Null Safety & The Billion-Dollar Mistake

Nullable types ?, safe call ?., Elvis operator ?:, and non-null assertion !!

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Tony Hoare invented the null pointer in 1965, later calling it his "billion-dollar mistake." Kotlin eliminates NullPointerExceptions at compile-time through its type system.

The Null Safety Operators

Non-nullable by default: var name: String = "Alice" cannot hold null. Attempting to set name = null fails compilation!

Nullable Types (?): To allow null, append a question mark: var name: String? = null.

Safe Call (?.): Executes method only if the reference is non-null: name?.length returns null instead of throwing an exception.

Elvis Operator (?:): Provides fallback default values: val len = name?.length ?: 0.

Non-null assertion (!!): Forces unwrap; throws an NPE if null. Avoid in production!

Idiomatic Null Handling in Kotlin

Try It Yourself »

fun printUserLength(username: String?) {
    // Safe call + Elvis operator fallback
    val length = username?.length ?: 0
    println("Username '$username' has length: $length")
}

printUserLength("AdaLovelace") // 11
printUserLength(null)          // 0 (No NullPointerException!)

The Elvis operator ?: provides graceful fallback values.

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Test Yourself With Exercise

+15 XP

Which operator in Kotlin provides a fallback default value if an expression evaluates to null?

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