Kotlin, known for its emphasis on null safety, introduces nullable types as a powerful feature to prevent null pointer exceptions. This concept is fundamental to writing robust and reliable Kotlin code.

What are Nullable Types?

In Kotlin, all types are non-nullable by default. Nullable types are explicitly declared using a question mark (?) after the type name. This allows variables to hold null values, providing a clear distinction between nullable and non-nullable types.

Declaring Nullable Types

To declare a nullable type, simply add a question mark after the type name:


var nullableString: String? = "Hello"
nullableString = null // This is allowed

var nonNullableString: String = "World"
// nonNullableString = null // This would cause a compilation error

Working with Nullable Types

Kotlin provides several ways to work safely with nullable types:

1. Safe Call Operator (?.)

The safe call operator allows you to call methods or access properties on nullable types without risking a null pointer exception:


val length = nullableString?.length // Returns null if nullableString is null

2. Elvis Operator (?:)

The Elvis Operator provides a default value when a nullable expression is null:


val length = nullableString?.length ?: 0 // Returns 0 if nullableString is null

3. Not-null Assertion Operator (!!)

The not-null assertion operator converts a nullable type to a non-null type, throwing an exception if the value is null. Use this operator with caution:


val length = nullableString!!.length // Throws NullPointerException if nullableString is null

Smart Casts

Kotlin's Smart Casts feature automatically casts nullable types to non-nullable within null-checked contexts:


if (nullableString != null) {
    // nullableString is automatically cast to String (non-nullable) in this scope
    println(nullableString.length)
}

Best Practices

Use nullable types judiciously, preferring non-nullable types when possible.
Leverage Kotlin's null safety features to handle nullable types safely.
Avoid overuse of the not-null assertion operator (!!), as it can lead to runtime exceptions.
Consider using the Safe Calls operator in combination with the Elvis operator for concise null handling.

Conclusion

Nullable types in Kotlin are a cornerstone of its null safety system. By understanding and properly utilizing nullable types, you can write more robust code, significantly reducing the risk of null pointer exceptions. This feature, combined with Kotlin's other null safety mechanisms, contributes to creating more reliable and maintainable applications.