java – Why does not a constant class static variable reconcile established value from another class?

Hello, I have a constant class in which there is a base url

    public static String BASE_URL = "";

I'm assigning this value of my login activity

    URLsAndConstants.BASE_URL + = enterUrlEditText.getText (). ToString ();

And then I'm doing this in my Constant class again

    public static string AUTHENTICATE_USER_BY_AD = BASE_URL + "/ MyApi";

Although I can register the value of BASE_URL after the assignment, I can not add it to the other static string, that is,
when I use this Public static string AUTHENTICATE_USER_BY_AD
in API does not concatenate the value of BASE_URL. What am I doing wrong and what should I do?