JComboBox选择更改

Question

everyone i am quite new to Java GUI, i am having an issue with a JComboBox , where it is firing when i removeAllItems from a combo box to refresh it, this is an issue because i am getting the selected items Details and populating a textboxes with them so as it is firing at that point i am getting a Null Pointer. Is there any simple(ish) way to have method on the ComboBox that is called when the selected item is changed not just when the combo box contents is changed?

Code

comboBox current method

private void customerComboActionPerformed(java.awt.event.ActionEvent evt) {                                              

   setDetails();

} 

Method for setting the Items in the combo Box

public void setCustomers()
{
customerCombo.removeAllItems();
for (Customer curr : Main.getNewCustomerList().getCustomers())
{

    customerCombo.addItem(curr);
}
}

method for setting the details

public void setDetails()
{
Customer selected = (Customer) customerCombo.getSelectedItem();
forenameText.setText(selected.getForename());
surnameText.setText(selected.getSurname());
costperkgText.setText(String.valueOf(selected.getDeliveryCost()));
line1Text.setText(String.valueOf(selected.getColAddress().getAddressLine1()));
line2Text.setText(String.valueOf(selected.getColAddress().getAddressLine2()));
cityText.setText(String.valueOf(selected.getColAddress().getCity()));
postcodeText.setText(String.valueOf(selected.getColAddress().getPostcode()));

}

Answer 1

You are not accounting for the case where there is no selection.

public void setDetails()
{
    Customer selected = (Customer) customerCombo.getSelectedItem();
    if (selected != null)
    {
        // there is a selection so use it
    }
    else
    {
        // for example, clear the text boxes
    }
}

We would also expect that changing the contents of the combo box might change its selection so we shouldn't ignore it.

Answer 2

I like to set a flag. The key is making sure the flag doesn't get stuck to false.

private volatile boolean fire = true;

public void setItems(Object[] items) {
    try {
        fire = false; // Don't fire updates
        updateItems(items);
    } finally {
        fire = true; // always reset no matter what!
    }
}

private JComboBox create() {
    JComboBox cb = new JComboBox();
    cb.addActionListener( new ActionListener() {
        public void actionPerformed( ActionEvent e ) {
            if(fire) {
                notifyListeners(); 
            }
        }
    });
}

You have to make sure this isn't being called by multiple threads, but since Swing isn't thread safe this should be happening anyway.