Android 如何转换行为主体<；可选<；列表<；文件>&燃气轮机&燃气轮机；去科特林？

我在RxJava中有BehaviorSubject，但我无法将其转换为Kotlin

public class Test {

private final BehaviorSubject<Optional<List<File>>> fileList = BehaviorSubject.createDefault(Optional.absent());

void test() {
        File file = new File("");
        fileList.getValue().get().add(file);
    }

}

---

Kotlin区分不可变列表（默认）和可变列表。改为声明一个可变列表

class Test {

private val fileList = BehaviorSubject.createDefault(Optional.absent<List<File>>())

internal fun test() {
    val file = File("")
    fileList.value.get().add(file)
    }

}

implementation 'io.reactivex.rxjava2:rxandroid:2.0.2'
implementation 'io.reactivex.rxjava2:rxjava:2.1.13'
implementation 'io.reactivex.rxjava2:rxkotlin:2.2.0'