HoRain云--Kotlin接口全解析-程序员充电站

// 简单接口 interface MyInterface { fun foo() fun bar() } // 带属性的接口 interface Named { val name: String } // 带默认实现的接口 interface Speaker { fun speak() { println("Speaking...") } }

1.2 实现接口

// 单一接口实现 class SimpleClass : MyInterface { override fun foo() { println("foo") } override fun bar() { println("bar") } } // 多个接口实现 class MultiInterfaceClass : MyInterface, Named, Speaker { override val name: String = "MultiClass" override fun foo() { println("foo in MultiInterfaceClass") } override fun bar() { println("bar in MultiInterfaceClass") } }

2. 接口属性

2.1 抽象属性

interface User { val email: String val nickname: String get() = email.substringBefore('@') } class PrivateUser(override val email: String) : User class SubscribingUser(val emailAddress: String) : User { override val email: String get() = emailAddress } // 使用 val privateUser = PrivateUser("alice@example.com") println(privateUser.email) // alice@example.com println(privateUser.nickname) // alice val subscribingUser = SubscribingUser("bob@example.com") println(subscribingUser.email) // bob@example.com println(subscribingUser.nickname) // bob

2.2 带访问器的属性

interface Person { val name: String val age: Int val description: String get() = "$name is $age years old" val isAdult: Boolean get() = age >= 18 } class Student(override val name: String, override val age: Int) : Person fun main() { val student = Student("Alice", 20) println(student.description) // Alice is 20 years old println(student.isAdult) // true }

3. 接口中的默认方法实现

interface Clickable { fun click() // 抽象方法 fun showOff() = println("I'm clickable!") // 带默认实现 } interface Focusable { fun setFocus(focused: Boolean) = println("I ${if (focused) "got" else "lost"} focus.") fun showOff() = println("I'm focusable!") } class Button : Clickable, Focusable { override fun click() = println("Button clicked") // 必须重写冲突的默认方法 override fun showOff() { super<Clickable>.showOff() super<Focusable>.showOff() } } fun main() { val button = Button() button.click() // Button clicked button.setFocus(true) // I got focus. button.showOff() // I'm clickable! // I'm focusable! }

4. 函数式接口（SAM 接口）

只有一个抽象方法的接口称为函数式接口（SAM - Single Abstract Method）。

// 使用 fun interface 声明函数式接口 fun interface IntPredicate { fun accept(i: Int): Boolean } // 传统方式实现 val isEven = object : IntPredicate { override fun accept(i: Int): Boolean = i % 2 == 0 } // 使用 lambda 表达式 val isEvenLambda: IntPredicate = { it % 2 == 0 } // 使用函数引用 fun isEvenMethod(i: Int): Boolean = i % 2 == 0 val isEvenRef: IntPredicate = ::isEvenMethod fun main() { println(isEven.accept(4)) // true println(isEvenLambda.accept(4)) // true println(isEvenRef.accept(4)) // true }

5. 接口继承

interface Base { fun print() } interface Derived : Base { fun printDerived() } class MyClass : Derived { override fun print() { println("Base implementation") } override fun printDerived() { println("Derived implementation") } } // 接口多继承 interface A { fun foo() { println("A") } } interface B { fun foo() { println("B") } fun bar() { println("bar") } } interface C : A, B { override fun foo() { super<A>.foo() super<B>.foo() println("C") } } class D : C { override fun bar() { println("D bar") } } fun main() { val d = D() d.foo() // A // B // C d.bar() // D bar }

6. 接口与抽象类比较

特性	接口	抽象类
多重继承	✓ 一个类可实现多个接口	✗ 只能继承一个抽象类
状态	✗ 不能有状态（属性不能初始化）	✓ 可以有状态（属性可初始化）
构造函数	✗ 没有构造函数	✓ 可以有主/次构造函数
默认实现	✓ 可以有方法默认实现	✓ 可以有方法实现
属性访问器	✓ 可以有属性访问器	✓ 可以有属性访问器
初始化块	✗ 不能有 init 块	✓ 可以有 init 块

7. 实际应用示例

7.1 策略模式

interface PaymentStrategy { fun pay(amount: Double) } class CreditCardPayment : PaymentStrategy { override fun pay(amount: Double) { println("Paying $$amount with Credit Card") } } class PayPalPayment : PaymentStrategy { override fun pay(amount: Double) { println("Paying $$amount with PayPal") } } class ShoppingCart(private var strategy: PaymentStrategy) { fun setPaymentStrategy(strategy: PaymentStrategy) { this.strategy = strategy } fun checkout(amount: Double) { strategy.pay(amount) } } fun main() { val cart = ShoppingCart(CreditCardPayment()) cart.checkout(100.0) // Paying $100.0 with Credit Card cart.setPaymentStrategy(PayPalPayment()) cart.checkout(200.0) // Paying $200.0 with PayPal }

