OOPs Concept in Java with Examples

Java, renowned for its simplicity, reliability, and portability, has been a cornerstone in the programming landscape for decades, largely due to its effective use of OOP (Object-Oriented Programming) concepts. The inception of Java, with its oops concepts in java, reshaped how developers approached programming, offering a platform-independent language that emphasizes security and simplicity. Through the implementation of what is oops in java, including its fundamental OOPs concepts like encapsulation, inheritance, and polymorphism, Java’s enduring popularity in various domains, from web applications to enterprise solutions, is a testament to its robustness and versatility. The concept of OOPs in java underpins its design philosophy, making it an ideal language for both beginners learning like the basics of OOPs in java and seasoned professionals delving into more advanced concepts of oops in java.

Java’s Object-Oriented Programming System (OOPS) is a fundamental aspect of its architecture, embodying key concepts such as classes, objects, inheritance, polymorphism, abstraction, and encapsulation. These java oops concepts are crucial for understanding how Java manages data and executes operations.

What is OOPs Concept?

Object-oriented programming is a core of Java Programming, which is used for designing a program using classes and objects. OOPs, can also be characterized as data controlling for accessing the code. In this approach, programmers define the data type of a data structure and the operations that are applied to the data structure.

Understanding Object-Oriented Concepts in Java:

In the world of Java programming, it’s essential to grasp the fundamental object-oriented concepts in Java. Object-Oriented Programming, or OOPs in Java, is not just a set of rules; it’s a powerful paradigm that drives Java’s design and development principles.

Exploring OOPs in Java: So, what exactly is OOPs in Java? Let’s break it down and understand the concept of OOPs through Java’s core principles.

Objects:

In Java, an object is more than just a runtime entity; it’s the embodiment of a class. These objects mirror real-world entities, complete with both state and behavior. Think of a ‘Car’ object based on a ‘Car’ class. It possesses attributes like color, brand, and speed, along with behaviors like ‘accelerate’ and ‘brake’. This is where Java OOP concepts come to life.

Classes:

A class in Java serves as a blueprint for creating objects. It bundles data (attributes) and behavior (methods) that define the object. Consider a ‘Car’ class; it defines attributes like color and methods like ‘accelerate()’. Essentially, a class is a template that characterizes what can be instantiated as an object, exemplifying OOPs concepts.

Abstraction:

Abstraction in Java involves concealing complexity while exposing only the essential aspects. It’s realized through abstract classes and interfaces. For instance, a ‘Vehicle’ interface declares a ‘move()’ method, but the actual implementation is left to classes like ‘Car’ or ‘Bike’. This focus on “what an object does” rather than “how it does it” is a core OOPs concept in Java.

Inheritance:

Inheritance is a mechanism where a new class (subclass) derives attributes and methods from an existing class (superclass). This fosters code reusability and establishes a hierarchical relationship between classes. For example, an ‘ElectricCar’ class can inherit traits from the ‘Car’ class, showcasing the practicality of Java OOPs concepts.

Polymorphism:

Polymorphism allows Java methods and objects to assume multiple forms. It finds common use in method overloading (same method name, different parameters) and method overriding (same method name and parameters in subclass as in parent class). Consider a ‘draw()’ method implemented differently in ‘Circle’, ‘Square’, and ‘Triangle’ classes as a prime example of polymorphism in Java.

Encapsulation:

Encapsulation is a pivotal OOPs principle in Java. It entails bundling data (attributes) and code (methods) into a cohesive unit. Moreover, it limits direct access to an object’s components, preventing inadvertent interference. Techniques like using private variables and offering public getter and setter methods exemplify Java OOPs concepts put into practice.

Java’s strength lies in its adherence to the OOP principles. Understanding the characteristics of OOPs in Java is pivotal for crafting efficient and modular code. These Java OOPs concepts form the foundation of modern software development, enabling developers to create elegant and scalable solutions.

OOPs Concepts in Java with Examples:

• Class Example: class Car { String color; void accelerate() { ... } }
• Object Example: Car myCar = new Car();
• Inheritance Example: class ElectricCar extends Car { ... }
• Polymorphism Example: Method overloading and overriding.
• Abstraction Example: Abstract classes and interfaces.
• Encapsulation Example: Private fields with public getters and setters.

Advanced OOP Concepts:

• Association, Aggregation, and Composition: These concepts define how objects interact and depend on each other.
• Coupling and Cohesion: Concepts focusing on the interdependence and responsibilities of classes in Java.

What is OOPs in Java?

In Java, OOPs refer to a framework based on objects and classes. Java OOPs concepts, with their emphasis on modularity and code reusability, make Java a versatile language for various applications.

What are Objects?  

Objects are always called instances of a class which are created from a class in java or any other language. They have states and behaviour.

These objects always correspond to things found in the real world, i.e., real entities. So, they are also called run-time entities of the world. These are self–contained which consists of methods and properties which make data useful. Objects can be both physical and logical data. It contains addresses and takes up some space in memory. Some examples of objects are a dog, chair, tree etc. 

