Delegation

DelegationExample.kt

/**
 * Our interface that has only one method [Delegation.print].
 * Who inherit [Delegation] might have different implementation.
 * So we are declaring [Delegation.print] without any kind of parameter.
 */
interface Delegation {
    fun print()
}

/**
 * Implementation of [Delegation]. We are printing an [Int] value.
 *
 * @param x [Int] value to print.
 */
class DelegationImpl(private val x: Int) : Delegation {
    override fun print() {
        println(x)
    }
}

/**
 * Another Implementation of [Delegation]. We are printing an [String] value.
 *
 * @param value [String] value to print.
 */
class DelegationImpl2(private val value: String) : Delegation {
    override fun print() {
        println(value)
    }
}

/**
 * Our delegation class. Using the keyword **by** we are delegating it.
 *
 * Note: [Derived] takes [Delegation] object as a parameter and return the same object.
 * So that client can access the public functions of [Delegation]
 */
class Derived(private val b: Delegation) : Delegation by b


/**
 * Our another delegation class.
 *
 * Note: We are overriding the public method [Delegation.print] and we are printing our message
 * instead of the [Delegation]'s one. [AnotherDerived.print] will always print our message.
 */
class AnotherDerived(private val b: Delegation) : Delegation by b {
    override fun print() {
        println("We are printing our custom message instead of the client one.")
    }
}

class DelegationExample {
    companion object {
        @JvmStatic
        fun main(args: Array<String>) {
            /**
             * Create instances of [Delegation]
             */
            val delegation1 = DelegationImpl(5)
            val delegation2 = DelegationImpl2("Test")

            /**
             * Accessing [Delegation.print] by using [Derived] class instead of directly accessing it.
             */
            Derived(delegation1).print() // Output -> 5
            Derived(delegation2).print() // Output -> "Test"

            /**
             * Another delegation to print our custom message instead of "Test".
             */
            AnotherDerived(delegation2).print() // Output -> "We are printing our custom message instead of the client one."
        }
    }
}