7.2 观察者模式

interface Observer { fun update(message: String) } interface Observable { fun addObserver(observer: Observer) fun removeObserver(observer: Observer) fun notifyObservers(message: String) } class NewsAgency : Observable { private val observers = mutableListOf<Observer>() override fun addObserver(observer: Observer) { observers.add(observer) } override fun removeObserver(observer: Observer) { observers.remove(observer) } override fun notifyObservers(message: String) { observers.forEach { it.update(message) } } fun publishNews(news: String) { println("News published: $news") notifyObservers(news) } } class NewsChannel(val name: String) : Observer { override fun update(message: String) { println("$name received: $message") } } fun main() { val agency = NewsAgency() val channel1 = NewsChannel("CNN") val channel2 = NewsChannel("BBC") agency.addObserver(channel1) agency.addObserver(channel2) agency.publishNews("Kotlin 1.9 released!") // 输出: // News published: Kotlin 1.9 released! // CNN received: Kotlin 1.9 released! // BBC received: Kotlin 1.9 released! }

7.3 仓库模式

interface Repository<T> { fun getById(id: Int): T? fun getAll(): List<T> fun save(item: T) fun delete(id: Int) } class User(val id: Int, val name: String) class UserRepository : Repository<User> { private val users = mutableMapOf<Int, User>() override fun getById(id: Int): User? { return users[id] } override fun getAll(): List<User> { return users.values.toList() } override fun save(item: User) { users[item.id] = item } override fun delete(id: Int) { users.remove(id) } }

8. 接口委托

Kotlin 支持通过委托实现接口，这是实现装饰器模式的强大工具。

interface Printer { fun printDocument(document: String) } class LaserPrinter : Printer { override fun printDocument(document: String) { println("Laser printing: $document") } } // 通过委托实现接口 class LoggingPrinter(private val printer: Printer) : Printer by printer { override fun printDocument(document: String) { println("Log: Printing document") printer.printDocument(document) println("Log: Document printed") } } fun main() { val printer = LaserPrinter() val loggingPrinter = LoggingPrinter(printer) loggingPrinter.printDocument("Report.pdf") // 输出: // Log: Printing document // Laser printing: Report.pdf // Log: Document printed }

9. 内联接口（Kotlin 1.5+）

// 内联类实现接口 interface Printable { fun print(): String } @JvmInline value class Name(val s: String) : Printable { override fun print(): String = "Name: $s" } fun main() { val name = Name("Kotlin") println(name.print()) // Name: Kotlin }

10. 标记接口

标记接口是没有任何方法或属性的接口，用于标记类型。

// 标记接口 interface Entity interface Service interface Repository // 使用 class User : Entity class UserService : Service class UserRepository : Repository // 类型检查 fun process(entity: Entity) { when (entity) { is User -> println("Processing user") // 处理其他 Entity } }

11. 最佳实践

11.1 接口设计原则

// 1. 接口分离原则 - 多个小接口优于一个大接口 interface Readable { fun read(): String } interface Writable { fun write(data: String) } interface Deletable { fun delete() } // 2. 明确命名 interface UserAuthenticator // 好 interface Auth // 不够明确 // 3. 使用默认实现减少重复代码 interface JsonSerializable { fun toJson(): String fun isValidJson(): Boolean { return try { // JSON 验证逻辑 true } catch (e: Exception) { false } } }

11.2 接口 vs 抽象类选择

// 使用接口的场景 interface Vehicle { val maxSpeed: Int fun start() fun stop() } // 使用抽象类的场景 abstract class Animal { abstract val name: String val isAlive: Boolean = true // 可以有状态 abstract fun makeSound() open fun sleep() { // 可以有非 final 方法 println("$name is sleeping") } init { // 可以有初始化块 println("Animal created") } }

11.3 工厂模式示例

interface Shape { fun draw() fun area(): Double } class Circle(val radius: Double) : Shape { override fun draw() = println("Drawing circle with radius $radius") override fun area() = Math.PI * radius * radius } class Rectangle(val width: Double, val height: Double) : Shape { override fun draw() = println("Drawing rectangle ${width}x$height") override fun area() = width * height } object ShapeFactory { fun createShape(type: String, vararg params: Double): Shape? { return when (type.lowercase()) { "circle" -> if (params.size >= 1) Circle(params[0]) else null "rectangle" -> if (params.size >= 2) Rectangle(params[0], params[1]) else null else -> null } } } fun main() { val circle = ShapeFactory.createShape("circle", 5.0) val rectangle = ShapeFactory.createShape("rectangle", 4.0, 6.0) circle?.draw() // Drawing circle with radius 5.0 println(circle?.area()) // 78.53981633974483 rectangle?.draw() // Drawing rectangle 4.0x6.0 println(rectangle?.area()) // 24.0 }