrinter);
for(Printable object:printableObjects){
object.print();
}
}
}

Concept 6: Over-riding and Over-loading
• Over-riding: When you extend a class, the extending
class can provide it’s own implementation of a method.
This is called method over-riding.
• Over-loading: Having two methods with the same name
but different parameters is known as Over-loading.

Exercise 9: Over-riding
• Abstract class vehicle has a method named drive(). We can over-ride this method in class Ferrari to print more specific
information. Modify the code as below.
public abstract class Vehicle{
private Integer numberOfWheels;
public void setNumberOfWheels(Integer numberOfWheels){
this.numberOfWheels = numberOfWheels;
}
public Integer getNumberOfWheels(){
return this.numberOfWheels;
}
//Behaviour
}
}
//Over-riding the method
System.out.println("I am driving a Ferrari.");
}
}
public class Audi extends Vehicle{}
public class Main {
//main method
ferrari.drive();
Audi audi = new Audi();
audi.drive();
}
}
• Compile the above code and run it. Notice how, Ferrari prints more specific information “I am driving a Ferrari” and Audi still
prints the “I am driving” message. This is because the method drive() is over-ridden in Ferrari class.

Exercise 10: Over-loading
• Modify the code for class Ferrari and the main method as provided
below.
//Overridden method
System.out.println("I am driving a Ferrari.");
}
//Overloaded method
public void drive(Integer milesPerHour){
System.out.println("I am driving a Ferrari at " + milesPerHour + " miles per hour");
}
}
public class Main {
//main method
ferrari.drive();
ferrari.drive(80);
}
}
• Notice how class Ferrari has two methods with the same name
drive.

Concept 7: Encapsulation
• Encapsulation is the concept of hiding data and
restricting access to attributes and behaviour.
• There are 4 levels of access in Java.
1. default: access limited to classes within the same package
2. public: access available to all other classes
3. private: access available only within the same class
4. protected: access available to classes within the same package and
subclasses

Object Oriented Programming Concepts

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