kotlin null 默认值指针异常

Question

I have code like this one:

@Service
class SomeClass (
    private val departmentClient : DepartmentClient
) {
    fun someFunction(
        employee: Employee,
        department: Department = departmentClient.getById(employee.departmentId)
    ): Unit {
        here my code
    }
}

data class Employee(val departmentId: Long, val id: Long)
data class Department(val id: Long)

@Service
class DepartmentClient() {
    fun getById(id: Long): Department
}

When I don't pass the department parameter in someFunction, I expect that departmentClient.getById(employee.departmentId) will be called. The problem is that in some cases I get a null pointer exception in this line, but in others, I don't. All dependencies are injected by Spring.

Answer 1

The function getById is probably defined wrongly and at runtime it hits with a null-pointer-exception. Where does this function come from?

It should look like this:

class DepartmentClient() {
    fun getById(id: Long): Department?
}

That would mean you need to deal with department: Department? in here my code .

Answer 2

I think this happens when You assigned a variable null value You can fix the problem like this by using '?'or '?.' (safety operator) by putting '?' after a Type (Department)

fun someFunction(
        employee: Employee,
        department: Department? = departmentClient.getById(employee.departmentId)
    ): Unit {
        here my code
    }