How can I read the value of property in a Kotlin data class instance if the property name is only known at runtime?
:: is used for Reflection in kotlin. Class Reference val myClass = MyClass::class.
The KClass type is Kotlin's counterpart to Java's java. lang. Class type. It's used to hold references to Kotlin classes; you'll see what it lets you do with those classes in the “Reflection” section later in this chapter. The type parameter of KClass specifies which Kotlin classes can be referred to by this reference.
Properties vs fieldsField is just a class member variable that hold a value. It can be read-only or mutable and marked with any access modifier such as public or private . Property is more complex element that contain a private field and accessors. By accessors I mean getter and setter.
Here is a function to read a property from an instance of a class given the property name (throws exception if property not found, but you can change that behaviour):
import kotlin.reflect.KProperty1
import kotlin.reflect.full.memberProperties
@Suppress("UNCHECKED_CAST")
fun <R> readInstanceProperty(instance: Any, propertyName: String): R {
val property = instance::class.members
// don't cast here to <Any, R>, it would succeed silently
.first { it.name == propertyName } as KProperty1<Any, *>
// force a invalid cast exception if incorrect type here
return property.get(instance) as R
}
dependencies {
implementation(kotlin("reflect"))
}
// some data class
data class MyData(val name: String, val age: Int)
val sample = MyData("Fred", 33)
// and reading property "name" from an instance...
val name: String = readInstanceProperty(sample, "name")
// and reading property "age" placing the type on the function call...
val age = readInstanceProperty<Int>(sample, "age")
println(name) // Fred
println(age) // 33
You can do it through reflection, and it's all the same for data classes and normal ones.
The first option is just to use Java reflection:
val name = obj.javaClass
.getMethod("getName") // to get property called `name`
.invoke(obj)
You can even make an extension function:
inline fun <reified T : Any> Any.getThroughReflection(propertyName: String): T? {
val getterName = "get" + propertyName.capitalize()
return try {
javaClass.getMethod(getterName).invoke(this) as? T
} catch (e: NoSuchMethodException) {
null
}
}
It calls the public getter. To get a value of a private property you can modify this code using getDeclaredMethod
and setAccessible
. This will also work for Java objects with the corresponding getters (but it misses the convention of is
and has
getters for boolean
).
And usage:
data class Person(val name: String, val employed: Boolean)
val p = Person("Jane", true)
val name = p.getThroughReflection<String>("name")
val employed = p.getThroughReflection<Boolean>("employed")
println("$name - $employed") // Jane - true
kotlin-reflect
library which you should add to your project separately, here's its documentation. It will let you get actual Kotlin property value, ignoring Java getters.
You can use javaClass.kotlin
to get actual Kotlin class token, and then you get a property from it:
val name = p.javaClass.kotlin.memberProperties.first { it.name == "name" }.get(p)
This solution will work only for Kotlin classes, not for Java ones, but it is much more reliable if you need to work with Kotlin classes: it doesn't depend on the underlying implementation.
The answers above didn't work for me so I've created an extension function for this:
@Throws(IllegalAccessException::class, ClassCastException::class)
inline fun <reified T> Any.getField(fieldName: String): T? {
this::class.memberProperties.forEach { kCallable ->
if (fieldName == kCallable.name) {
return kCallable.getter.call(this) as T?
}
}
return null
}
This is an example call:
val valueNeeded: String? = yourObject.getField<String>("exampleFieldName")
Also include this in your app's build.gradle:
implementation "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
I wonder if it is possible to define the type of the field programmatically. You can get the type easily with:
kCallable.returnType
However you still have to assign the generic type explicitely:
getField<String>
instead of
getField<kCallable.returnType>
Edit: I ended up using the following:
when (prop.call(object)) {
is ObservableList<*> -> {}
is Property<*> -> {}
}
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With