This class can be used to implement any java interface. Any java vm method invocation matching the interface on the proxy object will automatically be passed to the c# implementation.

// Opens an android date picker dialog and grabs the result using a callback.
private static DateTime selectedDate = DateTime.Now;
class DateCallback : AndroidJavaProxy
{
	public DateCallback() : base("android.app.DatePickerDialog$OnDateSetListener") { }
	void onDateSet(AndroidJavaObject view, int year, int monthOfYear, int dayOfMonth)
	{
		selectedDate = new DateTime(year, monthOfYear+1, dayOfMonth);
	}
}
void OnGUI ()
{
	if (GUI.Button(new Rect (15, 15, 450, 75), string.Format("{0:yyyy-MM-dd}", selectedDate)))
	{
		var activity = new AndroidJavaClass("com.unity3d.player.UnityPlayer").GetStatic<AndroidJavaObject>("currentActivity");
		activity.Call("runOnUiThread", new AndroidJavaRunnable(() =>
		{
			new AndroidJavaObject("android.app.DatePickerDialog", activity, new DateCallback(), selectedDate.Year, selectedDate.Month-1, selectedDate.Day).Call("show");
		}));
	}
}