Kotlin Programming Basics

# CHAPTER 3

Kotlin Programming Basics #

1. Introduction #

2. Learning Objectives #

• Write and execute basic Kotlin scripts.

• Understand the concept of Type Inference.

• Use String interpolation to inject variables into text dynamically.

• Write basic Kotlin functions to encapsulate logic.

• Use the println() command for standard input/output.

3. The main Function #

// The entry point of every Kotlin program!
fun main() {
    // This command prints text to the developer console
    println("Hello, Android Developer!") 
}

*(Note: Unlike Java or C++, Kotlin does NOT require semicolons ; at the end of every line!)*

4. Variables and Type Inference #

fun main() {
    // We do NOT need to say this is a String. Kotlin figures it out!
    var playerName = "John Wick" 
    
    // Kotlin automatically knows this is an Integer (whole number)
    var score = 1500 
    
    println(playerName)
    println(score)
}

5. String Interpolation (The $ Symbol) #

fun main() {
    var hero = "Batman"
    var city = "Gotham"
    
    // The $ injects the variable perfectly into the string!
    println("The hero $hero protects the city of $city.")
    // Output: The hero Batman protects the city of Gotham.
}

If you need to perform math or logic inside the string, wrap it in curly braces ${}:

var apples = 5
println("I have ${apples * 2} pieces of fruit.") 
// Output: I have 10 pieces of fruit.

6. Comments in Kotlin #

// This is a single-line comment. The computer ignores it.

/*
   This is a multi-line comment.
   You can write paragraphs of explanation here.
   The compiler will ignore all of it!
*/

7. Basic Functions #

// 1. We define the function
fun greetUser() {
    println("Welcome to the application!")
    println("Loading your profile...")
}

fun main() {
    println("App Started")
    
    // 2. We 'call' or trigger the function
    greetUser() 
    
    println("App Ended")
}

8. Mini Project: The Coffee Shop Script #

fun main() {
    // Define the variables
    var customerName = "Alice"
    var coffeeType = "Mocha"
    var price = 4
    
    // Execute the logic using String Interpolation
    println("--- NEW ORDER ---")
    println("Customer: $customerName")
    println("Order: One large $coffeeType.")
    println("Total Due: $$price.00")
    println("Thank you, $customerName, your $coffeeType is being prepared!")
}

9. Common Mistakes #

• Forgetting the main Function: If you write a bunch of variables and println commands at the top level of a file, outside of a fun main() { ... } block, the Kotlin compiler will throw massive errors. Code that executes must be contained within a function!

• Using Semicolons: While Kotlin *allows* you to put a semicolon at the end of a line, doing so is considered terrible practice and violates Kotlin styling guidelines. Trust the compiler; drop the semicolons.

10. Best Practices #

• Naming Conventions: Variable and function names in Kotlin should always use camelCase. This means the first word is lowercase, and every subsequent word is capitalized with no spaces (e.g., playerName, calculateTotalScore()).

11. Exercises #

1. 1. Write a main function. Inside it, declare a variable containing your favorite color.

1. 2. Use println and String Interpolation ($) to print: "My favorite color is [your color here]".

12. Coding Challenges #

13. MCQ Quiz with Answers #

14. Interview Questions #

• Q: Explain the concept of "Type Inference" in Kotlin. How does it improve developer productivity compared to strictly typed legacy languages like Java or C++?

• Q: Contrast traditional string concatenation (using the + operator) with Kotlin's String Interpolation. What specific readability advantages does interpolation provide?

• Q: Describe the absolute necessity of the fun main() block within a standalone Kotlin application.

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #