JTable JComboBox默认在列表扩展时显示第一项

Question

I have been trying to determine why my JComboBox is displaying the 1st item in the list through numerous Google searches, but I'm struggling to find relevant help. It could be that I don't know the correct terminology (hence the overly specific title of this question) and thus not finding the information that would explain my issue. I checked out the JComboBox API, and few of the listeners and models that it uses, but they did not seem likely candidates.

The JComboBox in question is inside a JTable, so I am not aware if that changes the default behaviour of it. The code I am using is as below:

//row and col are final due to usage inside anonymous inner class
public TableCellEditor getCellEditor(final int row, final int col)
{
    String[] listItems = new String[arrayList.getSize()];
    int i = -1;

    for(String s : arrayList)
    {
        i++;
        listItems[i] = s;
    }

    JComboBox<String> box = new JComboBox<>(listItems);
    box.addItemListener(new ItemListener()
    {
        public void itemStateChanged(ItemEvent e)
        {
            if(e.getStateChange() == ItemEvent.SELECTED)
            {
                if(e.getItem().equals("Add/Edit Projectile"))
                {
                    //Where Editor is a JFrame that will be opened 
                    new Editor();
                }
            }
        }
    });

    DefaultCellEditor list = new DefaultCellEditor(box);
}

Please note that the Arraylist in my program does not contain Strings, but instead a more complicated set of custom objects that I believe would distract from the main issue.

I haven't included a Renderer for JComboBox's in the JTable as I was happy enough with the way it appeared, and figured that my problem was more going to be something I have neglected to implement in the model/implemented wrong.

I've also provided a couple of screenshots to better portray my problem. The first image is when the JComboBox is not selected, and simply displaying the currently selected item.

The second image is when I have just clicked the JComboBox to bring up the list. As depicted, it will immediately bring up that first item, no matter what it is.

If anyone has any suggestions as to where to look/solutions, I would be very grateful.

---

EDIT

My particular table has two columns, where the left column is a variable name, and the right column is the value associated with the variable. The tables role is to display the properties of a selected object, where each value for different variable for different objects are likely to not be the same.

In this particular case, the cell displays a JComboBox with all the available Projectiles in the game we are making. Each enemy has a different type of projectile it defaults to. So when I click on a different enemy in our game area, the table will display all of their current properties (defaults if they have not been changed).

Enemies do have a getter for the Projectile, so I could determine what the currently selected enemy is, get it's projectile, do a toString() to find how it is to be represented in the list, and do a setValueAt().

The only problem is at the moment it is always selecting the first item in the list when the list is expanded.

Answer 1

Unless the values for the JComboBox are dynamically generated for each row, you should be able to just prepare the CellEditor ahead of time, for example...

JComboBox cb = new JComboBox(new String[]{"1", "2", "3", "4"});
DefaultCellEditor editor = new DefaultCellEditor(cb);

JTable table = new JTable(new DefaultTableModel(5, 1));
table.getColumnModel().getColumn(0).setCellEditor(editor);

This will set the selected value of the editor to the value of the cell when the editing process starts

Updated

In the case where the combobox values are dynamically generate per row, you could do something more like...

JComboBox cb = new JComboBox();
DefaultCellEditor editor = new DefaultCellEditor(cb) {

    @Override
    public Component getTableCellEditorComponent(JTable table, Object value, boolean isSelected, int row, int column) {
        JComboBox editor = (JComboBox) getComponent();
        String[] listItems = new String[arrayList.getSize()];
        int i = -1;
        for (String s : arrayList) {
            i++;
            listItems[i] = s;
        }

        DefaultComboBoxModel model = new DefaultComboBoxModel(listItems);
        editor.setModel(model);
        editor.setSelectedItem(value);
        return editor;
    }
};

JTable table = new JTable(new DefaultTableModel(5, 1));
table.getColumnModel().getColumn(0).setCellEditor(editor);

Note the use of editor.setSelectedItem(value); , this will set the selected value to the cells current value...

You could also re-use the model, clearing it each time and re-filling it with new values. You might find this more efficient if you have a large number of rows as you won't need to constantly create a new model each time a cell is edited

Answer 2

Thow this is an oldie...

Your problem is most likely you don't implement "equals" in the class used in the combo.

The Combo needs to select the current item when it is being prepared and does so by iterating through the elements of the model and selects the first one that is equal to the value in the cell. If none is encountered then it leaves the combo as is (either first element or the last used element in a previous cell edit)

Answer 3

This is how you should default to the previously selected element:

//...
Object selectedItem = box.getSelectedItem();
//Add some elements to the jComboBox
box.setSelectedItem(selectedItem);