Class And Object In Java

Java Functions And Methods

Java Foundation

Java Flow Control

Java Inner Classes

Java 8 Features

Java File I/o

Java Multithreading

Java Arrays And Strings

Java Exception Handling

Java Classes

Java OOP's

Java Collections Framework

Introduction to Java Class and Object:

In Java, the fundamental building blocks of object-oriented programming are classes and objects. These concepts help us to model real-world entities and their interactions in a systematic and organized way.

Class

A class in Java serves as a blueprint or template for creating objects. It encapsulates data for the object and methods to manipulate that data. Essentially, a class defines the properties (attributes) and behaviors (methods) that its objects will have.

Object

An object is an instance of a class. While a class is a conceptual entity, an object is a concrete entity that exists in memory during runtime. Objects have state and behavior, which are defined by their class.

Class

What is a Class?

A class in Java is a user-defined data type that acts as a blueprint for objects. It defines the attributes and behaviors that the objects created from the class will have. Classes encapsulate data and operations on data into a single unit.

Attributes (Fields): These are variables within a class that hold the data or state of the objects. Fields can be of any data type, including other classes.
Methods: These are functions within a class that define the behavior of the objects. Methods can perform operations on the fields and may return values.

Purpose of a Class

The primary purpose of a class is to represent real-world entities in Java applications. By using classes, we can create multiple objects that share the same properties and behaviors.

Representation of Entities: Classes are used to represent entities such as Student, Employee, Product, Account, etc.
Data Representation: To represent the data of these entities, we use variables (fields) within the class.
Behavior Representation: To represent the behaviors or actions of these entities, we use methods within the class.

Class Analogy

Think of a class as a blueprint for a house. The blueprint defines the structure, layout, and design of the house. It specifies details such as the number of rooms, the layout, and other architectural features. However, the blueprint itself is not a house; it is just a plan for creating houses.

Code Example

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public class Student {    // Attributes (Fields)    private String name;    private int age;    private String studentId;​    // Constructor    public Student(String name, int age, String studentId) {        this.name = name;        this.age = age;        this.studentId = studentId;    }​    // Methods    public void study() {        System.out.println(name + " is studying.");    }​    public void attendClass() {        System.out.println(name + " is attending class.");    }​    // Getter and Setter methods    public String getName() {        return name;    }​    public void setName(String name) {        this.name = name;    }​    public int getAge() {        return age;    }​    public void setAge(int age) {        this.age = age;    }​    public String getStudentId() {        return studentId;    }​    public void setStudentId(String studentId) {        this.studentId = studentId;    }}

Object

What is an Object?

An object is an instance of a class. It is a concrete entity that exists in memory during runtime. Objects have a state (defined by the values of their attributes) and behavior (defined by the methods of the class).

State: The state of an object is the values of its attributes at any given time.
For example
```
for a Student object, the state could be the name, age, and student ID.
```
Behavior: The behavior of an object is the actions that the object can perform, as defined by the methods of the class. For example,
```
a Student object can perform actions such as studying or attending a class.
```

Object is Real, Class is Virtual

While a class is a virtual concept (a blueprint), an object is a real entity that exists during the execution of a program.

Object Analogy

Continuing with the house analogy, an object is like an actual house built from the blueprint. Each house (object) built from the same blueprint (class) can have different states (e.g., different paint colors, furniture) but follows the same design.

Code Example

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public class Main {    public static void main(String[] args) {        // Creating an object of the Student class        Student student1 = new Student("Alice", 20, "S12345");​        // Accessing attributes and methods of the object        System.out.println("Student Name: " + student1.getName());        System.out.println("Student Age: " + student1.getAge());        System.out.println("Student ID: " + student1.getStudentId());​        student1.study();        student1.attendClass();​        // Modifying attributes of the object        student1.setAge(21);        System.out.println("Updated Student Age: " + student1.getAge());    }}

Difference Between Class and Object

Feature	Class	Object
Definition	Blueprint or template for creating objects	Instance of a class
Nature	Conceptual entity	Physical entity that exists in memory
Creation	Defined using `class` keyword	Created using `new` keyword
Attributes	Defines attributes (fields) for objects	Holds specific values in attributes
Methods	Defines behaviors (methods) for objects	Can invoke methods defined in the class
Memory Allocation	No memory allocation at definition	Memory is allocated when an object is created
Example	`public class Car { String color; void drive() { ... } }`	`Car car1 = new Car(); car1.color = "Red"; car1.drive();`