Kotlin 开发 - lateinit 关键字

lateinit

1、基本介绍

lateinit 用于延迟初始化非空属性，避免声明时立即赋值或使用可空类型
lateinit 只能用于 var，不能用于 val
类型必须是非空的引用类型，不能是 Int、Boolean 等基本类型
声明时不能有初始值

何时不用 lateinit，可以用可空类型 var data: String? = null 替代，但每次使用都要 ?. 调用，不够简洁

2、演示

基本使用

kotlin 复制代码

class UserService {
    fun getUsers(): List<String> = listOf("Alice", "Bob")
}

class UserController {
    private lateinit var userService: UserService

    fun initialize() {
        userService = UserService()
    }

    fun showUsers() {
        if (::userService.isInitialized) {
            println(userService.getUsers())
        } else {
            println("service not ready")
        }
    }
}

kotlin 复制代码

val userController = UserController()

userController.initialize()
userController.showUsers()

复制代码

# 输出结果

[Alice, Bob]

kotlin 复制代码

val userController = UserController()

userController.showUsers()

复制代码

# 输出结果

service not ready

未初始化时访问会抛出 UninitializedPropertyAccessException 异常

kotlin 复制代码

class UserService {
    fun getUsers(): List<String> = listOf("Alice", "Bob")
}

class UserController {
    private lateinit var userService: UserService

    fun initialize() {
        userService = UserService()
    }

    fun showUsers() {
        println(userService.getUsers())
    }
}

kotlin 复制代码

val userController = UserController()

userController.showUsers()

复制代码

# 输出结果

kotlin.UninitializedPropertyAccessException: lateinit property userService has not been initialized

lateinit 不能用于 val，否则编译失败

kotlin 复制代码

class UserService {
    fun getUsers(): List<String> = listOf("Alice", "Bob")
}

class UserController {
    private lateinit val userService: UserService

    fun initialize() {
        userService = UserService()
    }

    fun showUsers() {
        if (::userService.isInitialized) {
            println(userService.getUsers())
        } else {
            println("service not ready")
        }
    }
}

复制代码

# 输出结果

'lateinit' modifier 'is allowed only on mutable properties'.

类型必须是非空的引用类型，不能是 Int、Boolean 等基本类型，否则编译失败

kotlin 复制代码

class Person {
    private lateinit var num: Int
}

复制代码

# 输出结果

'lateinit' modifier 'is not allowed on properties of primitive types'.

声明时不能有初始值，否则编译失败

kotlin 复制代码

class UserService {
    fun getUsers(): List<String> = listOf("Alice", "Bob")
}

class UserController {
    private lateinit var userService: UserService = UserService()

    fun initialize() {
        userService = UserService()
    }

    fun showUsers() {
        if (::userService.isInitialized) {
            println(userService.getUsers())
        } else {
            println("service not ready")
        }
    }
}