Master Core Java Programming From Scratch

What is Object-Oriented Programming?

Object-Oriented Programming, or OOP, is a programming approach that organizes software around objects containing state and behavior.

Java is primarily an object-oriented programming language. Classes define the structure and behavior of objects, while objects represent actual instances created from those classes.

What is a Class?

A class is a blueprint or template that defines the data and behavior that objects of that class can have.

class Student {

    String name;
    int age;

    void study() {

        System.out.println(name + " is studying");

    }

}

Here, Student is a class containing two fields and one method.

What is an Object?

An object is an instance of a class. It contains its own state and can use the behavior defined by its class.

Student student = new Student();

The new keyword creates an object, and the variable student stores a reference to that object.

Class vs Object

Fields

Fields are variables declared inside a class. They represent the state or properties of an object.

class Employee {

    String name;
    int age;
    double salary;

}

Methods

Methods define behavior that an object or class can perform.

class Employee {

    String name;

    void work() {

        System.out.println(name + " is working");

    }

}

Complete Class Example

class Student {

    String name;
    int age;

    void displayDetails() {

        System.out.println("Name: " + name);
        System.out.println("Age: " + age);

    }

}

Creating Objects

public class Main {

    public static void main(String[] args) {

        Student student = new Student();

        student.name = "Samadhan";
        student.age = 22;

        student.displayDetails();

    }

}

Name: Samadhan
Age: 22

Multiple Objects

A single class can be used to create many independent objects.

Student first = new Student();

first.name = "Sejal";
first.age = 21;


Student second = new Student();

second.name = "Pradnya";
second.age = 18;

Both objects have the same structure but maintain their own field values.

Object References

Variables that store objects normally contain references to those objects rather than the objects themselves.

Student student = new Student();

student.name = "Sejal";

student
   │
   │ reference
   ↓
┌────────────────────┐
│ Student Object     │
│ name = "Sejal"     │
│ age  = 0           │
└────────────────────┘

null Object Reference

A reference variable can contain null, meaning it does not currently refer to an object.

Student student = null;

Accessing Class Members

The dot operator . is used to access accessible fields and methods through an object reference.

Student student = new Student();

student.name = "Sejal";

student.displayDetails();

Default Values of Instance Fields

Instance fields receive default values when an object is created if they are not explicitly initialized.

Instance Members

A non-static field or method belongs to an object instance. Each object can have different instance-field values.

class Car {

    String color;

    void displayColor() {

        System.out.println(color);

    }

}

Car car1 = new Car();
Car car2 = new Car();

car1.color = "Red";
car2.color = "Blue";

car1.displayColor();
car2.displayColor();

Red
Blue

Static Members

A static field belongs to the class rather than to each individual object.

class Counter {

    static int count = 0;

    Counter() {

        count++;

    }

}

public class Main {

    public static void main(String[] args) {

        new Counter();
        new Counter();
        new Counter();

        System.out.println(Counter.count);

    }

}

3

The this Keyword

The this keyword refers to the current object. It is commonly used when a parameter has the same name as an instance field.

class Student {

    String name;

    void setName(String name) {

        this.name = name;

    }

}

State and Behavior

Object Creation Flow

Class definition
       ↓
new keyword
       ↓
Object created
       ↓
Constructor executes
       ↓
Reference returned
       ↓
Object can be used

Constructors are discussed in detail in the next lesson.

Example: CIIT Student👨‍🏫🤓

Consider a student-management example at CIIT Institute. The CIITStudent class represents a student enrolled in a course and stores student details and pending course fees.

class CIITStudent {

    String studentName;
    String courseName;
    double pendingFees;

    void payFees(double amount) {
        pendingFees -= amount;
        System.out.println("Rs. " + amount + " successfully paid for " + courseName + " course.");
    }

    void displayFeeStatus() {
        System.out.println("\n--- CIIT Institute Fee Receipt ---");
        System.out.println("Student Name : " + studentName);
        System.out.println("Course Name  : " + courseName);
        System.out.println("Pending Fees : Rs. " + pendingFees);
        System.out.println("----------------------------------");
    }
}

public class Main {

    public static void main(String[] args) {

        CIITStudent student = new CIITStudent();

        student.studentName = "Sam";
        student.courseName = "Core Java";
        student.pendingFees = 15000.0;

        student.displayFeeStatus();

        System.out.println("\n[Action: Student pays installment]");

        student.payFees(5000.0);

        student.displayFeeStatus();
    }
}

Output

--- CIIT Institute Fee Receipt ---
Student Name : Sam
Course Name  : Core Java
Pending Fees : Rs. 15000.0
----------------------------------

[Action: Student pays installment]
Rs. 5000.0 successfully paid for Core Java course.

--- CIIT Institute Fee Receipt ---
Student Name : Sam
Course Name  : Core Java
Pending Fees : Rs. 10000.0
----------------------------------

Best Practices

• Use meaningful class names such as Student, Employee, or BankAccount.
• Follow Java naming conventions.
• Keep one clear responsibility for each class.
• Prefer private fields with controlled access in production-oriented object models.
• Avoid unnecessary static state.

Interview Questions