Kotlin Null Safety Cheat Sheet

var name: String = "Kotlin"   // non-null, cannot be assigned null// name = null                // compile errorvar nickname: String? = "Kt"  // nullable, marked with ?nickname = null                // OKfun greet(name: String) {     // parameter cannot be null    println("Hello, $name")}fun greetSafe(name: String?) { // parameter may be null    println("Hello, ${name ?: "Guest"}")}

val nickname: String? = null// Safe call: returns null instead of throwing if the receiver is nullval length: Int? = nickname?.length// Elvis operator: provide a default when the left side is nullval displayName: String = nickname ?: "Anonymous"// Chaining safe callsdata class Address(val city: String?)data class User(val address: Address?)val user: User? = User(Address(null))val city = user?.address?.city ?: "Unknown"// Elvis with early return/throwfun process(value: String?) {    val v = value ?: return    println(v.uppercase())}

val nickname: String? = "Kt"// !! throws NullPointerException if the value is actually nullval length: Int = nickname!!.length// Use sparingly -- only when you are certain the value cannot be null// and a crash is the correct behavior if that assumption is wrong.fun getConfig(): String? = readConfigFile()// Prefer requireNotNull/checkNotNull for clearer failure messagesval config: String = requireNotNull(getConfig()) { "Config must be present" }

fun describe(value: String?) {    if (value != null) {        // Kotlin smart-casts value to non-null String inside this block        println(value.length)    }    // Also works with early return    if (value == null) return    println(value.uppercase())}// Smart casts don't work across mutable var properties that could change,// but work reliably with local val and immutable properties.

// Java method: public String getName() { return null; } // no @Nullable annotation// Kotlin sees getName() as a "platform type" String! (unofficial notation)val name = javaObject.getName() // compiles as if non-null, but can still be null// Two failure modes if the Java side actually returns null:val length = name.length // NPE happens HERE, at the point of use, not assignment// Defensive interop: treat platform types as nullable explicitlyval safeName: String? = javaObject.getName()val safeLength = safeName?.length ?: 0// Best practice: wrap Java APIs at the boundary with explicit nullabilityfun getUserNameSafely(java: JavaUser): String = java.getName() ?: ""// @NonNull / @Nullable annotations on the Java side make Kotlin enforce them properly

import kotlin.contracts.ExperimentalContractsimport kotlin.contracts.contract@OptIn(ExperimentalContracts::class)fun isValidEmail(value: String?): Boolean {    contract {        returns(true) implies (value != null)    }    return !value.isNullOrBlank() && value.contains("@")}fun send(email: String?) {    if (isValidEmail(email)) {        // compiler smart-casts email to non-null String here because of the contract        println(email.length)    }}// Standard library examples already using contracts:val s: String? = nullif (s.isNullOrEmpty()) {    // s is smart-cast-eligible as null OR empty inside this branch} else {    println(s.length) // s smart-cast to non-null String}

val listOfNullableInts: List<Int?> = listOf(1, null, 3) // elements may be nullval nullableListOfInts: List<Int>? = null // the list reference itself may be null// filterNotNull() drops nulls and returns List<Int>val nonNullInts: List<Int> = listOfNullableInts.filterNotNull()// mapNotNull combines map + filter out nulls in one passval doubled = listOfNullableInts.mapNotNull { it?.times(2) }// firstOrNull / find return null instead of throwing NoSuchElementExceptionval first: Int? = listOfNullableInts.firstOrNull { it != null && it > 1 }// getOrNull on arrays/lists avoids IndexOutOfBoundsExceptionval arr = arrayOf(1, 2, 3)val fourth: Int? = arr.getOrNull(10) // null, not a crash// generic type parameters are non-null by default; use T? to opt into nullabilityfun <T : Any> firstNotNull(list: List<T?>): T? = list.firstOrNull { it != null }

import kotlin.properties.Delegatesclass Config {    // lateinit: for var, must be set before first read, throws    // UninitializedPropertyAccessException if read too early. No primitives.    lateinit var apiKey: String    // by lazy: for val, computed once on first access, thread-safe by default    val expensiveValue: String by lazy {        println("computing...")        loadFromDisk()    }    // Delegates.notNull: like lateinit but works with primitives (Int, Boolean, ...)    var retryCount: Int by Delegates.notNull()    fun isApiKeyReady() = ::apiKey.isInitialized // check without triggering the exception}fun loadFromDisk(): String = "cached-value"