Kotlin Null Safety Cheat Sheet

Covers nullable types, the safe call and elvis operators, the not-null assertion, and platform types for handling null safely in Kotlin.

1 PageBeginnerMar 28, 2026

Nullable Types

The ? suffix marks a type as allowed to hold null.

kotlin

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"}")}

Safe Calls & Elvis Operator

Chaining nullable access without throwing.

kotlin

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())}

Not-Null Assertion

Forcing an unwrap when you're certain a value is non-null.

kotlin

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" }

Null Safety Operators

The full operator toolkit for working with nullable types.

? (nullable marker)- Declares that a type may hold null, e.g. String?
?. (safe call)- Calls a member only if the receiver is non-null; otherwise evaluates to null
?: (elvis operator)- Supplies a default value when the left-hand expression is null
!! (not-null assertion)- Forces unwrap; throws NullPointerException if the value is null
?.let { }- Executes the block only when the receiver is non-null, using it as the argument
as? (safe cast)- Casts to a type, returning null instead of throwing ClassCastException on failure
lateinit var- Defers initialization of a non-null var, avoiding a nullable type for late-bound properties

Smart Casts

The compiler automatically treats a checked nullable as non-null.

kotlin

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.

Pro Tip

Avoid `!!` in application code — it reintroduces the exact NullPointerException risk Kotlin's type system is designed to eliminate. Prefer `?.`, `?:`, or `requireNotNull()` with a descriptive message so failures are self-explanatory.

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