I have made my own C# classes in Unity, is there any way I can access these classes from the Android Activity that starts the UnityPlayer?

Example: I have a C# class called testClass in Unity:

class testClass{
    public static string myString="test string";
}

From the Android activity in Java I want to access that class:

string str=testClass.myString;

Is this possible? If so, how? Or is there some other way to do this? In the end I basically want to communicate between my Android activity and the UnityPlayer object.

Thanks in advance.

EDIT: Ok so I looked at building Android plugins for Unity but this wasn't satisfactory to me. I ended up building a socket client-server interface in Unity with C# and another one in Java for the Android app:

So Unity listens on port X and broadcasts on port Y

The Android activity listens on port Y and broadcasts on port X

This is necessary as both interfaces are running on the same host.

So that's how I solved my problem, but I'm open for any suggestions if anyone knows a better way of communicating between the Unityplayer and your app.