Java Basic Syntax

1. Structure of a Java Program

Every Java program has at least one class. If the class is public, the file name must match the class name (MyFirstJavaProgram.java).

public class MyFirstJavaProgram {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

What this does (line by line):

1. public class MyFirstJavaProgram — declares a class named MyFirstJavaProgram.
2. public static void main(String[] args) — entry point where the JVM begins execution.
3. System.out.println(...) — prints text to the console and appends a newline.

2. The main() Method — entry point

The JVM looks for a method with the exact signature below to start your program:

public static void main(String[] args)

Meaning of each keyword:

• public — accessible by the JVM from anywhere.
• static — JVM calls it without creating an object.
• void — method returns no value.
• String[] args — command-line arguments passed by the user.

3. Comments — documenting intent

Comments help other developers (and your future self) understand code. They are ignored by the compiler.

// Single-line comment
/* Multi-line comment
   spanning multiple lines */
/**
 * Javadoc comment — used to generate API docs
 */

Tip: Write short Javadocs for public classes and public methods; it helps users of your API.

4. Keywords — reserved words you cannot use as identifiers

Java has reserved words (keywords) that carry special meaning. Examples:

class, public, static, void, if, else, for, while, try, catch, return

Attempting to use a keyword as a variable or method name will cause a compile-time error.

5. Naming Conventions

• Classes: PascalCase — MyClass
• Methods: camelCase — calculateTotal()
• Variables: camelCase — totalAmount
• Constants: UPPER_SNAKE_CASE — MAX_RETRIES

Following these conventions improves readability and is expected in professional code.

6. Simple Runnable Example

Try this interactive example:

Run in Online Java Compiler

public class PrintingDemo {
    public static void main(String[] args) {
        int age = 25;
        String name = "Raju";
        System.out.println(name + " is " + age + " years old.");
    }
}

7. Common beginner mistakes

• Not matching public class name with file name (e.g., class MyApp must be in MyApp.java).
• Forgetting main method signature — JVM won't find your program entry point.
• Missing semicolons at the end of statements (;).
• Using keywords as variable names.

8. Quick practice (copy, paste & run)

Copy this into your Java file (TestDemo.java) and run it using javac and java:

Or try this example with Online Java Compiler

public class TestDemo {
    public static void main(String[] args) {
        int n = 5;
        System.out.println("Numbers 1 to " + n + ":");
        for (int i = 1; i <= n; i++) {
            System.out.println(i);
        }
    }
}