How to Handle Null in Kotlin: Safe Calls and Null Checks

In Kotlin, you handle null by declaring variables as nullable using ? and safely accessing them with ?. or providing defaults with the ?: operator. This prevents NullPointerException by forcing you to check for null before use.

Why This Happens

Kotlin does not allow null values in regular variables by default. If you try to assign or use a null value without marking the variable as nullable, the compiler will give an error. If you force null usage without checks, it causes a NullPointerException at runtime.

kotlin

fun main() {
    val name: String = null
    println(name.length)
}

Output

error: null can not be a value of a non-null type String

The Fix

To fix this, declare the variable as nullable by adding ? after the type. Use safe calls ?. to access properties or methods only if the value is not null. Use the Elvis operator ?: to provide a default value if null.

kotlin

fun main() {
    val name: String? = null
    println(name?.length ?: "No name provided")
}

Output

No name provided

Prevention

Always declare variables as nullable only when they can hold null. Use safe calls ?. and the Elvis operator ?: to handle null safely. Avoid using the not-null assertion !! unless you are sure the value is not null, as it throws exceptions otherwise. Enable Kotlin's null-safety linting in your IDE to catch unsafe null usage early.

Related Errors

Common related errors include NullPointerException when using !! on a null value, and compiler errors when assigning null to non-null types. To fix these, use nullable types and safe calls as shown above.

kotlin

fun main() {
    val text: String? = null
    println(text!!.length) // Throws NullPointerException
}

Output

Exception in thread "main" kotlin.KotlinNullPointerException

Key Takeaways

Declare variables as nullable with ? when they can hold null.

Use safe calls ?. to access nullable variables safely.

Use the Elvis operator ?: to provide default values for nulls.

Avoid using !! unless you are sure the value is not null.

Enable null-safety linting to catch null-related issues early.