Classes and Objects in Java OOPs: Learn with Examples

Java Class and Object: An Overview

Fundamental ideas in Java programming are Java Classes and objects, which resemble real-world things with states, behaviors, and characteristics. Classes, which include variables to hold object states and methods to specify their behavior, allow programmers to model their code after actual entities in the real world. Java's natural object-oriented programming capabilities are built on this powerful combination, which makes writing easier. If you want to get started with Java, you can check out classes and objects in Java, Java class and object examples, and class and object programs in Java in this helpful Java tutorial to kickstart your learning journey

Java Class

An essential part of the Java language is the class, which contains objects with similar properties and specifies their data and behavior. It provides structure and efficiency for Java developers and serves as the blueprint for building objects. Abstraction in Java is an important aspect of object-oriented programming, where an object is an abstract concept in Java; it has no physical representation like variables or other types of data but instead acts as a template for entities being modeled within a program. Java classes provide the framework for building models that incorporate essential concepts like inheritance, polymorphism, abstraction, & encapsulation. They act as the design manual for the construction of objects, making Java development essential.

Syntax:

access_modifier class<class_name>
{ 
    data member; 
    method; 
    constructor;
    nested class;
    interface;
}

Example of the class program in Java

class Student {
 int id; // data member (also instance variable)
 String name; // data member (also instance variable)

 public static void main(String args[])
 {
  Student s1 = new Student(); // creating an object of
         // Student
  System.out.println(s1.id);
  System.out.println(s1.name);
 }
}

A Student is defined by this Java class and has two instance variables (name and id). In the main method, a Student (s1) instance is created, and its uninitialized variables—which most likely have default values of 0 for id and null for name—are printed.

How to create a class in Java?

• Creating a class in Java is a simple yet powerful tool in the programming language.
• It involves listing classes, defining class variables and methods, and creating objects with the design of the Java class.
• The Constructor in Java plays a crucial role in this process.
• The create class process starts with writing the code for a Java file, making sure to specify that it is a public class.
• This allows other programs to find what that particular program consists of and how it should be used.
• After completing this step, create as many variables and methods as needed for the program.
• Finally, create an object from the original public java class and access its variables or methods for use in that application.
• With this information at hand, anyone can create Java classes efficiently with only a few steps to follow.

Object in java

• Object in Java is an important concept to understand when learning the Java programming language.
• Object in Java allows programmers to store and process data, making it a powerful and popular programming language.
• Object in Java consists of a hierarchy of classes that are used to create and define different types of objects which can then be manipulated by code.
• Object in Java is also used for Object Oriented Programming (OOP) as it enables programmers to break down complex problems into smaller subproblems that can be solved independently.
• Object in Java can be reused easily, allowing developers to save time and avoid rewriting the same code multiple times.
• Object in Java is versatile and easy to learn, making it a great choice for beginners and experienced coders alike.

How to create an object in Java?

There are many different ways to create object in Java, those are

Define a class:

public class Car {
   // Class variables
   String color;
   int modelYear;
   // Class constructor
   public Car(String color, int modelYear) {
      this.color = color;
      this.modelYear = modelYear;
   }
   // Class methods
   public void start() {
      System.out.println("The car is starting.");
   }
   public void stop() {
      System.out.println("The car is stopping.");
   }
}

The "Car" Java class provides methods to start and stop the vehicle as well as class variables for color and model year. It encompasses the characteristics and actions of cars.

Create an instance of the class:

• Once the developers have defined the class, then anyone can create an instance of the class by using the "new" keyword.
• For example, to create an instance of the Car class, can do the following:
• Car myCar = new Car("red", 2022);
• This will create a new instance of the Car class with the color "red" and model year 2022, and assign it to the variable "myCar".

Access the object:

• Once the developers have created the object, the user can access its variables and methods by using the dot (.) notation.
• For example, to access the color variable of the "myCar" object, anyone can do the following:
• String carColor = myCar.color;
• This will assign the value of the "color" variable of the "myCar" object to the "carColor" variable.
• Similarly, to call the "start" method of the "myCar" object, anyone can do the following:
• myCar.start();
• This will call the "start" method of the "myCar" object, which will print the message "The car is starting." to the console.

Different ways to create objects in Java

Using the new keyword: 

Example:

ClassName objectName = new ClassName();

With this line of code, an object of the class "ClassName" is created and assigned to the variable "objectName."

Using Class.forName() method: 

Example:

Class className = Class.forName("ClassName");
ClassName objectName = (ClassName) className.newInstance();

In this code, the class "ClassName" is dynamically loaded by "Class.forName("ClassName")" and then created by "className.newInstance()" and assigned to the variable "objectName" after being cast to the proper type. 

Using clone() method:

Example:

ClassName object1 = new ClassName();
ClassName object2 = object1.clone();

According to this example, "object1" generates an instance of "ClassName," whereas "object2 = object1.clone()" makes a new instance of "object2" that is a copy or clone of "object1." You can duplicate objects with the same characteristics and behaviors thanks to this.

Using object deserialization: 

Example:

ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);
ClassName objectName = (ClassName) objectInputStream.readObject();

In this example, the "ObjectInputStream" is initially set up to read serialized data from the "inputStream" and to deserialize that data into an instance of the "ClassName" class, which is then assigned to the variable "objectName." 

Using Factory Method:

Example:

public static ClassName createObject() {
    return new ClassName();
}
ClassName objectName = ClassName.createObject();

In the "ClassName" class, the static function "createObject" in this example defines a brand-new instance of "ClassName." The "createObject" function is then used, which is a standard pattern for generating objects with specified initialization logic, to create an instance of "ClassName" called "objectName".

Using Dependency Injection:

Example:

public class MyClass {
    private Dependency dependency;
    public MyClass(Dependency dependency) {
        this.dependency = dependency;
    }
}
MyClass objectName = new MyClass(new Dependency());

In this case, a constructor that initializes a private instance variable named "Dependency" and a class named "MyClass" is defined. Then, it establishes a dependency between the two classes by creating an instance of "objectName" of "MyClass" with a fresh instance of "Dependency" as a constructor argument.

Anonymous object in Java

• Anonymous objects in Java are incredibly useful for instantiating an object and using it within a single statement.
• They are typically used when an object is needed briefly, such as during method or constructor calls.
• These objects can be used to call methods or access fields without explicitly creating a variable for it, and often make coding simpler.
• Anonymous objects provide flexibility since many abstract classes do not require concrete implementations.
• Anonymous objects are incredibly practical when it comes to streamlining code and simplifying scripting through their ability to hide complexity and provide clean output with just the information required by the programmer.

Java Objects

Java objects, which represent actual real-world objects, are instances of classes. They include data (attributes) and behaviors (methods) associated with a particular class.

Declaring Objects (Also called instantiating a class)

In Java, declaring an object entails making instances of a class. This includes naming a variable, the class type, and the class constructor before using the "new" keyword.

Example

ClassName objectName = new ClassName();

• ClassName: ClassName is the name of the class.
• objectName: objectName is the name of the variable that holds the object.
• new ClassName(): new ClassName() creates a new instance of the class and assigns it to objectName.

Initializing a Java object

When a Java object is initialized, its initial state is established by giving its attributes (data members) values. Usually, constructors or setter methods are used for this. 

Example

public class Student {    private String name;
    private int age;    // Constructor to initialize the object
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
}// Initializing a Student object
Student student = new Student("Mahesh", 20);

The "Student" class is defined by this code and has a private name and age attributes as well as a constructor to initialize them. It then establishes the starting state of a "student" object with the name "Mahesh" and the age of 20.

Difference between class and object

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