This tutorial will help you to understand about Java OOP’S concepts with examples. Let’s discuss what are the features of Object Oriented Programming. Writing object-oriented programs involves creating classes, creating objects from those classes, and creating applications, which are stand-alone executable programs that use those objects.
A class is a template, blueprint,or contract that defines what an object’s data fields and methods will be. An object is an instance of a class. You can create many instances of a class. A Java class uses variables to define data fields and methods to define actions. Additionally,a class provides methods of a special type, known as constructors, which are invoked to create a new object. A constructor can perform any action, but constructors are designed to perform initializing actions, such as initializing the data fields of objects.
Objects are made up of attributes and methods. Attributes are the characteristics that define an object; the values contained in attributes differentiate objects of the same class from one another. To understand this better let’s take the example of Mobile as an object. Mobile has characteristics like a model, manufacturer, cost, operating system etc. So if we create “Samsung” mobile object and “IPhone” mobile object we can distinguish them from characteristics. The values of the attributes of an object are also referred to as the object’s state.
In this page, we will learn about the basics of OOPs. Object-Oriented Programming is a paradigm that provides many concepts such as inheritance, data binding, polymorphism, etc.
Simula is considered the first object-oriented programming language. The programming paradigm where everything is represented as an object is known as a truly object-oriented programming language.
Smalltalk is considered the first truly object-oriented programming language.
The popular object-oriented languages are Java, C#, PHP, Python, C++, etc.
OOPs (Object-Oriented Programming System)
Object means a real-world entity such as a pen, chair, table, computer, watch, etc. Object-Oriented Programming is a methodology or paradigm to design a program using classes and objects. It simplifies the software development and maintenance by providing some concepts:
- Object
- Class
- Inheritance
- Polymorphism
- Abstraction
- Encapsulation
1) Object
An object can be defined as an instance of a class, and there can be multiple instances of a class in a program. An Object contains both the data and the function, which operates on the data. For example - chair, bike, marker, pen, table, car, etc.
2) Class
The class is a group of similar entities. It is only an logical component and not the physical entity. For example, if you had a class called “Expensive Cars” it could have objects like Mercedes, BMW, Toyota, etc. Its properties(data) can be price or speed of these cars. While the methods may be performed with these cars are driving, reverse, braking etc.
3)Inheritance
An important feature of object-oriented programs is inheritance—the ability to create classes that share the attributes and methods of existing classes, but with more specific features. Inheritance is mainly used for code reusability. So you are making use of already written the classes and further extending on that. That why we discussed the code reusability the concept. In general one line definition, we can tell that deriving a new class from existing class, it’s called as Inheritance. You can look into the following example for inheritance concept. Here we have Mobile class extended by other specific class like Android and Blackberry.
package oopsconcept; public class Android extends Mobile{ //Constructor to set properties/characteristics of object Android(String man, String o,String m, int c){ super(man, o, m, c); } //Method to get access Model property of Object public String getModel(){ return "This is Android Mobile- " + model; } }
package oopsconcept; public class Blackberry extends Mobile{ //Constructor to set properties/characteristics of object Blackberry(String man, String o,String m, int c){ super(man, o, m, c); } public String getModel(){ return "This is Blackberry-"+ model; } }
4)Polymorphism
In Core, Java Polymorphism is one of easy concept to understand. Polymorphism definition is that Poly means many and morphos means forms. It describes the feature of languages that allows the same word or symbol to be interpreted correctly in different situations based on the context. There are two types of Polymorphism available in Java. For example, in English, the verb “run” means different things if you use it with “a footrace,” a “business,” or “a computer.” You understand the meaning of “run” based on the other words used with it. Object-oriented programs are written so that the methods having the same name works differently in different context. Java provides two ways to implement polymorphism.
Static Polymorphism (compile time polymorphism/ Method overloading):
The ability to execute different method implementations by altering the argument used with the method name is known as method overloading. In below program, we have three print methods each with different arguments. When you properly overload a method, you can call it providing different argument lists, and the appropriate version of the method executes.
package oopsconcept; class Overloadsample { public void print(String s){ System.out.println("First Method with only String- "+ s); } public void print (int i){ System.out.println("Second Method with only int- "+ i); } public void print (String s, int i){ System.out.println("Third Method with both- "+ s + "--" + i); } } public class PolymDemo { public static void main(String[] args) { Overloadsample obj = new Overloadsample(); obj.print(10); obj.print("Amit"); obj.print("Hello", 100); } }
Output:
First Method with only String-10 Second Method with only int-Amit Third Method with both-Hello -- 100
Dynamic Polymorphism (run time polymorphism/ Method Overriding)
When you create a subclass by extending an existing class, the new subclass contains data and methods that were defined in the original superclass. In other words, any child class object has all the attributes of its parent. Sometimes, however, the superclass data fields and methods are not entirely appropriate for the subclass objects; in these cases, you want to override the parent class members. Let’s take the example used in inheritance explanation.
package oopsconcept; public class OverridingDemo { public static void main(String[] args) { //Creating Object of SuperClass and calling getModel Method Mobile m = new Mobile("Nokia", "Win8", "Lumia",10000); System.out.println(m.getModel()); //Creating Object of Sublcass and calling getModel Method Android a = new Android("Samsung", "Android", "Grand",30000); System.out.println(a.getModel()); //Creating Object of Sublcass and calling getModel Method Blackberry b = new Blackberry("BlackB", "RIM", "Curve",20000); System.out.println(b.getModel()); } }
5) Abstraction
An abstraction is an act of representing essential features without including background details. It is a technique of creating a new data type that is suited for a specific application. For example, while driving a car, you do not have to be concerned with its internal working. Here you just need to concern about parts like steering wheel, Gears, accelerator, etc.
