Kotlin reflection for object class gives null

Question

I'm diving into reflection and I've met the following issue with this object class:

object Dict {

     fun getAll(): Map<String, List<String>> {
        return mapOf(
            "Mark" to listOf("Banana"),
            "John" to listOf("Strawbery", "Pie"),
            "Lily" to listOf("Apple", "Banana", "Pie"),
        )
    }
}

knowing the package canonical name of this class I can get this method:

val dictClass = Class.forName("com.example.util.Dict")
val getAllRelationsMethod = relationsClass.getMethod("getAll")
val peopleWithMeals = getAllRelationsMethod.invoke(null) // this line gives error

I got the following exception: java.lang.NullPointerException: null receiver

I'm passing the null into the invoke() method because it doesn't need a class instance to call (like static in java). Does the method call differs from Java here somehow? Didn't find a tip around the Internet so that's why I'm asking here. Thanks in advance! :)

gmetax · Accepted Answer · 2019-11-27 12:29:27Z

4

If I remember correctly you have to change it like that ..

@JvmStatic
fun getAll(): Map<String, List<String>> {
    return mapOf(
        "Mark" to listOf("Banana"),
        "John" to listOf("Strawbery", "Pie"),
        "Lily" to listOf("Apple", "Banana", "Pie"),
    )
}

JVMStatic

edited Nov 27, 2019 at 12:29

answered Nov 27, 2019 at 12:05

gmetax

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2 Comments

shurrok Over a year ago

Wow, that works. I thought I need to use this annotation only if my code is supposed to work with some other java code... Thanks!

gmetax Over a year ago

If I am correct, the part or reflection is java, so you want the method to be static in java that is why you need the annotation :)

Roland · Accepted Answer · 2019-11-27 13:37:07Z

1

As you can access (and have control over) both, the caller, as also the target, you probably shouldn't use reflection. @JvmStatic is a solution to this, but only if you can alter the target class.

If you can't and still need reflection, the following may help you:

val dictClass = Class.forName("com.example.util.Dict")
val getAllRelationsMethod = relationsClass.getMethod("getAll")
val objectInstance = dictClass.getField("INSTANCE").get(null) // get value of static field INSTANCE (which every object has)
val peopleWithMeals = getAllRelationsMethod.invoke(objectInstance)

It first extracts the object's instance with which you can call the object functions.

If you know that the object is always Dict and you imported it appropriately, you can also just use the following to call a function of/on it:

val dictClass = Class.forName("com.example.util.Dict")
val getAllRelationsMethod = relationsClass.getMethod("getAll")
val peopleWithMeals = getAllRelationsMethod.invoke(Dict) // Dict is an object and here the actual instance is used...

If it isn't very generic what you want to do, then omit reflection. Maybe having appropriate interfaces may help you accomplish something similar instead of using reflection?

answered Nov 27, 2019 at 13:37

Roland

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8 Comments

shurrok Over a year ago

It is very generic indeed. But thanks for all the tips :)

Roland Over a year ago

if it is very generic, then @JvmStatic is probably no solution to it... and if it is, it is probably not that generic ;-)

shurrok Over a year ago

How can you know that without knowing the context of the project? :D

Roland Over a year ago

experience (if you can set @JvmStatic everywhere, then you can also introduce an appropriate other structure) ... but as I do not know for sure I used "probably" ;-)

shurrok Over a year ago

I assure you that the reflection is exactly what I need in my strict example. I just want to point that the question wasn't about if I should use the reflection or not but how to get the object function from it ;-)

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