Smart Casts

The Kotlin compiler is smart enough to perform type casts automatically in most cases, including:

1. Casts from nullable types to their non-nullable counterparts.
2. Casts from a supertype to a subtype.

import java.time.LocalDate
import java.time.chrono.ChronoLocalDate

fun main() {
    val date: ChronoLocalDate? = LocalDate.now()    // 1
    
    if (date != null) {
        println(date.isLeapYear)                    // 2
    }
    
    if (date != null && date.isLeapYear) {          // 3
        println("It's a leap year!")
    }
    
    if (date == null || !date.isLeapYear) {         // 4
        println("There's no Feb 29 this year...")
    }
    
    if (date is LocalDate) {
        val month = date.monthValue                 // 5
        println(month)
    }
}

1. Declares a nullable variable.
2. Smart-cast to non-nullable (thus allowing direct access to isLeapYear).
3. Smart-cast inside a condition (this is possible because, like Java, Kotlin uses short-circuiting).
4. Smart-cast inside a condition (also enabled by short-circuiting).
5. Smart-cast to the subtype LocalDate.

This way, you can automatically use variables as desired in most cases without doing obvious casts manually.