[Kotlin] Introduction to Kotlin Programming Language

📋 Here are the notes summarizing what I learned from the course!

Introduction to Kotlin Programming Language

Objectives:

• Review the basics of Kotlin programming language including variables, control structures, functions, classes, and collections.
• Set up Android Studio to use Kotlin and build apps.
• Develop Android apps using Kotlin.

What is Kotlin?

Kotlin is a statically typed programming language for modern multiplatform applications, meaning type checking is done at compile-time. It is 100% interoperable with Java and Android.

Basic Syntax

• Package specification should be at the top of the source file.

  package my.demo
  import java.util.*
  

Defining Functions

• Function with two Int parameters returning Int:

  fun sum(a: Int, b: Int): Int {
      return a + b
  }
  

• Function with an expression body and inferred return type:

  fun sum(a: Int, b: Int) = a + b
  

• Function returning no meaningful value (Unit return type can be omitted):

  fun printSum(a: Int, b: Int) {
      println("sum of $a and $b is ${a + b}")
  }
  

Defining Variables

• Read-only local variable:

  val a: Int = 1  // immediate assignment
  val b = 2       // `Int` type is inferred
  val c: Int      // Type required when no initializer is provided
  c = 3           // deferred assignment
  

• Mutable variable:

  var x = 5
  x += 1
  

Using String Templates

• Example of simple name in template and arbitrary expression in template:

  var a = 1
  val s1 = "a is $a"
  a = 2
  val s2 = "${s1.replace("is", "was")}, but now is $a"
  

Using Conditional Expressions

• Using if as an expression:

  fun maxOf(a: Int, b: Int) = if (a > b) a else b
  

Using Nullable Values and Checking for Null

• Function that returns a nullable Int:

  fun parseInt(str: String): Int? {
      // implementation
  }
  

Using Collections

• Iterating over a collection and checking if it contains an object:

  val items = listOf("apple", "banana", "kiwi")
  for (item in items) {
      println(item)
  }
  when {
      "orange" in items -> println("juicy")
      "apple" in items -> println("apple is fine too")
  }
  

Classes

• Example of a simple class:

  class Person(firstName: String, lastName: String, personAge: Int) {
      val upperCaseFirstName = firstName.toUpperCase()
      val upperCaseLastName = lastName.toUpperCase()
      var age = personAge
  
      init {
          println("Uppercase First Name = $upperCaseFirstName")
          println("Age = $age")
      }
  }
  

Data Classes

• Example of a data class:

  data class User(val name: String, val age: Int)
  

References:

• Kotlin Programming Language Official Site
• Basic Syntax
• Coding Conventions
• Basic Types
• Classes and Objects
• Introduction to Kotlin on Baeldung
• Keyword Reference in Kotlin
• Google Codelabs: Build Your First Android App in Kotlin

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