All programming languages provide abstractions. It can be argued that the complexity of the problems you’re able to solve is directly related to the kind and quality of abstraction. An essential element of object-oriented programming is an abstraction. Humans manage complexity through abstraction. When you drive your car you do not have to be concerned with the exact internal working of your car(unless you are a mechanic). What you are concerned with is interacting with your car via its interfaces like steering wheel, brake pedal, accelerator pedal etc. Various manufacturers of car have different implementation of the car working but its basic interface has not changed (i.e. you still use the steering wheel, brake pedal, accelerator pedal etc to interact with your car). Hence the knowledge you have of your car is abstract.
A powerful way to manage abstraction is through the use of hierarchical classifications. This allows you to layer the semantics of complex systems, breaking them into more manageable pieces. From the outside, a car is a single object. Once inside, you see that the car consists of several subsystems: steering, brakes, sound system, seat belts, heating, cellular phone, and so on. In turn, each of these subsystems is made up of more specialized units. For instance, the sound system consists of a radio, a CD player, and/or a tape player. The point is that you manage the complexity of the car (or any other complex system)through the use of hierarchical abstractions.
An abstract class is something which is incomplete and you can not create an instance of the abstract class. If you want to use it you need to make it complete or concrete by extending it. A class is called concrete if it does not contain any abstract method and implements all abstract method inherited from abstract class or interface it has implemented or extended. By the way, Java has a concept of abstract classes, abstract method but a variable can not be abstract in Java.
Let's take an example of Java Abstract Class called Vehicle. When I am creating a class called Vehicle, I know there should be methods like start() and Stop() but don't know start and stop mechanism of every vehicle since they could have different start and stop mechanism e.g some can be started by a kick or some can be by pressing buttons.
The advantage of Abstraction is if there is a new type of vehicle introduced we might just need to add one class which extends Vehicle Abstract class and implement specific methods. The interface of start and stop method would be same.
package oopsconcept; public abstract class VehicleAbstract { public abstract void start(); public void stop(){ System.out.println("Stopping Vehicle in abstract class"); } } class TwoWheeler extends VehicleAbstract{ @Override public void start() { System.out.println("Starting Two Wheeler"); w } } class FourWheeler extends VehicleAbstract{ @Override public void start() { System.out.println("Starting Four Wheeler"); } }
package oopsconcept; public class VehicleTesting { public static void main(String[] args) { VehicleAbstract my2Wheeler = new TwoWheeler(); VehicleAbstract my4Wheeler = new FourWheeler(); my2Wheeler.start(); my2Wheeler.stop(); my4Wheeler.start(); my4Wheeler.stop(); } }
Output:
Starting Two Wheeler Stopping Vehicle in abstract class Starting Four Wheeler Stopping Vehicle in abstract class
6)Encapsulation
Binding (or wrapping) code and data together into a single unit are known as encapsulation. For example capsule, it is wrapped with different medicines.
A java class is the example of encapsulation. Java bean is the fully encapsulated class because all the data members are private here.
7) Association
Association is a relationship between two objects. It defines the diversity between objects. In this OOP concept, all object have their separate lifecycle, and there is no owner. For example, many students can associate with one teacher while one student can also associate with multiple teachers.
8) Aggregation
In this technique, all objects have their separate lifecycle. However, there is ownership such that child object can’t belong to another parent object. For example consider class/objects department and teacher. Here, a single teacher can’t belong to multiple departments, but even if we delete the department, the teacher object will never be destroyed.
9) Composition
A composition is a specialized form of Aggregation. It is also called "death" relationship. Child objects do not have their lifecycle so when parent object deletes all child object will also delete automatically. For that, let’s take an example of House and rooms. Any house can have several rooms. One room can’t become part of two different houses. So, if you delete the house room will also be deleted.
Advantages of OOPS:
OOP offers easy to understand and a clear modular structure for programs.
Objects created for Object-Oriented Programs can be reused in other programs. Thus it saves significant development cost.
Large programs are difficult to write, but if the development and designing team follow OOPS concept then they can better design with minimum flaws.
It also enhances program modularity because every object exists independently.
What is the difference between an object-oriented programming language and object-based programming language?
Object-based programming language follows all the features of OOPs except Inheritance. JavaScript and VBScript are examples of object-based programming languages.
Do You Know?
- Can we overload the main method?
- A Java Constructor returns a value but, what?
- Can we create a program without main method?
- What are the six ways to use this keyword?
- Why is multiple inheritance not supported in Java?
- Why use aggregation?
- Can we override the static method?
- What is covariant return type?
- What are the three usages of Java super keyword?
- Why use instance initializer block?
- What is the usage of a blank final variable?
- What is a marker or tagged interface?
- What is runtime polymorphism or dynamic method dispatch?
- What is the difference between static and dynamic binding?
- How downcasting is possible in Java?
- What is the purpose of a private constructor?
- What is object cloning?
What will we learn in OOPs Concepts?
- Advantage of OOPs
- Naming Convention
- Object and class
- Method overloading
- Constructor
- static keyword
- this keyword with six usage
- Inheritance
- Aggregation
- Method Overriding
- Covariant Return Type
- super keyword
- Instance Initializer block
- final keyword
- Abstract class
- Interface
- Runtime Polymorphism
- Static and Dynamic Binding
- Downcasting with instanceof operator
- Package
- Access Modifiers
- Encapsulation
- Object Cloning
