出现错误：(55, 41) Kotlin：类型推断失败

Question

我收到Error:(55, 41) Kotlin: Type inference failed. Expected type mismatch: inferred type is (Mutable)List<Optional<Address!>!>! but List<Address>? was expected

我将数据保存到我的 build.gradle 的 mongoDB 文档中：

implementation 'org.springframework.boot:spring-boot-starter-actuator'
implementation 'org.springframework.boot:spring-boot-starter-data-mongodb'
implementation 'org.springframework.boot:spring-boot-starter-data-rest'
implementation 'org.springframework.boot:spring-boot-starter-hateoas'
implementation 'org.springframework.boot:spring-boot-starter-web'
implementation 'com.fasterxml.jackson.module:jackson-module-kotlin'

我的文档对象

@Document(collection = "address")
class Address {
    @Id
    var id: String? = null
    var number: String? = null
    var street: String? = null
    var neighborhood: String? = null
    var locality: String? = null
    var region: String? = null
    var country: String? = null
    var code: String? = null
    var google_place_id: String? = null

    constructor()
}

和

@Document(collection = "person")
class Person {
    @Id
    var id: String? = null
    var name: String? = null
    var lastName: String? = null
    var phone: String? = null
    var email: String? = null
    var password: String? = null
    @DBRef(db = "mlm")
    var address: List<Address>? = null

    constructor()
}

我有接口

@Repository
interface PersonRepository : MongoRepository<Person, String>

和

@Repository
interface AddressRepository : MongoRepository<Address, String>

我的控制器工作
我的播种机/井测试真的要看看如何做到这一点，这就是我遇到问题的地方

class DbSeeder {
    @Autowired
    private val personRepository: PersonRepository? = null
    @Autowired
    private val addressRepository: AddressRepository? = null

    fun addressLoading() {
        val address1 = Address()
        address1.id = "5cb2e9424274072ec4bb4199"
        address1.number = "1"
        address1.street = "Microsoft Way"
        address1.neighborhood = "Redmond"
        address1.locality = "King County"
        address1.region = "Washington"
        address1.code = "425"
        address1.country = "United States"
        address1.google_place_id = "5644+83 Centurion"

        val address2 = Address()
        address2.id = "5cb2e9424274072ec4bb4198"
        address2.number = "1600"
        address2.street = "Amphitheatre Parkway"
        address2.neighborhood = ""
        address2.locality = "Mountain View"
        address2.region = "California"
        address2.country = "United States"
        address2.code = "94043"
        address2.google_place_id = "CWC8+Q9 Mountain View, California, USA"

        val address = Arrays.asList(address1, address2)
        this.addressRepository!!.insert(address)
    }

    fun personLoading() {
        val personDocument = Person()
        personDocument.id = "5cb2e9424274072ec4bb4197"
        personDocument.name = "William"
        personDocument.lastName = "Gates"
        personDocument.phone = "1081010810"
        personDocument.email = "bill.gates@gmail.com"
        personDocument.password = "bill-secret"

        val personAddressDBRef = addressRepository!!.findById("5cb2e9424274072ec4bb4199")

        personDocument.address = Arrays.asList(personAddressDBRef)

    //  val personDBRef = personRepository!!.save(personDocument)  // If the ObjectID is requires else ware
        personRepository!!.save(personDocument)
    }
}

要保存我的地址，我需要找到与 ObjectID 相关的地址，然后将 ObjectID 保存为个人文档中的 DBRef

val personAddressDBRef = addressRepository!!.findById("5cb2e9424274072ec4bb4199")
personDocument.address = Arrays.asList(personAddressDBRef) // << Error here

在最后一行我收到错误Error:(55, 41) Kotlin: Type inference failed. Expected type mismatch: inferred type is (Mutable)List<Optional<Address!>!>! but List<Address>? was expected 我似乎不知道如何继续
我在 GitHub 上提供了它https://github.com/Morons/gofetchbyidandinsertdbref.git
任何帮助将不胜感激

Answer 1

您需要阅读https://kotlinlang.org/docs/reference/java-interop.html#null-safety-and-platform-types 以了解推断类型的编写方式。但问题很简单：

1. personAddressDBRef 具有 Optional<Address> 类型，因为这是 findById 返回的内容。

2. 所以Arrays.asList(personAddressDBRef) 是List<Optional<Address>>（同样，请参阅上面的链接了解为什么您会看到更复杂的类型）。

3. 您不能将personDocument.address 设置为List<Optional<Address>>，而是需要List<Address>。

因此您需要决定如何将Optional<Address> 转换为List<Address>。一种方法是

personDocument.address = personAddressDBRef.map { listOf(it) }.orElseGet { listOf() }