What is Java and why learn it?

₹20 at the chai stall

What is Java?

Words you'll hear in every Java lesson

Java at a glance

The Java family: 4 platforms and 4 types of applications

“Java” is not one product. It's a language plus a set of platforms, each tuned for a different kind of device.

The four Java platforms

The four types of Java applications

From zero to your first running Java program

Here's what happens on your side and what happens inside the computer at each step.

1. Install a JDK

   Download a free JDK such as Eclipse Temurin or Oracle JDK. Choose an LTS version (21 or 25). In a hurry? Skip this and use the live editor in Part 7.

   The installer copies javac (the compiler), java (the launcher that starts the JVM) and thousands of ready-made library classes onto your disk.

2. Check it worked

   java -version
   javac -version

   Your operating system searches the folders listed in the PATH setting for programs named java and javac, runs them, and they print their version.

3. Write the program

   Open any editor (VS Code, IntelliJ IDEA, even Notepad) and save this as Main.java. The file name must match the class name.

   The editor saves plain text characters to your disk. To the computer it's just letters, not yet a program.

4. Compile it

   javac Main.java

   javac reads the whole file, checks the grammar and the types, and writes a new file Main.class containing bytecode. No message means success.

5. Run it

   java Main

   Namaste from Java!
   The JVM starts, loads Main.class into memory, checks the bytecode is safe, finds the main method and runs it line by line.

Write once, run anywhere: both views

Press Play or step through with Next. The left side shows what you see on screen. The right side shows what's happening behind it at the same moment.

Java vs C vs Python: how each one runs

Where Java runs in your daily life

You use Java-powered systems many times a day without knowing it. Here are seven places, and what Java does in each.

Watch it with an example: one UPI payment

You pay Rs 60 at the college canteen. Follow the payment and see where Java code is doing the work.

Examples are simplified to show where Java fits. Each company's exact technology choices vary.

Your first Java program

Read the program and its line-by-line meaning first, then open the live editor and try each challenge.

public class Main {
    public static void main(String[] args) {
        System.out.println("Namaste from Java!");
    }
}

• public class MainEvery Java program lives inside a class. Here the class is named Main, so the file must be Main.java.
• public static void main(String[] args)The entry point: the JVM always starts running from this exact method. Lesson 1.6 explains each word.
• System.out.println(...)Prints text to the console and then moves to a new line (ln = line).
• "Namaste from Java!"A string: text inside double quotes.
• ; and { }Every statement ends with a semicolon. Curly braces group code into blocks.

What the JVM does at every line

Step through this slightly bigger program. Watch the highlighted line on the left and the memory on the right. The stack holds methods and their small variables, and the heap holds objects such as strings. Both are covered in depth in Module 8.

Worked examples: real problems, solved and traced in memory

Three tiny programs of the kind Java runs every day in shops, banks and phones. Each one uses only what you have seen so far: println, a few variables and arithmetic. Run them, then step through to see each value appear on main's stack.

Masala dosa Rs 60, Filter coffee Rs 25

--- Canteen Bill ---
Masala dosa   Rs 60
Filter coffee Rs 25
Total         Rs 85

goal 8000, walked 5340

Walked: 5340 steps
Remaining: 2660 steps
Done: 66%

message: "Your OTP for login is 482913. Do not share it with anyone. Valid for 5 minutes."

Characters: 79
SMS parts needed: 1

Check yourself

Quick revision

Interview and viva questions

Practice problems

Short programs that use println, variables and arithmetic. Every output is fixed, so you can check yours exactly. The last three are small business tasks.

(none)

Name: Riya Sharma
City: Pune

public class Main {
    public static void main(String[] args) {
        System.out.println("Name: Riya Sharma");
        System.out.println("City: Pune");
    }
}

45, 30

Sum = 75

public class Main {
    public static void main(String[] args) {
        int a = 45;
        int b = 30;
        System.out.println("Sum = " + (a + b));
    }
}

7 days × 24 hours

Hours in a week: 168

public class Main {
    public static void main(String[] args) {
        int days = 7;
        int hoursPerDay = 24;
        System.out.println("Hours in a week: " + days * hoursPerDay);
    }
}

155

2 h 35 min

public class Main {
    public static void main(String[] args) {
        int total = 155;
        System.out.println(total / 60 + " h " + total % 60 + " min");
    }
}

78, 85, 92

Average: 85.0

public class Main {
    public static void main(String[] args) {
        int sum = 78 + 85 + 92;
        double avg = sum / 3.0;
        System.out.println("Average: " + avg);
    }
}

6 km

Fare: Rs 102.5

public class Main {
    public static void main(String[] args) {
        double extraKm = 6 - 1.5;
        double fare = 26 + extraKm * 17;
        System.out.println("Fare: Rs " + fare);
    }
}

240 units

Units: 240
Bill: Rs 1780

public class Main {
    public static void main(String[] args) {
        int units = 240;
        int bill = units * 7 + 100;
        System.out.println("Units: " + units);
        System.out.println("Bill: Rs " + bill);
    }
}

1450, 4 people

Each pays: Rs 362
Left over: Rs 2

public class Main {
    public static void main(String[] args) {
        int bill = 1450;
        int people = 4;
        System.out.println("Each pays: Rs " + bill / people);
        System.out.println("Left over: Rs " + bill % people);
    }
}

Real-world scenario questions (asked in company interviews)

Companies rarely ask “define Java”. They describe a situation from the job and watch how you think. Read each scenario, decide your answer first, then open the model answer.