what is delegation in kotlin

Delegation in kotlin means, Delegating the responsibility to other objects.

Output

David is studying
<--------------->
David is studying

Code

fun main(args: Array<String>) {

    val david = David()
    val demoOne = SchoolDemoOne(david)
    val demoTwo = SchoolDemoTwo(david)

    demoOne.studying()
    println("<--------------->")
    demoTwo.studying()

}

/**
 * Studying can perform the action of studying
 */
interface Student {
    fun studying()
}


/**
 * Concrete implementation for a student
 */
class David : Student {
    override fun studying() {
        println("David is studying")
    }
}

/**
 * Delegation done but by not using `By` feature of kotlin
 */
class SchoolDemoOne(
    private val student : Student
): Student{
    override fun studying() {
        student.studying()
    }
}

/**
 * Delegation done but by using `By` feature of kotlin
 * Notice we removed the boilerplate code
 */
class SchoolDemoTwo(
    private val student : Student
): Student by student