Null xavfsizligi

fun main() {
    val bor: String = "matn"
    val yoq: String? = null

    println(bor.length)
    println(yoq)
}

// error: null can not be a value of a non-null type
val bor: String = null

val yoq: String? = null
// error: only safe (?.) or non-null (!!) calls allowed
println(yoq.length)

fun main() {
    val bor: String? = "Namangan"
    val yoq: String? = null

    println(bor?.length)
    println(yoq?.length)
    println(yoq?.uppercase()?.reversed())
}

fun main() {
    val yoq: String? = null
    val bor: String? = "matn"

    println(yoq ?: "standart qiymat")
    println(bor ?: "standart qiymat")
    println(yoq?.length ?: 0)

    val uzunlik = bor?.length ?: -1
    println(uzunlik)
}

fun uzunlik(matn: String?): Int {
    val toza = matn?.trim() ?: return -1
    return toza.length
}

fun main() {
    println(uzunlik("  salom  "))
    println(uzunlik(null))
}

fun main() {
    val bor: String? = "matn"
    println(bor!!.length)
}

fun tavsif(matn: String?): String {
    if (matn == null) {
        return "qiymat yo'q"
    }
    return "uzunligi ${matn.length}, katta harfda ${matn.uppercase()}"
}

fun main() {
    println(tavsif("salom"))
    println(tavsif(null))
}

fun main() {
    val ism: String? = "Malika"
    val yoq: String? = null

    ism?.let {
        println("Uzunlik: ${it.length}")
        println("Katta harfda: ${it.uppercase()}")
    }

    yoq?.let {
        println("bu chiqmaydi")
    }

    val natija = ism?.let { it.length * 2 } ?: 0
    println(natija)
}

fun main() {
    val qiymat: Any = "matn"

    val matn: String? = qiymat as? String
    val son: Int? = qiymat as? Int

    println(matn)
    println(son)
    println(son ?: -1)
}

fun main() {
    val ballar: List<Int?> = listOf(85, null, 92, null, 78)

    println(ballar)
    println(ballar.filterNotNull())
    println(ballar.filterNotNull().sum())
    println(ballar.count { it != null })

    val ismlar: List<String>? = null
    println(ismlar?.size ?: 0)
    println(ismlar.orEmpty())
}

data class Manzil(val shahar: String, val kocha: String?)
data class Foydalanuvchi(val ism: String, val manzil: Manzil?)

fun kochaNomi(f: Foydalanuvchi?): String =
    f?.manzil?.kocha ?: "ko'rsatilmagan"

fun main() {
    val a = Foydalanuvchi("Malika", Manzil("Namangan", "Navoiy"))
    val b = Foydalanuvchi("Aziza", Manzil("Andijon", null))
    val c = Foydalanuvchi("Nodira", null)

    println(kochaNomi(a))
    println(kochaNomi(b))
    println(kochaNomi(c))
    println(kochaNomi(null))
}