code: public void add(ApplicationWindow window) {
          if (activeWindow == null) // first window will be set as activeWindow
              setActiveWindow(window);
  
          if (!windows.contains(window)) {
              windows.add(window);
              window.setWindowManager(this);
              setChanged();
              notifyObservers();
          }
      }
docs: Adds the given window to the set of windows managed by this window
  manager. Does nothing is this window is already managed by this window
  manager.
  
  @param window
  the window