使用伴侣对象返回Kotlin中的类的实例

Question

Android Studio 3.0 RC2
Kolin 1.1.51

I creating an Android App, and I want to return the instance of the class that extends Application and access the equivalent you would do using a static in Java.

class BusbyMoviesMainApplication : Application() {
    companion object {
        private val instance: BusbyMoviesMainApplication = BusbyMoviesMainApplication()

        @JvmStatic
        fun getBusbyInstance(): BusbyMoviesMainApplication {
            return instance
        }
    }
}

I am accessing it like this. However, getCacheDir returns null

BusbyMoviesMainApplication.getBusbyInstance().getCacheDir()

I can't see what I am doing wrong.

In Java I have done like this before, which works, I am trying to do the same in Kotlin:

public class BusbyMoviesApplication extends Application {
    private static BusbyMoviesApplication mBusbyMoviesApplication;

    public static BusbyMoviesApplication getInstance() {
        return mBusbyMoviesApplication;
    }

    @Override
    public void onCreate() {
        super.onCreate();

        mBusbyMoviesApplication = BusbyMoviesApplication.this;
    }
}

Answer 1

In the Kotlin code, you're calling the constructor of the Application subclass directly instead of letting the framework create it for you. You could instead do what you did in the Java code, and initialize the instance property in the Application 's onCreate method (plus I shortened the getter a bit):

class BusbyMoviesMainApplication : Application() {
    companion object {
        private lateinit var instance: BusbyMoviesMainApplication

        @JvmStatic
        fun getBusbyInstance() = instance
    }

    override fun onCreate() {
        super.onCreate()
        instance = this
    }
}

---

Based on the discussion in the comments below, this would perhaps be a more idiomatic solution for the getter:

class BusbyMoviesMainApplication : Application() {
    companion object {
        @JvmStatic
        lateinit var instance: BusbyMoviesMainApplication
            private set
    }

    override fun onCreate() {
        super.onCreate()
        instance = this
    }
}