Java/Android/Kotlin: Reflection on private Field and call public methods on it

To access private properties and functions of a class in Kotlin here are two useful extension functions:

inline fun <reified T> T.callPrivateFunc(name: String, vararg args: Any?): Any? =
    T::class
        .declaredMemberFunctions
        .firstOrNull { it.name == name }
        ?.apply { isAccessible = true }
        ?.call(this, *args)

inline fun <reified T : Any, R> T.getPrivateProperty(name: String): R? =
    T::class
        .memberProperties
        .firstOrNull { it.name == name }
        ?.apply { isAccessible = true }
        ?.get(this) as? R

Usage Example:

class SomeClass {

    private val world: World = World()

    private fun somePrivateFunction() {
        println("somePrivateFunction")
    }

    private fun somePrivateFunctionWithParams(text: String) {
        println("somePrivateFunctionWithParams()  text=$text")
    }
}

class World {
    fun foo(): String = "Test func"
}

// calling private functions:

val someClass = SomeClass()
someClass.callPrivateFunc("somePrivateFunction")
someClass.callPrivateFunc("somePrivateFunctionWithParams", "test arg")

// getting private member and calling public function on it:

val world = someClass.getPrivateProperty<SomeClass, World>("world")
println(world?.foo())

To use reflection in Kotlin add dependency:

implementation "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"

If you like readable reusable easy to understand solution:

Define somewhere:

fun <T> Any.privateField(name: String): T {
    val field = this::class.java.getDeclaredField(name)
    field.isAccessible = true
    @Suppress("UNCHECKED_CAST")
    return field.get(this) as T
}

And use elsewhere:

val <T : TextInputLayout> T.startIconView: CheckableImageButton
    get() = privateField("startIconView")

It’s possible to make private fields accessible using reflection. The following examples (both written in Kotlin) show it...

Using Java Reflection:

val hello = Hello()
val f = hello::class.java.getDeclaredField("world")
f.isAccessible = true
val w = f.get(hello) as World
println(w.foo())

Using Kotlin Reflection:

val hello = Hello()
val f = Hello::class.memberProperties.find { it.name == "world" }
f?.let {
    it.isAccessible = true
    val w = it.get(hello) as World
    println(w.foo())
}