How to access Kotlin companion object in Java?

Ops, I got it. Just use the below.

MainApplication application = MainApplication.Companion.get(mContext);

You can add @JvmStatic annotation to the method in companion object to make Kotlin generate a static method.

class MainApplication : Application() {
    companion object {
        @JvmStatic fun get(context: Context): MainApplication {
            return context.applicationContext as MainApplication
        }
    }
}

you can then access it from Java like before converting to Kotlin:

MainApplication application = MainApplication.get(mContext);

EDIT: I feel obliged to add something I learned recently: @JvmStatic doesn't actually move where the method gets generated. It duplicates it, by generating a static method for Java in addition to the method on the companion object. Personally I think this isn't great and it can have some implications depending on a use case, so something worth knowing.

By omitting the name of your companion object, the name Companion must be used to access the methods.

Example:

class MyClass1 {
    companion object Object1 {
        fun method1 {
        }
    }
}

class MyClass2 {
    companion object {
        fun method2 {
        }
    }
}

To invoke the first companion object method you would do the following:

MyClass1.method1()

To invoke the second:

MyClass2.Companion.method2()

See the Kotlin docs on Companion Objects for details.