Complete Java Course ☕
Master Java from basics to Object-Oriented Programming
Level 1: Introduction to Java
What is it? Java is a robust, high-level, class-based programming language.
Features:
- Platform Independent: Write Once, Run Anywhere (WORA).
- Object-Oriented: Everything is organized around objects and data.
Where Java is used: Android apps, enterprise software, large-scale web applications, and embedded systems.
Try This 🔥
Think about an Android app you use daily. It was likely built using Java!
✅ You learned this concept!
Level 2: Basic Program Structure
What is it? Every Java program must be enclosed in a class and contain a main method to execute.
public class Main
{
public static void main(String[] args)
{
System.out.println("Hello World");
}
}
Explanation:
class: A blueprint for the code.main method: The starting point where the program begins executing.System.out.println: The command used to print text to the screen.
Try This 🔥
Change "Hello World" to your own name and observe the output.
✅ You wrote your first Java program!
Level 3: Variables & Data Types
What is it? Variables are containers for storing data. In Java, you MUST specify the data type.
int age = 18;
double marks = 85.5;
char grade = 'A';
String name = "Rajan";
Explanation: int holds whole numbers, double holds decimals, char holds a single character in single quotes, and String holds text in double quotes.
Try This 🔥
Create a String variable for your favorite subject and an int for your expected score.
✅ You learned this concept!
Level 4: Operators
What is it? Symbols used to perform mathematical or logical operations.
int x = 10;
int y = 5;
int result = x + y;
Explanation: Java supports standard operators like + (add), - (subtract), * (multiply), and / (divide).
Try This 🔥
Change the operator to * and print the result using System.out.println(result);.
✅ You learned this concept!
Level 5: Conditionals
What is it? Blocks of code that only run if a specific condition is evaluated as true.
if (x > 0)
{
System.out.println("Positive");
}
Explanation: We use the if keyword to test a condition inside parentheses (). If true, the code inside the curly braces {} runs.
Try This 🔥
Add an else block to handle the scenario where x is less than 0.
✅ You learned this concept!
Level 6: Loops
What is it? Structures that repeat a specific block of code as long as a condition is met.
for (int i = 0; i < 5; i++)
{
System.out.println(i);
}
Explanation: A for loop initializes a variable (i = 0), sets a condition to check before each loop (i < 5), and increments the variable after each cycle (i++).
Try This 🔥
Modify the loop condition to i < 10 to print numbers from 0 to 9.
✅ You learned this concept!
Level 7: Methods
What is it? Reusable blocks of code that perform a specific task when called.
int add(int a, int b)
{
return a + b;
}
Explanation: A method has a return type (like int) and parameters. Method Overloading is when you create multiple methods with the exact same name but different parameters (e.g., add(int a, int b, int c)).
Try This 🔥
Create a method named multiply that takes two integers and returns their product.
✅ You learned this concept!
Level 8: Arrays & Strings
What is it? Arrays store multiple values of the same type. Strings handle text sequences.
int[] arr = {1, 2, 3};
for (int i = 0; i < arr.length; i++)
{
System.out.println(arr[i]);
}
Explanation: arr.length automatically gives the size of the array. The loop goes through each index systematically to print the values.
Try This 🔥
Create a String array containing three fruit names, and loop through to print them.
✅ You learned this concept!
Level 9: Object-Oriented Programming (OOP)
What is OOP? OOP is a programming approach using objects and classes to model real-world concepts.
- 1. Encapsulation: Hiding internal data and requiring access via methods (like getter/setter).
- 2. Abstraction: Hiding complex implementation details and showing only essentials.
- 3. Inheritance: A new class taking on the properties of an existing class.
- 4. Polymorphism: One action occurring in different ways (like method overloading).
class Car
{
String brand;
void show()
{
System.out.println(brand);
}
}
Try This 🔥
Add an int year variable to the Car class and print it inside the show() method.
✅ You learned this concept!
Level 10: Classes & Objects
What is it? A Class is a blueprint. An Object is a real, usable instance of that blueprint.
Car c = new Car();
c.brand = "BMW";
c.show();
Explanation: We use the new keyword to create an object of the Car class. We then access its variables and methods using the dot (.) operator.
Try This 🔥
Create a second Car object named c2, set its brand to "Audi", and call its show() method.
✅ You learned this concept!
Level 11: Constructors
What is it? A special method that runs automatically when an object is created, used to set initial values.
class Car
{
Car()
{
System.out.println("Created");
}
}
Explanation: The constructor must have the exact same name as the class and does not have a return type (not even void).
Try This 🔥
Create an object of this Car class to see the "Created" message automatically print.
✅ You learned this concept!
Level 12: Access Modifiers
What is it? Keywords that set the visibility and security level of classes, variables, and methods.
- public: The code is accessible from any other class.
- private: The code is accessible ONLY within the declared class (Key for Encapsulation).
- protected: The code is accessible in the same package and subclasses.
Try This 🔥
Try making a variable private and accessing it from another class. You will get an error!
✅ You learned this concept!
Level 13: Exception Handling
What is it? Managing runtime errors so your application doesn't completely crash.
try
{
int x = 10 / 0;
}
catch (Exception e)
{
System.out.println("Error");
}
Explanation: The program attempts the code in the try block. Since dividing by zero is impossible, it catches the error and runs the catch block instead.
Try This 🔥
Change the division to 10 / 2. The catch block will be skipped because there is no error.
✅ You learned this concept!
Level 14: Practical Programs
What is it? Combining everything you've learned into full, working Java applications.
1. Simple Calculator Logic
int a = 10;
int b = 5;
System.out.println(a + b);
System.out.println(a - b);
2. Student Class Implementation
class Student
{
String name;
Student(String n)
{
name = n;
}
void display()
{
System.out.println(name);
}
}
Try This 🔥
Open the compiler, combine the Student class, create a main method, instantiate a Student object, and run display().
✅ You learned this concept!