When we treat animals as objects, it has states like colour, name, breed etc., and behaviours such as eating, wagging the tail etc.

Suppose, we have created a class called My book, we specify the class name followed by the object name, and we use the keyword new.

Object Example 1:

Public class Mybook {
int x=10;
Public static void main (String args []) {
Mybook Myobj= new Mybook ();
System.out.println(MyObj.x);
}
}

In the above example, a new object is created, and it returns the value of x which may be the number of books.

Mybook Myobj= new Mybook ();

 This is the statement used for creating objects.

System.out.println(Myobj.x);

This statement is used to return the value of x of an object.

We can also create multiple objects in the same class and we can create in one class and access it in another class. This method is used for better organization of classes and always remember that name of the java file and the class name remains the same. 

Example 2:

The below example shows how multiple objects are created in the same class and how they are accessed from another class.

• Mybook.java

Public class Mybook {
int x=10;
int y=8;
}

• Count.java

Class Count {
Public static void main (String [] args)
{
Mybook myobj1 = new myobj1();
          Mybook myobj2 = new myobj2();
           System.out.println (myobj1.x);
System.out.println (myobj2.y);
}
}

When this program is compiled, it gives the result as 10, and 8 respectively.

What are Classes?

Classes are like object constructors for creating objects. The collection of objects is said to be a class. Classes are said to be logical quantities. Classes don’t consume any space in the memory. Class is also called a template of an object. Classes have members which can be fields, methods and constructors. A class has both static and instance initializers.

A class declaration consists of:

1. Modifiers: These can be public or default access.
2. Class name: Initial letter.
3. Superclass: A class can only extend (subclass) one parent.
4. Interfaces: A class can implement more than one interface.
5. Body: Body surrounded by braces, { }.

A class keyword is used to create a class. A simplified general form of the class definition is given below:

class classname {
type instance variable 1;
type instance variable 2;
.
.
.
type instance variable n;
type methodname 1 (parameter list) {
// body od method 
}
type methodname 2 (parameter list) {
// body od method 
}
type methodnamen (parameter list) {
// body od method 
}
 }

The variables or data defined within a class are called instance variables. Code is always contained in the methods. Therefore, the methods and variables defined within a class are called members of the class. All the methods have the same form as the main () these methods are not specified as static or public. 

What is Abstraction?  

Abstraction is a process which displays only the information needed and hides the unnecessary information. We can say that the main purpose of abstraction is data hiding. Abstraction means selecting data from a large number of data to show the information needed, which helps in reducing programming complexity and efforts.  

There are also abstract classes and abstract methods. An abstract class is a type of class that declares one or more abstract methods. An abstract method is a method that has a method definition but not implementation. Once we have modelled our object using data abstraction, the same sets of data can also be used in different applications—abstract classes, generic types of behaviours and object-oriented programming hierarchy. Abstract methods are used when two or more subclasses do the same task in different ways and through different implementations. An abstract class can have both methods, i.e., abstract methods and regular methods.

Now let us see an example related to abstraction.

Suppose we want to create a student application and ask to collect information about the student.

We collect the following information.  

• Name 
• Class
• Address
• Dob
• Fathers name
• Mothers’ names and so on. 

We may not require every information that we have collected to fill out the application. So, we select the data that is required to fill out the application. Hence, we have fetched, removed, and selected the data, the student information from large data. This process is known as abstraction in the oops concept.

Abstract class example:

//abstract parent class 
    	Abstract class animal {
    	 //abstract method 
   	  public abstract void sound ( ) ;
    	 }
   	 Public class lion extends animal {
  	  Public void sound ( ) {
System.out.println (“ roar “ );
}
public Static void main ( String args [ ] ) {
 animal obj = new lion ( );
obj. sound ();
}
}

Output: 
Roar

What is Inheritance?

Inheritance is a method in which one object acquires/inherits another object’s properties, and inheritance also supports hierarchical classification. The idea behind this is that we can create new classes built on existing classes, i.e., when you inherit from an existing class, we can reuse methods and fields of the parent class. Inheritance represents the parent-child relationship. To know more about this concept check the free inheritance in java course.

For example, a whale is a part of the classification of marine animals, which is part of class mammal, which is under that class of animal. We use hierarchical classification, i.e., top-down classification. If we want to describe a more specific class of animals such as mammals, they would have more specific attributes such as teeth; cold-blooded, warm-blooded, etc. This comes under the subclass of animals whereas animals come under the superclass. The subclass is a class which inherits properties of the superclass. This is also called a derived class. A superclass is a base class or parental class from which a subclass inherits properties.

We use inheritance mainly for method overriding and R:

To inherit a class, we use the extend keyword.

There are five types of inheritance single, multilevel, multiple, hybrid and hierarchical. 

• Single level  

In this one class i.e., the derived class inherits properties from its parental class.  This enables code reusability and also adds new features to the code. Example: class b inherits properties from class a.

Class A is the base or parental class and class b is the derived class.

