Kotlin Somo la 31: Objects na Companion Objects

Utangulizi

Katika lugha ya Java, tunatumia static kuunda method au property inayohusiana na class nzima badala ya object.
Lakini Kotlin haina neno kuu static, badala yake inatumia:

Object Declaration → hutengeneza singleton.
Companion Object → hutoa static-like members ndani ya class.

? 1. Object Declaration (Singletons)

Object declaration hutumika kuunda class moja tu yenye instance moja.
Mfano:

object Database {
    val url = "jdbc:mysql://localhost:3306/mydb"
    fun connect() {
        println("Umeunganishwa na database kwa URL: $url")
    }
}

fun main() {
    Database.connect()
}

➡️ Database hapa ni singleton – kuna instance moja tu wakati wote wa program.

? 2. Object Expressions (Anonymous Objects)

Hizi hutumika kutengeneza object ya muda (anonymous) bila kuunda class.
Mfano:

fun main() {
    val listener = object {
        val id = 1
        fun onClick() = println("Button imebonyezwa!")
    }

    listener.onClick()
}

➡️ Hii ni sawa na kuunda class ndogo haraka kwa matumizi ya muda mfupi.

? 3. Companion Objects

Companion Object ni object maalum ndani ya class ambayo hufanya kazi kama static members.

class User(val name: String) {
    companion object {
        fun createDefault(): User {
            return User("Mgeni")
        }
    }
}

fun main() {
    val u1 = User("Ali")
    val u2 = User.createDefault()

    println(u1.name) // Ali
    println(u2.name) // Mgeni
}

➡️ createDefault() inaweza kupatikana moja kwa moja kupitia User.createDefault() bila kuunda object mpya.

? 4. Companion Objects with Factory Pattern

Companion objects mara nyingi hutumika kutengeneza factory methods.

class Account(val id: Int, val balance: Double) {
    companion object Factory {
        fun create(id: Int): Account {
            return Account(id, 0.0)
        }
    }
}

fun main() {
    val acc = Account.create(101)
    println("Akaunti mpya: id=${acc.id}, salio=${acc.balance}")
}

➡️ Hapa tumetumia companion object kwa kutengeneza akaunti mpya kwa njia rahisi.

? 5. Companion Objects na Constants

Badala ya kuunda static final kama Java, tunatumia const val ndani ya companion object.

class Config {
    companion object {
        const val VERSION = "1.0.0"
    }
}

fun main() {
    println(Config.VERSION) // 1.0.0
}

? 6. Ulinganisho wa Object na Companion Object

Kipengele	Object Declaration	Companion Object
Instance	Singleton ya class nzima	Static-like members ndani ya class
Matumizi	Global utilities (mfano: Database, Logger)	Factory methods, constants, helpers
Upatikanaji	Moja kwa moja: `Database.connect()`	Kupitia class: `User.createDefault()`