OOPS Concepts or Object-Oriented Programming Concepts are very important. Without having an idea about OOPS concepts, you will not be able to design systems in the object-oriented programming model.
Table of Contents
OOPS Concepts
Core OOPS concepts are:
- Abstraction
- Encapsulation
- Polymorphism
- Inheritance
- Association
- Aggregation
- Composition
Let’s look into these object-oriented programming concepts one by one. We will use Java programming language for code examples so that you know how to implement OOPS concepts in java.
Abstraction
Abstraction is the concept of hiding the internal details and describing things in simple terms. For example, a method that adds two integers. The internal processing of the method is hidden from the outer world. There are many ways to achieve abstraction in object-oriented programmings, such as encapsulation and inheritance.
A Java program is also a great example of abstraction. Here java takes care of converting simple statements to machine language and hides the inner implementation details from the outer world.
Encapsulation
Encapsulation is the technique used to implement abstraction in object-oriented programming. Encapsulation is used for access restriction to class members and methods.
Access modifier keywords are used for encapsulation in object oriented programming. For example, encapsulation in java is achieved using private, protected and public keywords.
Polymorphism
Polymorphism is the concept where an object behaves differently in different situations. There are two types of polymorphism – compile time polymorphism and runtime polymorphism.
Compile-time polymorphism is achieved by method overloading. For example, we can have a class as below.
public class Circle {
public void draw(){
System.out.println("Drwaing circle with default color Black and diameter 1 cm.");
}
public void draw(int diameter){
System.out.println("Drwaing circle with default color Black and diameter"+diameter+" cm.");
}
public void draw(int diameter, String color){
System.out.println("Drwaing circle with color"+color+" and diameter"+diameter+" cm.");
}
}
Here we have multiple draw methods but they have different behavior. This is a case of method overloading because all the methods name is same and arguments are different. Here compiler will be able to identify the method to invoke at compile-time, hence it’s called compile-time polymorphism.
Runtime polymorphism is implemented when we have “IS-A” relationship between objects. This is also called a method overriding because subclass has to override the superclass method for runtime polymorphism.
If we are working in terms of the superclass, the actual implementation class is decided at runtime. The compiler is not able to decide which class method will be invoked. This decision is done at runtime, hence the name as runtime polymorphism or dynamic method dispatch.
package com.journaldev.test;
public interface Shape {
public void draw();
}
package com.journaldev.test;
public class Circle implements Shape{
@Override
public void draw(){
System.out.println("Drwaing circle");
}
}
package com.journaldev.test;
public class Square implements Shape {
@Override
public void draw() {
System.out.println("Drawing Square");
}
}
Shape is the superclass and there are two subclasses Circle and Square. Below is an example of runtime polymorphism.
Shape sh = new Circle();
sh.draw();
Shape sh1 = getShape(); //some third party logic to determine shape
sh1.draw();
In the above examples, java compiler doesn’t know the actual implementation class of Shape that will be used at runtime, hence runtime polymorphism.
Inheritance
Inheritance is the object-oriented programming concept where an object is based on another object. Inheritance is the mechanism of code reuse. The object that is getting inherited is called superclass and the object that inherits the superclass is called subclass.
We use extends keyword in java to implement inheritance. Below is a simple example of inheritance in java.
package com.journaldev.java.examples1;
class SuperClassA {
public void foo(){
System.out.println("SuperClassA");
}
}
class SubClassB extends SuperClassA{
public void bar(){
System.out.println("SubClassB");
}
}
public class Test {
public static void main(String args[]){
SubClassB a = new SubClassB();
a.foo();
a.bar();
}
}
Association
Association is the OOPS concept to define the relationship between objects. The association defines the multiplicity between objects. For example Teacher and Student objects. There is a one-to-many relationship between a teacher and students. Similarly, a student can have a one-to-many relationship with teacher objects. However, both student and teacher objects are independent of each other.
Aggregation
Aggregation is a special type of association. In aggregation, objects have their own life cycle but there is ownership. Whenever we have “HAS-A” relationship between objects and ownership then it’s a case of aggregation.
Composition
The composition is a special case of aggregation. The composition is a more restrictive form of aggregation. When the contained object in “HAS-A” relationship can’t exist on its own, then it’s a case of composition. For example, House has-a Room. Here the room can’t exist without the house. Composition is said to be better than inheritance, read more at Composition vs Inheritance.
That’s all for a quick round-up on OOPS concepts.
References: https://docs.oracle.com/javase/tutorial/java/concepts/
Super….. Thanks Pankaj
Hi Pankaj, thank you very much for so much information about Java and all in the same place. It was great idea doing that. I have read lots of your articles.
Allow me to point something that doesn’t sound right, when you say:
“Shape is the superclass and there are two subclasses Circle and Square.” shouldn’t it be:
“Shape is an interface and Circle and Square are implementations of it, which give us an Interface polymorphism.” ?
The idea here is to show the inheritance. The Shape can also be a class and Circle/Square classes extend it.
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Thank you for sharing such an informative and useful post with us.
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How to answer this code
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Thanks for the simple and wonderful explanations. Can you please add examples for association, aggregation and compositions as well? That would be very helpful.
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Thanks. very well explained,
Nice Arcticle, Well explained with examples
Nice explanation with great examples. Loved it
Thank you , Nice article.
Can you please explain about Coupling and Cohession?
I only know one thing about your question. At the software engineering industry, according to modern software development concepts, all developed software should be highly cohesive and loosely coupled.
Cohesion refers to what the class (or module) can do. Low cohesion would mean that the class does a great variety of actions – it is broad, unfocused on what it should do. High cohesion means that the class is focused on what it should be doing, i.e. only methods relating to the intention of the class.
As for coupling, it refers to how related or dependent two classes/modules are toward each other. For low coupled classes, changing something major in one class should not affect the other. High coupling would make it difficult to change and maintain your code; since classes are closely knit together, making a change could require an entire system revamp.
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Thank you for article.can you explain difference between composition and aggregation with example?
many ways to achieve abstraction in object oriented programming, such as encapsulation and inheritance. this is corrects by inheritance are interface
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Thank you,
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Thumbs up.