Syntax: 

Class a {
…
}
Class b extends class a {
…
}

• Multilevel

This one class is derived from another class which is also derived from another class i.e., this class has more than one parental class, hence it is called multilevel inheritance.

Syntax:

Class a {
….
}
Class b extends class a {
….
}
Class c extends class b {
… 
}

• Hierarchical level 

In this one parental class has two or more derived classes or we can say that two or more child classes have one parental class.

Syntax:

Class a {
…
}	
Class b extends class a {
..
}
Class c extends class a {
..
}

• Hybrid inheritance

This is the combination of multiple and multilevel inheritances and in java, multiple inheritances are not supported as it leads to ambiguity and this type of inheritance can only be achieved through interfaces.

Consider that class a is the parental or base class of class b and class c and in turn, class b and class c are parental or a base class of class d. Class b and class c are derived classes from class a and class d is derived class from class b and class c.

The following program creates a superclass called add and a subclass called sub, using extend keyword to create a subclass add.

// a simple example of inheritance 
//create a superclass
Class Add {
int my;
int by;
void setmyby (int xy, int hy) {
my=xy;
by=hy;
}
}
/create a sub class
class b extends add {
int total;
void sum () {
public Static void main (String args [ ] ) {
b subOb= new b ( );
subOb. Setmyby (10, 12);
subOb. Sum ( ) ;
System.out.println(“total =” + subOb. Total);
}
} 

It gives output as – total = 22

What is Polymorphism?

Polymorphism refers to many forms, or it is a process that performs a single action in different ways. It occurs when we have many classes related to each other by inheritance. Polymorphism is of two different types, i.e., compile-time polymorphism and runtime polymorphism. One of the examples of Compile time polymorphism is that when we overload a static method in java. Run time polymorphism also called a dynamic method dispatch is a method in which a call to an overridden method is resolved at run time rather than compile time. In this method, the overridden method is always called through the reference variable. By using method overloading and method overriding, we can perform polymorphism. Generally, the concept of polymorphism is often expressed as one interface, and multiple methods. This reduces complexity by allowing the same interface to be used as a general class of action. 

Example:

public class Bird {
…
Public void sound ( ) {
System.out.println ( “ birds sounds “ );
}
}
public class pigeon extends Bird {
…
@override
public void sound ( ) {
System.out.println( “ cooing ” ) ;
}
}
public class sparrow extends Bird ( ) {
….
@override 
Public void sound ( ){
System.out.println( “ chip ” ) ;
}
}

In the above example, we can see common action sound () but there are different ways to do the same action. This is one of the examples which shows polymorphism.

Polymorphism in java can be classified into two types:

1. Static / Compile-Time Polymorphism
2. Dynamic / Runtime Polymorphism

What is Compile-Time Polymorphism in Java?

Compile-Time polymorphism in java is also known as Static Polymorphism. to resolved at compile-time which is achieved through the Method Overloading.

What is Runtime Polymorphism in Java?

Runtime polymorphism in java is also known as Dynamic Binding which is used to call an overridden method that is resolved dynamically at runtime rather than at compile time. 

What is Encapsulation?

Encapsulation is one of the concepts in OOPs concepts; it is the process that binds together the data and code into a single unit and keeps both from being safe from outside interference and misuse. In this process, the data is hidden from other classes and can be accessed only through the current class’s methods. Hence, it is also known as data hiding. Encapsulation acts as a protective wrapper that prevents the code and data from being accessed by outsiders. These are controlled through a well-defined interface. 

Encapsulation is achieved by declaring the variables as private and providing public setter and getter methods to modify and view the variable values. In encapsulation, the fields of a class are made read-only or write-only. This method also improves reusability. Encapsulated code is also easy to test for unit testing.

Example:

class animal {
// private field 
private int age;
//getter method 
Public int getage ( ) {
return age;
}
//setter method 
public void setAge ( int age ) {
this. Age = age;
}
}
class Main {
public static void main (String args []);
//create an object of person 
Animal a1= new Animal ();
//change age using setter 
A1. setAge (12);
// access age using getter 
System.out.println(“ animal age is ” + a1. getage ( ) );
}
}

Output: Animal age is 12

In this example, we declared a private field called age that cannot be accessed outside of the class.

To access age, we used public methods. These methods are called getter and setter methods. Making age private allows us to restrict unauthorized access from outside the class. Hence this is called data hiding. 

Advanced OOP Concepts

Coupling and Cohesion

• Coupling: In OOP, coupling refers to the degree of direct knowledge one class has of another. This concept is crucial in understanding how classes interact within a system. There are two types of coupling – tight and loose.
  • Tight Coupling: Here, classes are highly dependent on each other. This is generally discouraged as it makes the system more rigid and less adaptable to change. For example, if Class A directly uses data or methods of Class B, any change in Class B can significantly impact Class A.
  • Loose Coupling: This is the more desirable form of coupling in OOP. In this scenario, classes are designed to interact with each other but do so in a way that reduces their direct dependencies. For instance, using interfaces or abstract classes can help achieve loose coupling, allowing a class to communicate with another class without needing to know the intricate details of its implementation.
• Cohesion: Cohesion refers to how closely related and focused the responsibilities of a class or module are. High cohesion is generally preferable as it indicates that a class is designed to do a specific job well without taking on responsibilities that should be delegated to other classes.
  • High Cohesion: In a highly cohesive system, each class has a clearly defined role, making the system easier to maintain, test, and understand. For example, a class designed solely for database interactions should not contain business logic or UI code.
  • Low Cohesion: Low cohesion suggests that a class performs a wide variety of actions and is not focused on a specific task. This often leads to classes that are difficult to understand, maintain, and can lead to redundant code.

