Question

I have strange stacktrace when click to submit button (this code is rendered but don’t work form processing). Example class:

public class SamplePage extends WebPage {

    private List<String> list = Arrays.asList(new String[] { "item1", "item2", "item3" });
    private List<String> selectedItem = Arrays.asList(new String[] { "item1" });

    public SamplePage(final PageParameters parameters) {
        super(parameters);

        Form<?> form = new Form<Void>("form");
        form.add(new Button("submin") {
            @Override
            public void onSubmit() {
                System.out.println("Selected");
                for (String tag : selectedItem)
                    System.out.println(tag);
            }
        });
        ListMultipleChoice<String> selector = new ListMultipleChoice<>("itemSelector",
            new PropertyModel<List<String>>(this, "selectedItem"), list);
        add(form);
        form.add(selector);
    }
    public List<String> getSelectedItem() {
        return selectedItem;
    }
    public void setSelectedItem(List<String> selectedItem) {
        this.selectedItem = selectedItem;
    }
} 

Caused by: java.lang.UnsupportedOperationException
    at java.util.AbstractList.remove(AbstractList.java:161)
    at java.util.AbstractList$Itr.remove(AbstractList.java:374)
    at java.util.AbstractList.removeRange(AbstractList.java:571)
    at java.util.AbstractList.clear(AbstractList.java:234)
    at org.apache.wicket.markup.html.form.FormComponent.updateCollectionModel(FormComponent.java:1531)
    at org.apache.wicket.markup.html.form.ListMultipleChoice.updateModel(ListMultipleChoice.java:369)
    at org.apache.wicket.markup.html.form.Form$FormModelUpdateVisitor.component(Form.java:221)
    at org.apache.wicket.markup.html.form.Form$FormModelUpdateVisitor.component(Form.java:192)
    at org.apache.wicket.util.visit.Visits.visitPostOrderHelper(Visits.java:273)
    at org.apache.wicket.util.visit.Visits.visitPostOrderHelper(Visits.java:261)
    at org.apache.wicket.util.visit.Visits.visitPostOrder(Visits.java:244)
    at org.apache.wicket.markup.html.form.FormComponent.visitComponentsPostOrder(FormComponent.java:388)
    at org.apache.wicket.markup.html.form.Form.internalUpdateFormComponentModels(Form.java:1701)
    at org.apache.wicket.markup.html.form.Form.updateFormComponentModels(Form.java:1666)
    at org.apache.wicket.markup.html.form.Form.process(Form.java:827)
    at org.apache.wicket.markup.html.form.Form.onFormSubmitted(Form.java:762)
    at org.apache.wicket.markup.html.form.Form.onFormSubmitted(Form.java:692)
... 31 more

How I understand problem is in definition model, but I don’t understand why (I tried a lot of ways and didn’t check problem). When I created DropDownChoice in same way, all worked correctly, but in this case I used

new PropertyModel<String>(this, "field")

in contrast to

new PropertyModel<List<String>>(this, "selectedItem")

I think it’s very silly mistake and ask your help.

Answer 1

Arrays.asList() returns a fixed size list, you (i.e. Wicket) cannot remove elements form it.

Use another list instead:

private List<String> selectedItem = new ArrayList<String>();

public SamplePage(final PageParameters parameters) {
    ....
    selectedItem.add("item1");