Question

I am trying to design a model in which I have a List of List of child object having same super class. This model will be a Model of a ListView. The super class is:

public class Element implements Serializable {

    private static final long serialVersionUID = 10121L;
    
    private String value;

    public Element(String value) {
        this.value = value;
    }

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }

    @Override
    public String toString() {
        return "Element [value=" + value + "]";
    }   
}

One of the sub class is:

public class DateElement extends Element {

    private static final long serialVersionUID = 10122L;

    private DateTime date;
    
    public DateElement(String value, DateTime date) {
        super(value);
        this.date = date;
    }

    public DateTime getDate() {
        return date;
    }

    public void setDate(DateTime date) {
        this.date = date;
    }

    @Override
    public String toString() {
        return "DateElement [date=" + date + ", value=" + getValue() + "]";
    }   
}

I am instantiating the List of List as:

this.userMonitorMap = new ArrayList<List<? extends Element>>(0);

Adding a List of DateElement in userMonitorMap as:

List<DateElement> dateElements = getDateElements();
userMonitorMap.add(dateElements);

Upto this portion I am having no trouble. But the population of the ListView has problem, the code snippet is:

        ListView row = new ListView("row", userMonitorMap) {
            
            @Override
            protected void populateItem(ListItem rowItem) {
                List<? extends Element> columnMap = (List<? extends Element>) rowItem;
                ListView column = new ListView("column", columnMap) {
                    
                    @Override
                    protected void populateItem(ListItem columnItem) {
                        Element element = (Element) columnItem; //The compile time error is here
                    }
                };
                rowItem.add(column);
            }
        };

I have commented the problem in my code. How can I achieve this? What are the changes do I need?

The eclipse IDE red marked that line and the error message is: Cannot cast from ListItem to Element

Thanks and Regards.

Answer 1

The ListItem isn’t the object in the list itself, it contains a model which contains your object. You should be using:

Model<Element> elementModel = columnItem.getModel();

or

Element element = columnItem.getModelObject();

Take a look at these examples

Your code should read:

ListView row = new ListView(“row”, userMonitorMap) {

        @Override
        protected void populateItem(ListItem rowItem) {
            List<? extends Element> columnMap = rowItem.getModelObject();
            ListView column = new ListView("column", columnMap) {

                @Override
                protected void populateItem(ListItem columnItem) {
                    Element element = columnItem.getModelObject();
                }
            };
            rowItem.add(column);
        }
    };