Association, Aggregation, and Composition

• Association: This is a broad term that defines a relationship between two separate classes which is established through their objects. The association can be of various types like one-to-one, one-to-many, many-to-one, and many-to-many. For example, a single ‘Teacher’ object may be associated with multiple ‘Student’ objects in a one-to-many relationship.
• Aggregation: This is a specialized form of association where the relationship is a “has-a” type but the lifetime of the owned object doesn’t depend on the owner. It represents a whole-part relationship. For instance, a ‘Department’ class can have a list of ‘Professor’ objects, but the existence of ‘Professor’ objects is not strictly tied to the ‘Department’.
• Composition: This is also a type of association but with a stronger relationship. In composition, the lifetime of the owned object is managed by the owner, meaning if the owner object is destroyed, the owned objects are also destroyed. It represents a stronger whole-part relationship. A classic example is the relationship between a ‘House’ and ‘Room’; if the ‘House’ is destroyed, the ‘Rooms’ within it also cease to exist.

Coupling in Java

Coupling refers to the relationship between two classes. It indicates the knowledge one object or class has of another. That means that if one class changes its properties or behaviour, it will affect the dependent changes in the other class. Therefore, these changes will depend upon the level of interdependence the two classes have between them. There are two types of coupling, namely tight coupling, and loose coupling.

• Tight coupling: If one class is strongly interrelated to another class, it is said to have a tight coupling with that class. 

public class College{
public void status() {
System.out.println("College is open today");
}
}
public class Student{
College obj = new College();
public void goToCollege() {
obj.status();
}
}

In the above code example, the student class is dependent on the college class. That is, any change in the college class requires student classes to change. Here, therefore, student class and college class are tightly coupled with each other.

• Loose coupling: If one class is weakly interrelated to another class, it is said to have loose coupling with that class. Loose coupling is preferred over tight coupling. A class can achieve this with the help of interfaces, as shown below. 

public interface College{
void status();
}
class CollegeStatus1 implements College{
public void status() {
System.out.println("College is open monday to friday");
}
}
class CollegeStatus2 implements College{
public void status() {
System.out.println("College is open on saturday");
}
}
public class Student{
College obj = new CollegeStatus1();
public void goToCollege() {
obj.status();
}
}

In the above code example, CollegeStatus1 and CollegeStatus2 are loosely coupled. Here, student class is not directly or tightly coupled with a CollegeStatus1 or CollegeStatus2 class. By applying a dependency injection mechanism, the loose coupling implementation is achieved to allow a student to go to college with any class which has implemented a college interface. In addition, it means we can use CollegeStatus2 whenever the college is open on Saturday.

Cohesion in Java

Java Cohesion measures how the methods and the attributes of a class are meaningfully and strongly related to each other and how focused they are on performing a single well-defined task for the system. This is used to indicate the degree to which a class has a single, well-focused responsibility. More cohesive classes are good to keep them for code reusability. Low cohesive classes are difficult to maintain as they have a less logical relationship between their methods and properties. It is always better to have highly cohesive classes to keep them well focused for a single work.

• Low Cohesion: In the following code, we have a class called Book. But it is less cohesive because it comprises less focussed and independent attributes and methods to the class. This class should contain information related to the Book. Therefore, the person’s name and age method are making this classless cohesive.

class Book{
int price = 299; //related attribute
String name = "Sam"; //unrelated attribute
//related methods to Book class
public String author(String name) {
return name;
}
public String title(String subject) {
return subject;
}
public int id(int number) {
return number;
}
//unrelated methods to Book class
public int age(int age) {
return age;
}
}

• High Cohesion: When the class has a single well-defined purpose or task, it is said to be highly cohesive. So, in the above example code, if we remove the information related to the person, then the class becomes highly cohesive, as shown below.

class Book{
int price = 299; //related attribute
//related methods to Book class
public String author(String name) {
return name;
}
public String title(String subject) {
return subject;
}
public int id(int number) {
return number;
}
}

Association in Java

Association is a relation between two separate classes that establishes with the help of their Objects. It specifies the relationship between two or more Objects. Association can be one-to-one, one-to-many, many-to-one, and many-to-many. Let us understand this with real-world examples, suppose the relationship between the bus and the passengers. A bus can have only one driver(one-to-one). Many passengers can associate with the single bus(many-to-one). A single passenger can associate with many different buses(one-to-many). Also, many passengers can associate with the many different buses(many-to-many). One object is associated with another object to use the functionality and services provided by another object. 

