Master Kotlin for Android: A Quick Start Guide for Mobile Developers

Adding Kotlin Support to a Project

To start using Kotlin, create a new Android project and select Kotlin as the language, or add the required Kotlin dependencies to an existing project's build.gradle files.

buildscript {
    ext.kotlin_version = '1.3.41'
    repositories { ... }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.5.0-beta05'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
}

Variables

Kotlin requires variables to be initialized or marked as lateinit. By default variables are non‑null; to allow null you add a ? after the type, creating a nullable type.

var view: View // must be initialized
lateinit var view: View // will be initialized later
var name: String? = null // nullable variable

Accessing a nullable variable requires safe calls ( ?.) or non‑null assertions ( !!).

view?.setBackgroundColor(Color.RED)
view!!.setBackgroundColor(Color.RED)

Functions

Functions are declared with fun. The return type follows the parameter list; if omitted, the function returns Unit (similar to void).

fun cook(name: String): Food { ... }
fun main() { /* Unit is implicit */ }

Parameters can be nullable, and you must handle nullability when calling the function.

Classes and Objects

Classes are declared with class. By default they are final; use open to allow inheritance. The override keyword replaces Java’s @Override annotation.

open class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) { ... }
}

Instantiate objects without the new keyword:

val activity: Activity = NewActivity()

Getters, Setters, and Backing Fields

Kotlin properties automatically generate getters and setters. You can customize them using get() and set(value), accessing the underlying field with field.

var name: String = "Mike"
    get() = field + " nb"
    set(value) { field = "Cute " + value }

Visibility Modifiers

Properties and classes are public by default. Use private, protected, or internal as needed.

Type Checking and Casting

Use is for type checks; the compiler smart‑casts within the block. For explicit casts, use as (unsafe) or as? (safe, returns null on failure).

if (activity is NewActivity) {
    activity.action()
}

(activity as? NewActivity)?.action()

Exercises

Create a new Kotlin Android project with an Empty Activity and add a non‑null View property initialized in onCreate.

Declare a function that takes a View? parameter, passes the previously created View to it, and prints the view’s id.