XStream arrayList към и от XML

Не знам къде е проблема в момента. За първи път използвам xml и имам някои проблеми с поставянето на ArrayList в xml файл и вземането му от него.

Намерих това и се опитах да го направя по същия начин: Как да конвертирате списък с обекти в XML документ с помощта на XStream

но за съжаление не успях. Ето какво имам досега: Клас, който съдържа ArrayList:

public class ElbowList{

private ArrayList<Elbow> elbows = new ArrayList<>();

public ElbowList(){
    elbows = new ArrayList<Elbow>();
}

public void setElbows(ArrayList<Elbow> elbows){
    this.elbows.clear();
    this.elbows = elbows;
}

public ArrayList<Elbow> getElbows() {
    return elbows;
}

public void add(Elbow elbow){
    elbows.add(elbow);
}
}

Запазване в XML:

MainFrame mainFrame = (MainFrame) SwingUtilities.getWindowAncestor(SetupPanel.this);
            ElbowList elbowList = (ElbowList) mainFrame.getObjects().get(2); //get ElbowList object
            XStream xstream = new XStream();
            xstream.alias("elbow", Elbow.class);
            xstream.alias("elbows", ElbowList.class);
            xstream.addImplicitCollection(ElbowList.class, "elbows", Elbow.class);

            String xml = xstream.toXML(elbowList.getElbows());
            System.out.println(xml);

            try {
                PrintWriter out = new PrintWriter("Save.xml");
                out.println(xml);
                out.close();
            } catch (FileNotFoundException ex) {
                Logger.getLogger(SetupPanel.class.getName()).log(Level.SEVERE, null, ex);
            }

И този по-горе всъщност изглежда работи правилно. Заредете XML файл, там имам извикване за изключение:

try {
                XStream xstream = new XStream();
                FileReader reader = new FileReader("Save.xml");

                MainFrame mainFrame = (MainFrame) SwingUtilities.getWindowAncestor(SetupPanel.this);
                ElbowList elbowList = (ElbowList) mainFrame.getObjects().get(2);
                elbowList.setElbows((ArrayList<Elbow>) xstream.fromXML(reader));//exception occurs here

                SpacePanel spacePanel = (SpacePanel) mainFrame.getObjects().get(1);
                spacePanel.repaint();

            } catch (IOException ex) {
                Logger.getLogger(SetupPanel.class.getName()).log(Level.SEVERE, null, ex);
            }

Изключение получавам:

Exception in thread "AWT-EventQueue-0" com.thoughtworks.xstream.converters.ConversionException: elbow : elbow
---- Debugging information ----
message             : elbow
cause-exception     : com.thoughtworks.xstream.mapper.CannotResolveClassException
cause-message       : elbow
class               : java.util.ArrayList
required-type       : java.util.ArrayList
converter-type      : com.thoughtworks.xstream.converters.collections.CollectionConverter
path                : /list/elbow
line number         : 2
version             : 1.4.7
-------------------------------
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:79)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:79)
    at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:50)
    at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:134)
    at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:32)
    at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1185)
    at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1169)
    at com.thoughtworks.xstream.XStream.fromXML(XStream.java:1040)
    at view.SetupPanel$2.actionPerformed(SetupPanel.java:78)
(...)

Не разбирам защо има изключение за преобразуване, за мен всичко изглежда правилно, ако се стигне до този ред, когато се появи изключение. Нямам повече идеи какво не е наред, моля помогнете.