Consider the following code below:

//class bus
class Bus
{
private String name;
// bus name
Bus(String name)
{
this.name = name;
}
public String getBusName()
{
return this.name;
}
}

//passenger class
class Passenger
{   
// passenger name
private String name;
// passenger seat id number
private int seatId;
Passenger(String name, int seatId)
{
this.name = name;
this.seatId = seatId;
}
public String getPassengerName()
{
return this.name;
}
public int getPassengerId()
{
return this.seatId;
}
}

//Association between both the
//classes in the main method
class Demo
{
public static void main (String[] args)
{
Bus bus = new Bus("Shree Travels");
        Passenger psg = new Passenger("Sneha", 52);
System.out.println(psg.getPassengerName() + " with seat number " + psg.getPassengerId()
+ " is a passenger of " + bus.getBusName());
}
}

Output:

Sneha with seat number 52 is a passenger of Shree Travels

Explanation:

In the above example, two separate classes Bus and Passenger, are associated through their Objects inside the class Demo. In this way, we can establish the relationship between two different classes by using the concept of association. A bus can have many passengers, So it is a one-to-many relationship.

Association is of two types, they are:
1. Aggregation
2. Composition

Let’s discuss the two in detail.

Aggregation

Java Aggregation is a weak association and represents a relationship between an object containing other objects. This represents a part of a whole relationship where a part can exist without a whole. Let’s take an example of the relationship between Group and Person. A Person may belong to multiple Groups. Hence a Group can have multiple Persons. But if we delete a Group, the Person object will not destroy. Aggregation represents the Has-A relationship, unidirectional association, i.e., a one-way relationship. For instance, the group can have persons, but vice versa is not possible and thus unidirectional. In this section, both entries can survive individually, which means ending one entity will not affect the other entity. Hence, both objects are independent in aggregation.

Considering the following code example:

import java.util.*;

//person class
class Person
{
private String name;
private int age ;
Person(String name, int age)
{
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
}

/* Group class contains the list of person
Objects. It is associated with the person
class through its Object(s). */

//group class
class Group
{
private String groupName;
private List<Person> persons;
Group(String groupName, List<Person> persons)
{
this.groupName = groupName;
this.persons = persons;
}
}

//main method
class Demo
{
public static void main (String[] args)
{   
//creating objects of person class
Person a = new Person("Tanmay", 17);
Person b = new Person("Sam", 18);
Person c = new Person("Pitu", 19);
Person d = new Person("Khushi", 20);
//making a list of persons belongs to social welfare group
List<Person> p1 = new ArrayList<>();
p1.add(a);
p1.add(c);
//making a list of persons belongs to drama fest group
List<Person> p2 = new ArrayList<>();
p2.add(b);
p2.add(d);
//creating objects of group class
Group swGrp = new Group("Social Welfare", p1);
Group dfGrp = new Group("Drama Fest", p2);
//before deleting drama fest group
System.out.println("List of persons in Drama Fest group:");
for(Person p : p2) {
System.out.println("Person name: " + p.getName() + ", Age:" + p.getAge() + ", Group: Drama Fest");
}
//deleting drama fest group
dfGrp = null;
//after deleting drama fest group
//person list will not destroy
System.out.println("List of persons after deleting Drama Fest group:");
for(Person p : p2) {
System.out.println("Person name: " + p.getName() + ", Age: " + p.getAge());
}
}
}

Output:

List of persons in Drama Fest group:

Person name: Sam, Age:18, Group: Drama Fest

Person name: Khushi, Age:20, Group: Drama Fest

List of persons after deleting Drama Fest group:

Person name: Sam, Age: 18

Person name: Khushi, Age: 20

Explanation:

Here, we can see that the two classes Person and Group, are associated with each other with the help of objects. There are two groups social welfare and drama fest. We created these groups by using the person class. The group has a list of persons. We have two people Sam and Khushi, in the Drama Fest group as shown in the output. Afterwards, we deleted this group by setting the instance of group equals to null. But, our list of persons remains undestroyed due to the weak association, i.e., aggregation, even after the group was deleted.

Composition in Java

Java Composition is an association that represents a part of a whole relationship where a part cannot exist without a whole. Let’s take an example of the relationship between School and Room. The school object consists of several rooms. Whenever the school object destroys automatically, all the room objects will be destroyed, i.e., without the existing school object, there is no chance of an existing dependent object. So these are strongly associated, and this relationship is called composition. If a whole is deleted, then all parts are deleted. So composition represents the part-of relationship. 

Whenever there is a composition between two entities, the created object cannot exist without the other object. Thus, in composition, both entities are dependent on each other.

