Kotlin Documentation | Inheritance

Kotlin Documentation 시리즈에 대해

Category: Concepts - Classes and objects

문서 링크: https://kotlinlang.org/docs/inheritance.html

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Any

• 모든 클래스는 Any를 상속함
  • equals()
  • hashCode()
  • toString()

Open

• open: 클래스를 상속하기 위해 사용한다.

open class Base(p: Int)

class Derived(p: Int) : Base(p)

open class Shape {
    open fun draw() { /*...*/ }
    fun fill() { /*...*/ }
}

class Circle() : Shape() {
    override fun draw() { /*...*/ }
}

• final: override를 막고 싶을 때 사용한다.

open class Rectangle() : Shape() {
    final override fun draw() { /*...*/ }
}

Constructor

class MyView : View {
    constructor(ctx: Context) : super(ctx)

    constructor(ctx: Context, attrs: AttributeSet) : super(ctx, attrs)
}

Overriding properties

open class Shape {
    open val vertexCount: Int = 0
}

class Rectangle : Shape() {
    override val vertexCount = 4
}

interface Shape {
    val vertexCount: Int
}

class Rectangle(override val vertexCount: Int = 4) : Shape // Always has 4 vertices

Derived class initialization order

open class Base(val name: String) {

    init { println("Initializing a base class") }

    open val size: Int = 
        name.length.also { println("Initializing size in the base class: $it") }
}

class Derived(
    name: String,
    val lastName: String,
) : Base(name.replaceFirstChar { it.uppercase() }.also { println("Argument for the base class: $it") }) {

    init { println("Initializing a derived class") }

    override val size: Int =
        (super.size + lastName.length).also { println("Initializing size in the derived class: $it") }
}

Argument for the base class: Hello
Initializing a base class
Initializing size in the base class: 5
Initializing a derived class
Initializing size in the derived class: 10

→ 파생 클래스 프로퍼티를 베이스 클래스에서 사용 불가능

→ When designing a base class, you should therefore avoid using open members in the constructors, property initializers, or init blocks.

Superclass

open class Rectangle {
    open fun draw() { println("Drawing a rectangle") }
    val borderColor: String get() = "black"
}

class FilledRectangle : Rectangle() {
    override fun draw() {
        super.draw()
        println("Filling the rectangle")
    }

    val fillColor: String get() = super.borderColor
}

class FilledRectangle: Rectangle() {
    override fun draw() {
        val filler = Filler()
        filler.drawAndFill()
    }

    inner class Filler {
        fun fill() { println("Filling") }
        fun drawAndFill() {
            super@FilledRectangle.draw() // Calls Rectangle's implementation of draw()
            fill()
            println("Drawn a filled rectangle with color ${super@FilledRectangle.borderColor}") // Uses Rectangle's implementation of borderColor's get()
        }
    }
}

Overriding rules

• super<ClassName> → 여러 클래스를 구현, 상속 시

open class Rectangle {
    open fun draw() { /* ... */ }
}

interface Polygon {
    fun draw() { /* ... */ } // interface members are 'open' by default
}

class Square() : Rectangle(), Polygon {
    // The compiler requires draw() to be overridden:
    override fun draw() {
        super<Rectangle>.draw() // call to Rectangle.draw()
        super<Polygon>.draw() // call to Polygon.draw()
    }
}