If Statement Executing When False

Question

When running (in run or debug mode) my project, I get an ArrayIndexOutOfBounds error, which makes sense. What doesn't is that I check if the index >= 0, and despite it saying the index is -1, somehow the code inside the if still runs.

Code:

...
// Contact List
lstContacts = new JList();
lstContacts.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
lstContacts.setPreferredSize(new Dimension(200, 200));
lstContacts.setMinimumSize(new Dimension(50, 50));
_contactList = _dbi.GetContactList();
_selectedIndex = -1; // An int declared earlier
lstContacts.setListData(_contactList.toArray());
lstContacts.addListSelectionListener(new ListSelectionListener()
{
    public void valueChanged(ListSelectionEvent e)
    {

        System.out.println();
        System.out.println("lstContacts.getSelectedIndex: " + lstContacts.getSelectedIndex());
        System.out.println("!e.getValueIsAdjusting: " + (!e.getValueIsAdjusting()));
        System.out.println("getselectedindex > 0:   " + (lstContacts.getSelectedIndex() > 0));
        System.out.println("Both: " + (!e.getValueIsAdjusting() && (lstContacts.getSelectedIndex() > 0)));

        // Filter out mid-actions
        if(!e.getValueIsAdjusting() && (lstContacts.getSelectedIndex() > 0))
        {

            if(pnlDetail.isVisible())
            {
                saveCurrentContact();
            }
            else
            {
                pnlDetail.setVisible(true);
            }

            System.out.println("  Both: " + (!e.getValueIsAdjusting() && (lstContacts.getSelectedIndex() > 0)));
            _selectedIndex = lstContacts.getSelectedIndex();
            System.out.println("  _selectedIndex: " + _selectedIndex);
            System.out.println("  lstContacts.getSelectedIndex: " + lstContacts.getSelectedIndex());
            PersonalContact sc = (PersonalContact)_contactList.get(_selectedIndex); //crashes here
            showContact(sc);
        }
    }
});
...

I inserted three dummy contacts in the list to begin with. Clicking on one runs fine, but clicking on another throws the error. In the error below, I clicked on the second entry.

Console output:

...
lstContacts.getSelectedIndex: 2
!e.getValueIsAdjusting: true
getselectedindex > 0:   true
Both: true
Entry ID [2] modified.

lstContacts.getSelectedIndex: -1
!e.getValueIsAdjusting: true
getselectedindex > 0:   false
Both: false
  Both: false
  _selectedIndex: -1
  lstContacts.getSelectedIndex: -1
Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: -1
    at java.util.ArrayList.elementData(Unknown Source)
    at java.util.ArrayList.get(Unknown Source)
    at main.ContactPanel$2.valueChanged(ContactPanel.java:203)
    at javax.swing.JList.fireSelectionValueChanged(Unknown Source)
    at javax.swing.JList$ListSelectionHandler.valueChanged(Unknown Source)
        ... [continued]

It looks like it runs correctly, and then runs again and crashes. What (probably obvious thing) am I missing? Thank you for your time and any help you can provide.

Answer 1

It is pretty clear:

Both: false
Both: false
_selectedIndex: -1
lstContacts.getSelectedIndex: -1
Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: -1

The selected index is -1 is the same as doing:

_contactList.get(-1);

Remember, almost all collections in java need an index >= 0.

I think you should modify your condition in order to check if the user have (or not) selected an item in the list so you can handle the error. Something like:

if(lstContacts.getSelectedIndex() >= 0){
  _selectedIndex = lstContacts.getSelectedIndex();
  PersonalContact sc = (PersonalContact)_contactList.get(_selectedIndex);
  showContact(sc);
}

Hope it helps.

Happy coding!

Efra