Consider the following code example:

import java.util.*;   
// activity room class
class ActivityRoom {  
    public String subject;   
    public int id;   
    
    ActivityRoom(String subject, int id)   
    {   
        this.subject = subject;   
        this.id = id;   
    }   
    
}   
// department class   
class Department {   
private String name;
    //list of activity rooms in a department.   
    private List<ActivityRoom> ar; 
    
    Department(List<ActivityRoom> ar)  
    {  
        this.ar = ar;  
    }   
    // Getting total number of colleges  
    public List<ActivityRoom> getActivityRoomsInDepartment()   
    {   
        return ar;   
    }   
}   
class Demo {   
    public static void main(String[] args)   
    {   
        // Creating the Objects of activity room class.   
     ActivityRoom a1 = new ActivityRoom("Technical", 601);   
     ActivityRoom a2 = new ActivityRoom("Business", 602);   
     ActivityRoom a3 = new ActivityRoom("Economics", 603);  
     
        // making the list of activity rooms.   
        List<ActivityRoom> act = new ArrayList<ActivityRoom>();   
        act.add(a1);   
        act.add(a2);   
        act.add(a3);  
        
        // Creating the Object of department class. 
        Department d = new Department(act); 
        
        // making the list of activity rooms in department.   
        List<ActivityRoom> arlist = d.getActivityRoomsInDepartment();   
        for (ActivityRoom a : arlist) {   
            System.out.println(a.subject + " activity room with id " + a.id);   
        }  
        
    }   
}

Output:

Technical activity room with id 601

Business activity room with id 602

Economics activity room with id 603

Explanation:

Here we have two classes Activity room and Department. A department composed of different subject activity rooms. So, If the department gets destroyed, then All activity rooms within that department will be destroyed, i.e., the activity room can not exist without the department. That’s why it is composition.

Methods in Java

Java method is a block of code or collection of statements grouped together to complete a certain job or operation. This is used to achieve the reusability of code and can be utilized many times. It also gives easy modification and readability of code. A method is executed only when we call or invoke it. We have two categories of methods in java, i.e., pre-defined and user-defined. Predefined methods are the methods that are already defined in the Java class libraries. When a particular method is written by the user or programmer, it is known as a user-defined method. User-defined methods can be modified according to the requirement.

Let’s discuss:

• Static method in Java
• The abstract method in Java
• Finalize method in Java
• Equals method in Java

Static Method in Java

A method that has the static keyword in the declaration is known as the static method. In other words, a method that belongs to a class rather than an instance of a class is known as a static method. We can also create a static method by using the keyword static before the method name. The main benefit of a static method is that we can invoke the static method without even creating an object. It can access static data members and also change their values and is also used to create an instance method. The main() method is a common example of the static method.

Example:

public class Demo  
{  
public static void main(String[] args)   
{  
displaymethod();  
}  
static void displaymethod()   
{  
System.out.println("It is an example of static method.");  
}  
}  

Output:

It is an example of a static method.

Abstract Method in Java

A method that is declared with keyword abstract is called an abstract method. The abstract method does not have an implementation or body, or block of code. The abstract method must always be declared in an abstract class, or we can say that if a class has an abstract method, it should be declared abstract. If a class has an abstract method, it should be declared abstract, but vice versa is not true, which means that an abstract class doesn’t need to have an abstract method compulsory. Also, If a normal class extends an abstract class, then the class must have to implement all the abstract parent class’s abstract methods, or it has to be declared abstract.

Example:

//abstract class area
abstract class Area{
 /* These two are abstract methods, the child class
  * must implement these methods
  */
 public abstract int areaSquare(int s);
 public abstract int areaRectangle(int l, int b);
 //Normal method 
 public void display(){
System.out.println("Normal method in abstract class Area");
 }
}
//Normal class extends the abstract class
class Demo extends Area{

 /* If we don't provide the implementation of these two methods, the
  * program will throw compilation error.
  */
 public int areaSquare(int s){
return s*s;
 }
 public int areaRectangle(int l, int b){
return l*b;
 }
 public static void main(String args[]){
Area a = new Demo();
System.out.println("Area of square " + a.areaSquare(9));
System.out.println("Area of rectangle " + a.areaRectangle(3,4));
a.display();
 }
}

Output:

Area of square 81

Area of rectangle 12

The normal method in abstract class Area

Final Method in Java

A method that is declared final is called a final method. We cannot override a final method. This means the child class can still call the final method of the parent class without any problem, but it cannot override it. This is because the main purpose of making a method final is to stop the modification of the method by the sub-class.

Example:

class DemoParent{  
final void method(){
System.out.println("Parent class final method");
}  
}  
     
class Demo extends DemoParent{  
//error
void method(){
System.out.println("final method modified inside child class");
}  
     
public static void main(String args[]){  
Demo d = new Demo();  
d.method();  
}  
}

The above code will throw an error as we are trying to modify the final method inside the child class(demo) of the parent class(demoParent).

Instead of modifying the method, we can use it as shown below:

class DemoParent{  
final void method(){
System.out.println("Parent class final method");
}  
}  
     
class Demo extends DemoParent{
public static void main(String args[]){  
Demo d = new Demo();  
d.method();  
}  
}

Output:

Parent class final method

Equals Method in Java

As the name suggests in java, .equals() is a method used to compare two objects for equality. The .equals() method in java is used to check if the two strings have similar values. It checks them character by character. One should not confuse .equals() method with == operator. The String equals() method compares the two given strings based on the content of the string, whereas the == operator is used for address comparison. If all the contents of both the strings are the same, then .equals() returns true otherwise, it returns false. If all characters are not matched, then it returns false. 

