The TreeItem API exposes the getChildren() method. The method is not marked final, therefore, I can override it.
The behavior I was expecting is that, if I override this method, I can pass a different observable list then the default one.

The below JFXTreeItem extends TreeItem by overriding getChildren(). However, when used in a treeView, I do not get children displayed on screen.

-----------------------------------------------------------------------------------------------------------------------------------
public class JFXTreeItem<T> extends TreeItem<T> {

    private ObservableList<TreeItem<T>> children;

    public JFXTreeItem(T value, Node graphic) {
        super(value, graphic);
        children = FXCollections.observableArrayList();
    }


    @Override
    public ObservableList<TreeItem<T>> getChildren() {
        return children;
    }
}
-----------------------------------------------------------------------------------------------------------------------------------

A closer look at the TreeItem code reveals two problems:
1- getChildren() in TreeItem is setting a changeListener (which is private and not exposed to subclasses). Therefore any overriding of getChildren() will loose this changeListener. I am referring to the code below from TreeItem :

public ObservableList<TreeItem<T>> getChildren()
  {
    if (this.children == null) {
      this.children = FXCollections.observableArrayList();
      this.children.addListener(this.childrenListener);
    }

    if (this.children.isEmpty()) return this.children;

    checkSortState();

    return this.children;
  }

2- The code of TreeItem does not consistently call getChildren(), and at times will call <this.children>. Since the only exposed API is getChildren(), the value in this.children becomes irrelevant if the developer overrided getChildren(). As an example, the sort method :

 void sort()
  {
    sort(this.children, this.lastComparator, this.lastSortMode);
  }


The only way where changing the children list works is by a hack through reflection. Something like the code below:

----------------------------------------------------------------------------------------------------------------------------------------
   public JFXTreeItem(T value, Node graphic) {
        super(value, graphic);

        ObservableList<TreeItem<T>> children = FXCollections.observableArrayList();

        try {
            Field childrenField = TreeItem.class.getDeclaredField("children");
            childrenField.setAccessible(true);
            childrenField.set(this, children);

            Field declaredField = TreeItem.class.getDeclaredField("childrenListener");
            declaredField.setAccessible(true);
            children.addListener((ListChangeListener<? super TreeItem<T>>) declaredField.get(this));

        } catch (Exception e) {
            e.printStackTrace();
        }
------------------------------------------------------------------------------------------------------------------------------------------

This code works, and items are displayed on the screen correctly.

However, I don't think as an end developer I should resort to this tweak.
Either the getChildren() remains available for overriding , and the internals of TreeItem get fixed.
Or getChildren() should be declared final, and therefore the developer knows that overriding it won't work.