How to mock a CompletableFuture in Kotlin - kotlin

Question

We have some code to read data from DynamoDB:
suspend fun getKeys(owner: String): Set<String> {
...
val query = ...
query.subscribe { page -> foo(page) }.await()
return ...
}
The subscribe{} above is defined in AWS SDK:
default CompletableFuture<Void> subscribe(Consumer<T> consumer)
I'd like to unit test the logic in this function, for subscribe I don't care about it, hopefully just mock and do nothing.
I tried mock a callback in my unit test (irrelevant code removed):
class BucketRepoTest(
#Mock private val query: SdkPublisher<Page<DbBucket>>
) {
#Test
fun `get keys should be working`() {
val callback = mock<(Page<DbBucket>) -> Unit>()
val result = mock<CompletableFuture<Void>>()
whenever(query.subscribe(callback)).thenReturn(result)
runBlocking {
val keys = data.getKeys("Charlie")
assert(keys.isEmpty())
}
}
}
But when I run the test I got NPE:
query.subscribe { page -> foo(page) } must not be null
java.lang.NullPointerException: query.subscribe { page -> foo(page) } must not be null
at com.myApp.getKeys(myfile.kt:75)
at ...
Any idea how to fix it?

Answer 1

The result cannot be void.
val result = mock<CompletableFuture>()
try to create a mock data and pass as result
val result = the object or data type you want to more
Example
data Result( Charlie = "john", age = "23")
then
class BucketRepoTest(
#Mock private val query: SdkPublisher<Page<DbBucket>> ) {
#Test
fun `get keys should be working`() {
val callback = mock<(Page<DbBucket>) -> Unit>()
val result = Result()
whenever(query.subscribe(callback)).thenReturn(result)
runBlocking {
val keys = data.getKeys("Charlie")
assert(keys.isEmpty())
}
}
}