Let us understand this with the help of an example:

public class Demo {
    public static void main(String[] args)
    {
        String s1 = "GreatLearning";
        String s2 = "GreatLearning";
        String s3 = new String("GreatLearning");
        System.out.println(s1 == s2); // true
        System.out.println(s1 == s3); // false
        System.out.println(s1.equals(s2)); // true
        System.out.println(s1.equals(s3)); // true
    }
}

Even though s1 and s3 are created with the same field(content), they are pointing to two different objects in memory. Hence at different addresses. Therefore == operator gives false and .equals() method gives true as both contain similar content greatLearning.

Message Passing in Java

Message Passing in terms of computers is a communication phenomenon between the processes. It is a kind of communication used in object-oriented programming. Message passing in Java is the same as sending an object, i.e., a message from one thread to another thread. It is utilized when threads do not have shared memory and are not able to share monitors or any other shared variables to communicate. In message passing calling program sends a message to a process and relies on that process to run its own functionality or code. Message passing is easy to implement, has faster performance, and we can build massive parallel models by using it. 

There are two types of it: Synchronous and Asynchronous.

• Synchronous message passing occurs when the objects run at the same time.
• In the case of an Asynchronous message passing, the receiving object can be down or busy when the requesting object sends the message.

Can Polymorphism, Encapsulation and Inheritance work together?

When we combine inheritance, polymorphism and encapsulation to produce a programming environment, this environment supports the development of far more robust and scalable programs that do the program-oriented model. A well-designed or model of the hierarchy of classes is the basis for reusing the code in which we have spent our time and effort developing and testing.  Encapsulation allows us to migrate our implementations over time without breaking that code which depends on our classes’ public interfaces. Polymorphism allows us to create readable, clean, sensible code.

As we know, it is through the applications of encapsulation, polymorphism and inheritance that individual parts are transformed into an object; for example, it may be a car, mobile phone etc. This is true in the case of computer programs. Through object-oriented principles, the various parts of complex programs are brought together to form a cohesive, robust, maintainable whole.

Many of the features supplied by java are part of its built-in class libraries which do use encapsulation, polymorphism, and inheritance extensively. 

Let us consider a real-world example. Humans are a form of inheritance from one standpoint, whereas cars are more like programs we write. All drivers rely on inheritance to drive different types of vehicles. People interface with the features of cars of all types as we have many different types of vehicles, and some have differences. The implementation of engines, brakes etc., comes under encapsulation and finally comes to polymorphism. We get a wide area of options on the same vehicle as to the anti-lock braking system, traditional braking system or power braking system. The same vehicle as many forms of the braking system is called polymorphism. This example shows us how encapsulation, inheritance and polymorphism are combined.   

Advantages of OOPs Concept 

Some of the advantages are:

• Re-usability

When we say re-usability, it means that “write once, use it multiple times” i.e., reusing some facilities rather than building it again and again, which can be achieved by using class. We can use it n number of times whenever required.

• Data redundancy 

It is one of the greatest advantages in oops. This is the condition which is created at the data storage when the same piece of data is held at two different places. If we want to use similar functionality in multiple classes, we can just write common class definitions for similar functionalities by inheriting them.

• Code maintenance

It is easy to modify or maintain existing code as new objects which can be created with small differences from the existing ones. This helps users from doing rework many times and modifying the existing codes by incorporating new changes to it.

• Security

Data hiding and abstraction are used to filter out limited exposure which means we are providing only necessary data to view as we maintain security.

• Design benefits 

The designers will have a long and more extensive design phase, which results in better designs. At a point of time when the program has reached critical limits, it will be easier to program all non-oops separately.

• Easy troubleshooting

Using encapsulation objects is self-constrained. So, if developers face any problem easily it can be solved. And there will be no possibility of code duplicity. 

• Flexibility 
• Problem-solving

Disadvantages of OOPs Concept 

• Effort – A lot of work is put into creating these programs.

• Speed – These programs are slower compared to other programs.

• Size – OOPs programs are bigger when compared to other programs.

Differences between Object-Oriented Programming, Procedural Oriented Programming?

You can learn more about oops concepts by taking a free course in oops concepts in C++.

Difference between an object-oriented programming language and an object-based programming language?

Comparison with Other Languages: Java vs. Python vs. C++

While Java, Python, and C++ all support OOP, their approaches and capabilities differ significantly, catering to different programming needs and scenarios. Java offers a pure OOP experience with a strong emphasis on safety and portability. Python provides a more flexible but less structured OOP approach, making it ideal for rapid development. C++, on the other hand, offers a powerful but complex OOP experience with its close-to-hardware capabilities.

