如何将“空”选择添加到ComboBox？

Question

I have a ComboBox populated with a custom object. However, I need to allow the selection of no selection (or a null value). I have tried comboBox.getItems().add(null) but that does not add an empty selection.

How do I add a blank selection at the top to essentially allow a user to "deselect" all items?

Answer 1

You could add a placeholder element. Instead of checking for deselected items you simply need to test reference equality. Depending on the class design you may need to use a custom cell factory to display the no text for this item:

public class CustomItem {

    private final String text;

    public CustomItem(String text) {
        this.text = text;
    }

    public String getText() {
        return text;
    }

}

final CustomItem emptyPlaceholder = new CustomItem(null);
ComboBox<CustomItem> combo = new ComboBox<>();
combo.getItems().addAll(emptyPlaceholder, new CustomItem("foo"), new CustomItem("bar"));
combo.setCellFactory(lv -> new ListCell<CustomItem>() {

    @Override
    protected void updateItem(CustomItem item, boolean empty) {
        super.updateItem(item, empty);

        setText((empty || item == null || item == emptyPlaceholder)
                ? ""
                : item.getText());
    }

});
combo.setButtonCell(combo.getCellFactory().call(null));