Null Safety

Null Safety

Kotlin's type system distinguishes between nullable and non-nullable types. By default, variables cannot hold null:

var name: String = "Alice"
name = null  // Compile error!

To allow null, add ? to the type:

var name: String? = "Alice"
name = null  // OK

Safe Calls (?.)

The safe call operator ?. calls a method only if the value is not null. If it is null, the whole expression returns null:

val name: String? = "HELLO"
println(name?.lowercase())  // hello

val empty: String? = null
println(empty?.lowercase()) // null

Elvis Operator (?:)

The Elvis operator ?: provides a default value when the left side is null:

val name: String? = null
val result = name ?: "stranger"
println(result) // stranger

Non-Null Assertion (!!)

The !! operator asserts that a value is not null. If it is null at runtime, it throws an exception:

val name: String? = "Hello"
println(name!!.length) // 5

Use !! sparingly — prefer safe calls or null checks instead.

Smart Casts

When you check for null with if, Kotlin smart-casts the variable to non-nullable inside the block:

val name: String? = "Alice"
if (name != null) {
    println(name.length) // Smart cast: name is String here
}

The let Scope Function

Use ?.let { } to execute a block only when a value is non-null. Inside the block, the value is available as it:

val name: String? = "Alice"
name?.let { println("Hello, $it") } // Hello, Alice

val empty: String? = null
empty?.let { println("Hello, $it") } // not executed

Your Turn

Write a function greet that takes a nullable String? parameter. If the name is not null, print "Hello, <name>!" using the Elvis operator to provide "World" as a default. Then in main, call it with "Alice", null, and "Bob".