我正在尝试在 Kotlin 中模拟 MongoCollection<Document> 类。
```kotlin
class MyTest {
  val mockCollection: MongoCollection<Document> = mock(MongoCollection<Document>::class.java)
}

val mockCollection: MongoCollection<Document> = mock(MongoCollection::class.java) as MongoCollection<Document>

MongoCollection<Document> mockCollection = (MongoCollection<Document>) mock(MongoCollection.class);

<details>
<summary>英文:</summary>
I am trying to mock MongoCollection&lt;Document&gt; class in kotlin

it given an error **only classes are allowed on the left side of a class literal**
on researching a bit I found this
https://stackoverflow.com/questions/46401104/only-classes-are-allowed-on-the-left-hand-side-of-a-class-literal
I tried create a type for **MongoCollection<Document>** and then passing it to the mock but it gives an error as the mock is of **Type**.
I also tried casting the mock to Document as below

but that gives an **NullpointerException** exception during access of MongoCollection inside implementation of code.
 
I have tried both the 
* Mockito kotlin library -https://mvnrepository.com/artifact/com.nhaarman.mockitokotlin2/mockito-kotlin/2.2.0
* Java mockito core - https://mvnrepository.com/artifact/org.mockito/mockito-core/3.5.13
and both of them have the same error?
I tired writing the same test in java and casting of generics work in it.

 
Does anyone have any experience of mocking generic classes in Kotlin?
</details>
# 答案1
**得分**: 1
我为您编写了以下示例：
```kotlin
interface Mosi<T> {
    fun mos(): T
}

class ExampleUnitTest {
    val mosi = Mockito.mock(Mosi::class.java)
    @Test
    fun test() {
        `when`(mosi.mos()).thenReturn("Mosi")
        assertEquals(mosi.mos(), "Mosi")
    }
}

class ExampleUnitTest {
    val mosi = Mockito.mock(Mosi::class.java) as Mosi<String>
    @Test
    fun test() {
        `when`(mosi.mos()).thenReturn("Mosi")
        assertEquals(mosi.mos(), "Mosi")
    }
}

class ExampleUnitTest {
    val mosi = mock<Mosi<String>> {
        on { mos() }
            .thenReturn("Mosi")
    }
    @Test
    fun test() {
        assertEquals(mosi.mos(), "Mosi")
    }
}

解决方案
---
创建一个类似于 Mockito 的扩展：
    inline fun <reified T: Any> mock() = Mockito.mock(T::class.java)
用法
---
像这样使用扩展函数：
    val mockCollection = mock<MongoCollection<Document>>()
    val mockCollection: MongoCollection<Document> = mock() // 我更喜欢这个
我总是使用这个而不是原始的 `mock()` 函数，因为可以推断出 Java 类。节省了一些输入。
题外话
---
甚至用于模拟 Lambdas 也非常好！
    val callback: () -> Unit = mock()
    someClass.doSomething(callback)
    verify(callback).invoke()