設定初值
利用介面A打造類別B的過程,稱之為以「類別B」實作「介面A」
interface MyInterface {
    val prop: Int // abstract
    val propertyWithImplementation: String
        get() = "foo"
    fun foo() {
        print(prop)
    }
}
class Child : MyInterface {
    override val prop: Int = 29
}