Structure of a Java program (class, main, statements)

The canteen token

What is the structure of a Java program?

Words you'll meet in this lesson

The order inside a .java file

The building blocks, one by one

Every word of the main method

Three kinds of comments

Kinds of statements you'll write

Build the token program, one layer at a time

Instead of copying the whole program, add one structural piece per step and see why each one is there.

1. Start with an empty class

   public class Main {
   }

   This compiles, and javac even adds a default constructor. But java Main fails with “Main method not found”: the JVM has a building with no front door.

2. Add the main method

   public class Main {
       public static void main(String[] args) {
           int token = 42;
           System.out.println("Token #" + token);
       }
   }

   Token #42
   Now the launcher finds main, creates the args array, and runs the two statements in order, top to bottom.

3. Add a field and a helper method

   The canteen name belongs to the whole program, so it becomes a field. Printing a token is a job we may repeat, so it becomes a method.

   The field is static, so it's stored once with the class, not in main's stack frame. Calling printToken(token) pushes a new stack frame on top of main's.

4. Import a class from the library

   To print the date we use LocalDate from the java.time package, so we add import java.time.LocalDate; at the top.

   An import doesn't copy any code into your file. It only tells javac that LocalDate means java.time.LocalDate. In the .class file, the full name is stored in the constant pool.

5. Add comments and check the layout

   Add a short comment above the class and indent each block by four spaces. Your final program is in Part 7.

   javac throws comments and extra spaces away completely: the .class file is identical with or without them. They exist purely for the humans who read the code later, including you.

public static void main, word by word: both views

Step through the most famous line in Java. Left: the code, with one word highlighted. Right: the question the JVM is asking at that moment.

How real projects are structured

Company projects follow the same rules, just with many more files. Here's how the structure you learned scales up.

Watch it with an example: a three-class fee app

Real programs have many classes. Watch how the JVM starts from the one class that has main and loads the others only when they are needed.

Examples are simplified to show the structure. Project layouts vary between companies and frameworks.

The complete token program

Every structural piece from this lesson in one program. Read the notes, then try the challenges.

import java.time.LocalDate;

/* Prints a canteen token with today's date. */
public class Main {
    static String canteen = "Kharghar Canteen";

    public static void main(String[] args) {
        int token = 42;
        printToken(token);
        System.out.println("Date: " + LocalDate.of(2026, 10, 15));
    }

    // A helper method: prints one token line
    static void printToken(int number) {
        System.out.println(canteen + " - Token #" + number);
    }
}

• import java.time.LocalDate;Lets us write LocalDate instead of java.time.LocalDate. Must come before the class.
• /* … */ and // …Comments. javac ignores them completely.
• static String canteen = …;A field: declared in the class, outside every method, so every method can use it.
• printToken(token);A method call statement. Java jumps into printToken, runs it, and comes back.
• static void printToken(int number)A helper method with one parameter. It's static so main can call it without an object.
• { } and indentationBraces mark where each block starts and ends. Indentation is only for humans, but makes the blocks easy to see.

Follow the program from main into a method and back

Watch the stack: when main calls printToken, a new frame goes on top, and it's removed when the method returns. The static field lives with the class, not in any frame.

Worked examples: real problems, solved and traced in memory

Every Java program has the same skeleton: imports, a class, fields, methods and main. These three programs use each part for a real job. In the trace, watch static fields appear once for the class while each method call gets its own frame.

tokens 101, 102

Udupi Corner | Token #101
Udupi Corner | Token #102
Slips printed: 2

issued 2026-10-15, 14 days

Issued: 2026-10-15
Due:    2026-10-29

price 800

After discount: Rs 720.0
Final price:    Rs 756.0

Check yourself

Quick revision

Interview and viva questions

Practice problems

Use the parts of a Java program: class, main, static fields, constants, imports and methods. Every output is fixed. The last three are business tasks.

(none)

Welcome!
Welcome!

public class Main {
    static void greet() {
        System.out.println("Welcome!");
    }

    public static void main(String[] args) {
        greet();
        greet();
    }
}

1000

GST: Rs 180.0

public class Main {
    static final double GST = 0.18;

    public static void main(String[] args) {
        System.out.println("GST: Rs " + 1000 * GST);
    }
}

21

42

public class Main {
    static int doubleIt(int n) {
        return n * 2;
    }

    public static void main(String[] args) {
        System.out.println(doubleIt(21));
    }
}

2026-08-15

SATURDAY

import java.time.LocalDate;

public class Main {
    public static void main(String[] args) {
        System.out.println(LocalDate.of(2026, 8, 15).getDayOfWeek());
    }
}

(none)

Visits: 3

public class Main {
    static int visits = 0;

    static void visit() {
        visits = visits + 1;
    }

    public static void main(String[] args) {
        visit();
        visit();
        visit();
        System.out.println("Visits: " + visits);
    }
}

3, 7

3 km: Rs 44
7 km: Rs 76

public class Main {
    static int delivery(int km) {
        return 20 + 8 * km;
    }

    public static void main(String[] args) {
        System.out.println("3 km: Rs " + delivery(3));
        System.out.println("7 km: Rs " + delivery(7));
    }
}

(none)

Patil Electronics
GSTIN: 27ABCDE1234F1Z5

public class Main {
    static final String SHOP = "Patil Electronics";
    static final String GSTIN = "27ABCDE1234F1Z5";

    public static void main(String[] args) {
        System.out.println(SHOP);
        System.out.println("GSTIN: " + GSTIN);
    }
}

30000

Net: Rs 36960.0

public class Main {
    static double gross(double basic) {
        return basic * 1.40;
    }

    static double net(double gross) {
        return gross * 0.88;
    }

    public static void main(String[] args) {
        System.out.println("Net: Rs " + net(gross(30000)));
    }
}

Real-world scenario questions (asked in company interviews)

Structure questions in companies are about readability, maintainability and how programs talk to the systems around them. Decide your answer first, then open the model answer.