Kotlin Syntax and First Program

# CHAPTER 3

Kotlin Syntax and First Program #

1. Chapter Introduction #

2. Learning Objectives #

• Write and execute a basic Kotlin program.

• Understand the role of the main function.

• Print text to the console using print() and println().

• Write single-line and multi-line comments.

• Appreciate Kotlin's concise syntax compared to Java.

3. The "Hello, World!" Program #

Type the following code exactly:

fun main() {
    println("Hello, World!")
}

To run it, click the small green "Play" triangle next to fun main() in IntelliJ. *Output:*

Hello, World!

4. Code Breakdown #

#### A. The fun Keyword In Kotlin, fun stands for "function". It is used to declare a new function (a block of code designed to do a specific job).

#### B. The main Function main is a special name. It is the entry point of every Kotlin application. When the JVM starts your program, it specifically searches for a function named main and executes the code inside it first.

#### C. Curly Braces {} The curly braces {} define a "block" or "scope" of code. Everything between the opening { and the closing } belongs to the main function.

#### D. The println() Function println() prints the text inside the parentheses to the screen and automatically moves the cursor to a new line at the end. (If you use print() instead, it does not add a new line).

#### E. No Semicolons! Notice there is no semicolon ; at the end of the println statement! Unlike Java, C++, or C#, Kotlin infers the end of a statement via line breaks. You *can* use semicolons if you put two statements on the same line, but it is considered bad practice.

5. Comments in Kotlin #

Single-line comments start with //:

// This is a comment. It explains the code below.
fun main() {
    println("Hello") // This is an inline comment
}

Multi-line comments (block comments) use /* and */:

/* 
   This is a multi-line comment.
   It can span across several lines.
   Useful for disabling large chunks of code during testing.
*/
fun main() {
    println("Welcome")
}

6. Print vs Println #

fun main() {
    print("One ")
    print("Two ")
    println("Three")
    println("Four")
}

*Output:*

One Two Three
Four

Notice how "One", "Two", and "Three" end up on the same line, but "Four" is pushed to the next line because the println on "Three" added a line break.

7. Mini Project: Simple Console Layout #

fun main() {
    // Print a header
    println("======================")
    println("   DEVELOPER PROFILE  ")
    println("======================")
    
    // Print data
    println("Name: Kotlin Coder")
    println("Role: Android Developer")
    
    /* 
       End of program.
       The JVM will terminate the app now.
    */
}

8. Common Mistakes #

• Capitalizing fun: Kotlin is case-sensitive. Fun main() or FUN main() will cause a compiler error. Keywords must be lowercase.

• Using single quotes for strings: In Kotlin, "Hello" (double quotes) is a String (text). 'A' (single quotes) is a single Character. You cannot use single quotes for multiple letters!

9. Best Practices #

• Indentation: Standard Kotlin style uses 4 spaces for indentation inside a block {}. IntelliJ will format this automatically if you press Ctrl + Alt + L (Windows) or Cmd + Option + L (Mac).

• Omit Semicolons: Never use semicolons at the end of lines. Kotlin developers actively dislike seeing them in code.

10. Exercises #

1. 1. Create a Main.kt file.

1. 2. Write a main function.

1. 3. Use println() to output your favorite movie, book, and food on three separate lines.

1. 4. Add a single-line comment explaining what the code does.

11. MCQs with Answers #

Q8. Can you use single quotes ' ' to define a string of words (e.g., 'Hello World') in Kotlin? a) Yes b) No, single quotes are strictly for single Characters Answer: b) No, single quotes are for single Characters.

Q9. Prior to Kotlin 1.3, main required an array parameter (fun main(args: Array<String>)). Is this still strictly required? a) Yes b) No, you can now write fun main() without parameters if you don't need command line arguments. Answer: b) No, the parameterless main is now standard.

12. Interview Questions #

• Q: Compare the "Hello World" syntax of Java and Kotlin. What boilerplate code does Kotlin remove? (Answer: Kotlin removes the mandatory enclosing class wrapper, the public static modifiers, the args array parameter, and semicolons).

• Q: Why did the creators of Kotlin choose to make semicolons optional?

13. Summary #

14. Next Chapter Recommendation #