Java call method after class is initialized

I have a class X which is extended by various classes. X needs to know each subtype that exists of it at runtime. So my idea was to create a static method "static init()" in X that takes a Class as parameter. The problem is that a subclass of X is required to be initialized via its static-init-blocks before the call to init() happens. X for example may discover some of the static fields of the subclass which may be of any declared type. So take this code for example:

class X {
    X() {
        /*This would be one idea but does not work
          in this concrete example.*/
        init(getClass());
    }
    static void init(Class<? extends X> c) {
         if(c.getDeclaredField("a").get(null) == null) {
             throw new AssertionError();
         }
    }
}

class Y extends X {
    static X a = new X();
    static {
        /*Another idea. Works totally fine but
          I dont want to trust subclasses that
          they do that correctly*/
        init(Y.class);
    }
}

So what I am looking for is a way to somehow get my init()-method called as the last step in the static initialization of a class. Or any other way to prevent the AssertionError from happening.