Kotlin 中的 Android Room 错误

Question

I think the is a semantic issue in Room while working in Kotlin. Simple DAO query in Room can be obtained by

@Query("SELECT * FROM Users WHERE id = :id")
fun getUser(id: Int): User

Although, I defined return type as User , not User? , this query can still returns null, when there is no User with the given id .

So whenever you call this function, you definitely need to check for null return, as below

val user = userDao.getUser("someid")
if (user != null){
    return user
}
else {
    return DEFAULT_USER
}

But since we defined return type of getUser as User , @kotlin compiler suggest that null check is redundant .

Am I missing something? Can anyone give some feedback on this?

Answer 1

There is no semantic issue. If Your function can return a null, it should be defined in the function return type. So, your function signature should be like this

fun getUser(id: Int): User?

This means the function can return null and now your null checks should work fine.

Answer 2

Apparently it's already tracked issue in Google Issue Tracker And as a respond they developers say

Status: Won't Fix (Intended Behavior)

this is not about generated code though, you are the person who creates that Query method so you should declare it as nullable.

So, if we know that our code can return null , we should declare it as Nullable as

@Query("SELECT * FROM Users WHERE id = :id")
fun getUser(id: Int): User?