NullPointerException in Listener class - pointers

Question

Basically, I currently have this GUI:
What it's supposed to do is add all the highlighted items on the left scroll pane to the right scroll pane. However, whenever I press the "Add" button, I get a NullPointerException. The error happens in the ColumnMainPanel class under AddListener's actionPerformed when the allColsPanel.getList().getSelectedValuesList() gets executed.
My code looks like this:
In the main frame:
ColumnMainPanel columnsPanel = new ColumnMainPanel(allColumns);
frame.setContentPane(columnsPanel);
ColumnMainPanel:
public class ColumnMainPanel extends JPanel {
private AllColumnsPanel allColsPanel;
private SelectedColumnsPanel selColsPanel;
private CentralButtonsPane buttonPane;
public ColumnMainPanel() {
allColsPanel = null;
selColsPanel = null;
buttonPane = null;
}
public ColumnMainPanel(String[] allColumns) {
super(new BorderLayout());
this.dbe = d;
setSize(250, 700);
allColsPanel = new AllColumnsPanel(allColumns);
selColsPanel = new SelectedColumnsPanel(allColumns.length);
buttonPane = new CentralButtonsPane();
add(allColsPanel, BorderLayout.WEST);
add(buttonPane, BorderLayout.CENTER);
add(selColsPanel, BorderLayout.EAST);
}
class AddListener implements ActionListener {
#Override
public void actionPerformed(ActionEvent e) {
//int index = list.getSelectedIndex();
String[] columnsSelected;
// This next line is where I get the error:
List<String> selectedValues = allColsPanel.getList().getSelectedValuesList();
columnsSelected = new String[selectedValues.size()];
columnsSelected = (selectedValues).toArray(columnsSelected);
selColsPanel.addToList(columnsSelected);
}
}
//add RemoveListener and ExportListener here
}
AllColumnsPanel:
public class AllColumnsPanel extends ColumnMainPanel implements ListSelectionListener {
private JList<String> list;
private DefaultListModel<String> listModel;
private int numListItems;
public AllColumnsPanel(String[] allColumns) {
setSize(300, 300);
this.numListItems = allColumns.length;
listModel = createListModel(allColumns);
list = createList(listModel);
JScrollPane listScrollPane = new JScrollPane(list);
listScrollPane.setPreferredSize(new Dimension(300, 300));
add(listScrollPane);
}
public DefaultListModel<String> createListModel(String[] cols) {
DefaultListModel<String> result = new DefaultListModel<String>();
for(int i = 0; i < cols.length; i ++) {
result.addElement(cols[i]);
}
return result;
}
public JList<String> createList(DefaultListModel<String> listModel) {
JList<String> result = new JList<String>(listModel);
result.setSelectionMode(ListSelectionModel.MULTIPLE_INTERVAL_SELECTION);
result.setSelectedIndex(0);
result.addListSelectionListener(this);
result.setVisibleRowCount(5);
return result;
}
public JList<String> getList() {
return list;
}
public boolean isEmpty() {
return numListItems < 1;
}
}
CentralButtonsPane:
public class CentralButtonsPane extends ColumnMainPanel {
private static final String addString = "Add";
private static final String removeString = "Remove";
private static final String exportString = "Export";
public CentralButtonsPane() {
setLayout((new BoxLayout(this, BoxLayout.Y_AXIS)));
setSize(50, 40);
add(createAddButton());
add(createRemoveButton());
add(createExportButton());
}
public JButton createAddButton() {
JButton result = new JButton(addString);
result.setActionCommand(addString);
result.setSize(50, 20);
result.setAlignmentX(Component.CENTER_ALIGNMENT);
result.addActionListener(new AddListener());
return result;
}
}
I checked the allColsPanel right after I initialize it in the ColumnMainPanel constructor and it's not null. However, when I checked if allColsPanel, under the AddListener, is null, it returns true. What am I missing here? What am I doing wrong?
Here is the error I get:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at mhhls.him.dbtoexcelgui.main.ColumnMainPanel$AddListener.actionPerformed(ColumnMainPanel.java:53)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)