provideDelegate.java

class StringProvider {
    operator fun provideDelegate(
            thisRef: Main,
            prop: KProperty<*>
    ): ReadOnlyProperty<Main, String> {
        if(checkProperty(thisRef, prop.name)) {
            return object : ReadOnlyProperty<Main, String> {
                override fun getValue(thisRef: Main, property: KProperty<*>): String {
                    return "true string"
                }
            }
        } else {
            return object : ReadOnlyProperty<Main, String> {
                override fun getValue(thisRef: Main, property: KProperty<*>): String {
                    return "false string"
                }
            }
        }
    }

    private fun checkProperty(thisRef: Main, name: String) = name.length > 4
}

fun bindString() =  StringProvider()

class Main {
    val fake by bindString()
    val valid by bindString()
}

fun main(args: Array<String>) {
    Main().apply {
        println(fake) //false string
        println(valid) //true string
    }
}