Classes and objects are the building blocks of object-oriented programming in Java. A class defines the properties and behaviors of something, while an object represents an actual instance of that class.
In this article, you will learn what classes and objects are, how to create them, how they work together, and how they differ from each other.

Table of Contents

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1. What is a Class in Java?

A class in Java is a blueprint or template used to create objects. It defines the properties and behavior that objects of that class can have.
For example, a Car class can define properties such as color and speed, along with behaviors such as drive() and brake().

class Car {
    String color;
    int speed;
    void drive() {
        System.out.println("The car is driving.");
    }
}

Here, color and speed are variables, while drive() is a method defined inside the class.

2. What is an Object in Java?

An object in Java is an instance of a class. It represents a specific entity and contains its own values for the properties defined by the class.
For example, if Car is a class, you can create a Car object like this:

Car myCar = new Car();

Here, myCar is an object of the Car class. It can access the variables and methods defined in that class.
In simple terms, a class is a blueprint, while an object is something created from that blueprint.

3. How Classes and Objects Work Together?

A class defines what an object will contain and what it can do, while an object uses those definitions in a program.
For example:

class Student {
    String name;
    void study() {
        System.out.println(name + " is studying.");
    }
}
class Main {
    public static void main(String[] args) {
        Student student1 = new Student();
        student1.name = "Alex";
        student1.study();
    }
}

Here, Student is the class, while student1 is an object created from that class. The object can access the name variable and study() method defined in Student.
In short, a class provides the structure, and objects use that structure to represent individual instances.

4. How to create a Class in Java?

You can create a class in Java using the class keyword, followed by the class name and its body.
Syntax: 

class ClassName {
    // Variables
    // Methods
}

Example:

class Student {
    String name;
    int age;
    void displayInfo() {
        System.out.println(name + " is " + age + " years old.");
    }
}

In this example, Student is the class, while name and age are variables that describe a student. The displayInfo() method defines a behavior for the class.
Once a class is created, you can use it to create objects in your program.

5. How to create Objects in Java?

Objects are created from a class using the new keyword. The object can then access the variables and methods defined in its class.

 Syntax:

ClassName objectName = new ClassName();

Example:

Student student1 = new Student();
student1.name = "Alex";
student1.age = 20;
student1.displayInfo();

Here, student1 is an object of the Student class. The new Student() expression creates the object, while student1 stores its reference.
You can create multiple objects from the same class, and each object can have different values.

6. Class vs Object

Classes and objects are closely related, but they serve different purposes. A class defines the structure and behavior, while an object is an instance created from that class.

Parameter	Class	Object
Definition	A Class is a blueprint to create objects.	An Object is an instance of class.
Purpose	A class defines the properties and behaviors of objects.	An object uses those properties and behaviors in a program.
Creation	A class is declared using the class keyword.	An object is commonly created using the new keyword.
Memory	A class itself does not represent an individual object in memory.	An object occupies memory when it is created.
Variables	A class defines variables that its objects can have.	An object contains its own values for those variables.
Methods	A class defines methods that describe possible behaviors.	An object can call the methods defined by its class.
Example	class Student { } defines a Student class.	Student s1 = new Student ( );  creates a Student object.
Relationship	One class can be used to create multiple objects.	Each object belongs to a particular class.

In short, a class provides the blueprint, while an object is the actual instance created from that blueprint.

7. Instance Variables and Methods

Instance variables and methods are important parts of a Java class. They define the data and behavior that objects created from the class can have.

Instance Variables: 
Instance variables are variables declared inside a class but outside its methods. Each object gets its own copy of these variables.

class Student {
    String name;
    int age;
}

Here, name and age are instance variables. Different Student objects can have different values for them.

Instance Methods:
Instance methods are methods defined inside a class that describe what an object can do.

class Student {
    String name;
    void study() {
        System.out.println(name + " is studying.");
    }
}

Here, study() is an instance method that can be called using a Student object.

Student student1 = new Student();
student1.name = "Alex";
student1.study();

In short, instance variables store an object's data, while instance methods define its behavior.

8. Constructors in Classes

A constructor is a special method used to initialize an object when it is created. It has the same name as the class and does not have a return type.
Example

class Student {
    String name;
    int age;
    Student(String studentName, int studentAge) {
        name = studentName;
        age = studentAge;
    }
}

You can use this constructor while creating an object:

Student student1 = new Student("Alex", 20);

Here, the constructor assigns "Alex" to name and 20 to age when the object is created.
Java also provides a default constructor when no constructor is explicitly defined in the class.
In short, constructors make it easier to initialize objects with their required values when they are created.

9. Real-World Examples, Common Mistakes and Best Practises

Classes and objects can represent real-world entities in Java programs. For example, an Employee class can define properties such as name and salary, while each employee object can store different values.

class Employee {
    String name;
    double salary;
    void displayInfo() {
        System.out.println(name + " earns " + salary);
    }
}
class Main {
    public static void main(String[] args) {
        Employee emp1 = new Employee();
        emp1.name = "Alex";
        emp1.salary = 50000;
        emp1.displayInfo();
    }
}

Common Mistakes: 

1. Confusing classes and objects: A class is a blueprint, while an object is an instance of that class.
2. Forgetting new: Objects are commonly created using the new keyword.
3. Using incorrect class names: Class names should follow Java naming conventions, such as Student instead of student.
4. Accessing objects incorrectly: Variables and methods should be accessed through a valid object reference when they are instance members.

Best Practices:

1. Give classes clear and meaningful names.
2. Keep each class focused on a specific purpose.
3. Use constructors to initialize objects when appropriate.
4. Use meaningful names for variables and methods.
5. Keep data protected by using appropriate access modifiers such as private.

10. Conclusion 

Classes and objects form the foundation of object-oriented programming in Java. A class defines the structure and behavior, while objects provide usable instances of that class.
Understanding how to create and use them makes it easier to build organized, reusable, and maintainable Java programs.

11. Frequently Asked Questions

1. What is a class in Java?

A class is a blueprint that defines the properties and behaviors of objects.

2. What is an object in Java?

An object is an instance of a class that can use its variables and methods.

3. How do you create an object in Java?

You can commonly create an object using the new keyword, such as Student s1 = new Student();.

4. Can one class have multiple objects?

Yes, you can create multiple objects from the same class, with each object having its own values.