Instruction

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Interfaces

Inheritance is a powerful tool for code reuse, but it focuses on the “is-a” relationship between classes. A subclass is a specialized version of its superclass. Interfaces, on the other hand, introduce a different concept: the “is” relationship.

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interface Edible {
  val isItReallyEdible: Boolean
  
  fun taste(): String
}

Default Implementation

Remember how interfaces are all about defining behavior contracts? While interfaces primarily focus on what a class should do, they can sometimes help with how to do it. This is where default methods come in.

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  fun consumer(): String {
    return "Human"
  }

Implementation

Now you’ll create two classes implementing the Edible interface:

class Fruit : Edible {
  override val isItReallyEdible = true

  override fun taste(): String {
    return "Sweet"
  }

}

class Mushroom : Edible {
  override val isItReallyEdible = false
  
  override fun taste(): String {
    return "Delicious"
  }

  override fun consumer(): String {
    return "Pig"
  }
}

fun main() {
  val items = listOf<Edible>(Fruit(), Mushroom())
  items.forEach { item ->
    println("${item.javaClass.simpleName} really edible: ${item.isItReallyEdible}, taste: ${item.taste()}, consumed by ${item.consumer()}")
  }
}
// Fruit really edible: true, taste: Sweet, consumed by Human
// Mushroom really edible: false, taste: Delicious, consumed by Pig

Multiple Interfaces

Each class can implement multiple interfaces. It’s a valuable tool for building flexible, reusable, and well-designed code in Kotlin. It promotes loose coupling and clear separation of concerns, making your code easier to understand, maintain, and evolve. Here are just a few benefits:


interface Edible {
  fun taste(): String
}

interface Sweet {
  fun andSour(): String
}

class Fruit(val name: String, val sour: Boolean = false) : Edible, Sweet {
  override fun taste(): String {
    return "$name is very good"
  }

  override fun andSour(): String {
    return if(sour)"Maybe" else "Definitely not!"
  }
}

class Garlic : Edible {
  override fun taste(): String {
    return "delicious"
  }
}

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fun main() {
  // Error: Type mismatch: inferred type is Garlic but Sweet was expected
  val sweetItems = listOf<Sweet>(Fruit("Peach"), Garlic())
  sweetItems.forEach { item ->
    println("${item.javaClass.simpleName} sour: ${item.andSour()}")
  }
}

fun main() {
  val items = listOf<Edible>(Fruit("Peach"), Garlic())
  items.forEach { item ->
    println("${item.javaClass.simpleName} taste: ${item.taste()}")
  }
}
//Fruit taste: Peach is very good
//Garlic taste: delicious

fun main() {
  //   Same, but for Sweet
  val sweetItems = listOf<Sweet>(Fruit("Peach"), Fruit("Tomato", false))
  sweetItems.forEach { item ->
    // Error:
    // Unresolved reference: name
    // Unresolved reference: taste
    println("${item.name} taste: ${item.taste()}. Sour? ${item.andSour()}")
  }
}

fun printList(val sweets: List<Sweet>)

fun main() {
  // Same, but name is not available
  val sweetItems = listOf<Sweet>(Fruit("Peach"), Fruit("Tomato", true))
  sweetItems.forEach { item ->
    println("${item.javaClass.simpleName} is Sweet. Sour? ${item.andSour()}")
  }
}
//Fruit is Sweet. Sour? Definitely not!
//Fruit is Sweet. Sour? Maybe

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fun main() {
  // Same, but for Fruit
  val fruitItems = listOf<Fruit>(Fruit("Peach"), Fruit("Tomato", true))
  fruitItems.forEach { item ->
    println( "${item.name} taste: ${item.taste()}. Sour? ${item.andSour()}")
  }
}
//Peach taste: Peach is very good. Sour? Definitely not!
//Tomato taste: Tomato is very good. Sour? Maybe
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