Best Practices in Using OOP Principles in Java

1. Follow SOLID Principles:
   • Single Responsibility: Each class should have one and only one reason to change, meaning it should only have one job.
   • Open/Closed: Classes should be open for extension but closed for modification.
   • Liskov Substitution: Subclasses should be substitutable for their base classes.
   • Interface Segregation: Prefer many smaller, client-specific interfaces over one large, general-purpose interface.
   • Dependency Inversion: Depend on abstractions, not concretions.
2. Use Access Modifiers Effectively:
   • Properly utilize private, protected, and public access modifiers to encapsulate data and methods.
3. Implement Proper Encapsulation:
   • Avoid exposing class internals unnecessarily. Use getters and setters where appropriate.
4. Prefer Composition Over Inheritance:
   • Utilize composition to build complex functionalities using simple classes. Inheritance can lead to a rigid system, whereas composition offers more flexibility.
5. Avoid Deep Inheritance Trees:
   • Deep inheritance hierarchies can become hard to manage and understand. Prefer shallower hierarchies.
6. Use Polymorphism Wisely:
   • Apply polymorphism to enhance code flexibility and readability, but avoid overusing it, which can lead to complex and hard-to-track code.
7. Understand and Implement Design Patterns Appropriately:
   • Familiarize yourself with common design patterns and use them where applicable, but don’t force a pattern where it isn’t needed.

Common Mistakes to Avoid in OOP with Java

1. Overusing Inheritance:
   • Avoid using inheritance for code reuse alone. Inheritance should model a clear hierarchy.
2. Ignoring Encapsulation:
   • Exposing all class fields publicly and not using getters and setters can lead to maintenance issues.
3. Creating God Objects:
   • Avoid creating classes that try to do too much (known as God Objects). This is a sign of poor class design and low cohesion.
4. Misusing Polymorphism and Abstract Classes:
   • Implementing unnecessary polymorphic structures or abstract classes can complicate the codebase.
5. Neglecting the Principle of Least Knowledge (Law of Demeter):
   • A class should not know about the inner details of the objects it manipulates.
6. Not Using Interfaces Appropriately:
   • Avoid not leveraging interfaces to define clear contracts for your classes.
7. Forgetting about Code Smells and Refactoring:
   • Regularly refactor your code to address code smells – indicators of deeper problems in the code.

Performance Considerations in OOP and Java Code Optimization

Understanding the impact of Object-Oriented Programming (OOP) concepts on performance is crucial for optimizing Java code. While OOP offers significant benefits in terms of code readability, maintainability, and scalability, it can also introduce performance overheads if not used judiciously.

Impact of OOP Concepts on Performance

1. Memory Usage:
   • Objects and Classes: Each object in Java occupies memory. Excessive use of objects, especially large ones, can lead to increased memory consumption and reduced application performance.
   • Inheritance and Polymorphism: Deep inheritance hierarchies and extensive use of polymorphism can lead to increased memory usage and slower runtime performance due to dynamic method dispatch.
2. CPU Overhead:
   • Method Calls: Frequent method calls, especially in deep inheritance structures or complex polymorphic scenarios, can introduce CPU overhead.
   • Encapsulation and Accessor Methods: Overuse of getters and setters for encapsulation can result in additional method calls, impacting CPU performance.

Optimizing Java Code for Performance

1. Efficient Use of Objects:
   • Minimize object creation, especially in performance-critical sections of the code. Use object pooling or flyweight patterns for frequently used objects.
2. Optimizing Inheritance and Polymorphism:
   • Prefer composition over inheritance where appropriate to reduce complexity and memory footprint.
   • Use interfaces wisely and avoid deep inheritance trees to reduce the overhead of dynamic method dispatch.
3. Effective Memory Management:
   • Monitor and manage memory usage actively. Utilize Java profiling tools to identify and fix memory leaks.
   • Optimize data structures, preferring efficient collections like ArrayList over LinkedList when random access is frequent.
4. Code Refactoring for Performance:
   • Refactor code to eliminate unnecessary method calls, especially in loops or frequently called methods.
   • Use inline code for trivial methods where appropriate.
5. Lazy Initialization and Eager Loading:
   • Use lazy initialization for heavy resources when their immediate loading might impact startup performance.
   • Conversely, use eager loading to pre-load resources and reduce latency in critical sections of the application.
6. Avoiding Premature Optimization:
   • Focus on writing clean, maintainable OOP code initially.
   • Optimize only after identifying bottlenecks through profiling.
7. Leveraging Modern JVM Features:
   • Stay updated with the latest Java versions, which often include performance enhancements in the JVM.
   • Utilize JIT compilation and understand how Java’s HotSpot VM optimizes code at runtime.
8. Concurrency and Multithreading:
   • Use multithreading judiciously to improve application performance, especially for I/O bound or computationally intensive operations.
   • Ensure thread safety and avoid common pitfalls like deadlock or race conditions.

OOps in Java FAQ

Engaging in the study of Java programming suggests a keen interest in the realm of software development. For those embarking upon this journey with aspirations towards a career in this field, it is recommended to explore the following pages in order to acquire a comprehensive understanding of the development career path. To further enhance your skills and knowledge, explore online courses for Java programming and software development, which will provide a solid foundation for your career.