java - "Return type of ... is not a subtype of the return type of the overridden member"？

Question

我正在尝试对一个函数 ActionableNotification.java 进行子类化，其内容类似于:

package com.venmo.notifications.notifications;

import android.content.ContextWrapper;
import com.squareup.picasso.Target;

public abstract class ActionableNotification extends ContextWrapper implements Target {

    @NonNull
    protected abstract String getTrackingNotificationCategory();
    // ...
}

继承类的读法与此类似

class DeclineInsufficientFundsBalanceOnlyCardNotification(context : Context, intent : Intent) : ActionableNotification(context, intent) {

    override fun getTrackingNotificationCategory() = ""
    // ...
 }

但是，当我尝试编译时出现此错误:

/Users/kupeek/dev/venmo-android/p2p-app/src/main/java/com/venmo/notifications/notifications/DeclineInsufficientFundsBalanceOnlyCardNotification.kt: (21, 53): Return type of 'getTrackingNotificationCategory' is not a subtype of the return type of the overridden member '@NonNull protected/protected and package/ abstract fun getTrackingNotificationCategory(): String defined in com.venmo.notifications.notifications.ActionableNotification'

我不明白这个错误信息，因为Kotlin继承类的返回类型是字符串，基类也是如此？

Answer 1

也许用 JetBrains 注解 @NotNull 来注解抽象方法会更好。我发现它看了一下this 。希望这可以帮助您。