Object Declarations

object AppConfig {
    val appName = "MyApp"
    var debugMode = false
}

fun main() {
    println(AppConfig.appName)
    AppConfig.debugMode = true
    println("Debug: ${AppConfig.debugMode}")
}

object MathUtils {
    fun square(x: Int) = x * x
    fun cube(x: Int) = x * x * x
}

fun main() {
    println("Square of 5: ${MathUtils.square(5)}")
    println("Cube of 3: ${MathUtils.cube(3)}")
}

object Session {
    init {
        println("Session object created")
    }
    val id = "abc123"
}

fun main() {
    println("Before accessing Session")
    println("Session id: ${Session.id}")
}

interface ClickListener {
    fun onClick()
}

fun main() {
    val listener = object : ClickListener {
        override fun onClick() {
            println("Clicked!")
        }
    }
    listener.onClick()
}

1. Trying to pass constructor arguments to an object declaration; singletons declared with object cannot take constructor parameters.
2. Forgetting an object is lazily initialized on first access, not at program startup, which can matter for side effects in its init block.
3. Confusing object declarations (named singletons) with anonymous object expressions used for one-off implementations.

• object Name { ... } declares a singleton -- exactly one instance that is created lazily on first access.
• Singletons cannot have constructor parameters since there is no explicit call to create them.
• An anonymous object : SomeType { ... } (an object expression) creates a one-off instance implementing an interface or extending a class, often used where a Java anonymous class would be used.
• Object declarations are commonly used for utility functions, constants, and shared state that should exist exactly once.