Null Safety

In an effort to rid the world of NullPointerException, variable types in Kotlin don't allow the assignment of null. If you need a variable that can be null, declare it nullable by adding ? at the end of its type.

fun main() {
    var neverNull: String = "This can't be null"            // 1
    
    neverNull = null                                        // 2
    
    var nullable: String? = "You can keep a null here"      // 3
    
    nullable = null                                         // 4
    
    var inferredNonNull = "The compiler assumes non-null"   // 5
    
    inferredNonNull = null                                  // 6
    
    fun strLength(notNull: String): Int {                   // 7
        return notNull.length
    }
    
    strLength(neverNull)                                    // 8
    strLength(nullable)                                     // 9
}

{validate="false"}

1. Declares a non-null String variable.
2. When trying to assign null to non-nullable variable, a compilation error is produced.
3. Declares a nullable String variable.
4. Sets the null value to the nullable variable. This is OK.
5. When inferring types, the compiler assumes non-null for variables that are initialized with a value.
6. When trying to assign null to a variable with inferred type, a compilation error is produced.
7. Declares a function with a non-null string parameter.
8. Calls the function with a String (non-nullable) argument. This is OK.
9. When calling the function with a String? (nullable) argument, a compilation error is produced.

Working with Nulls

Sometimes Kotlin programs need to work with null values, such as when interacting with external Java code or representing a truly absent state. Kotlin provides null tracking to elegantly deal with such situations.

fun describeString(maybeString: String?): String {              // 1
    if (maybeString != null && maybeString.length > 0) {        // 2
        return "String of length ${maybeString.length}"
    } else {
        return "Empty or null string"                           // 3
    }
}

fun main() {
    println(describeString(null))
}

1. A function that takes in a nullable string and returns its description.
2. If the given string is not null and not empty, return information about its length.
3. Otherwise, tell the caller that the string is empty or null.