I don't know Java, but i think the problem is that you are writing B4.Log in a definition of a static class. Try to put it in a method and call the method using JavaObject
B4X:
#if Java
public static class MyAdListener extends com.google.android.gms.ads.AdListener {
String eventName = "nativead";
public void CallLog(){
BA.Log(eventName);
}
@Override
public void onAdClosed() {
processBA.raiseEventFromDifferentThread(null, null, 0, eventName + "_adclosed", false, null);
}