# Class BasicComboPopup

java.lang.Object
java.awt.Component
java.awt.Container
javax.swing.JComponent
javax.swing.JPopupMenu
javax.swing.plaf.basic.BasicComboPopup

All Implemented Interfaces:
  `ImageObserver`, `MenuContainer`, `Serializable`, `Accessible`, `MenuElement`, `ComboPopup`

Direct Known Subclasses:
  `MetalComboBoxUI.MetalComboPopup`

public class BasicComboPopup extends JPopupMenu implements ComboPopup
This is a basic implementation of the `ComboPopup` interface. This class represents the ui for the popup portion of the combo box.

All event handling is handled by listener classes created with the `createxxxListener()` methods and internal classes. You can change the behavior of this class by overriding the `createxxxListener()` methods and supplying your own event listeners or subclassing from the ones supplied in this class.

Warning: Serialized objects of this class will not be compatible with future Swing releases. The current serialization support is appropriate for short term storage or RMI between applications running the same version of Swing. As of 1.4, support for long term storage of all JavaBeans has been added to the `java.beans` package. Please see `XMLEncoder`.

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `class`
  `BasicComboPopup.InvocationKeyHandler`
  As of Java 2 platform v 1.4, this class is now obsolete and is only included for backwards API compatibility.
  `protected class`
  `BasicComboPopup.InvocationMouseHandler`
  A listener to be registered upon the combo box (not its popup menu) to handle mouse events that affect the state of the popup menu.
  `protected class`
  `BasicComboPopup.InvocationMouseMotionHandler`
  This listener watches for dragging and updates the current selection in the list if it is dragging over the list.
  `protected class`
  `BasicComboPopup.ItemHandler`
  This listener watches for changes to the selection in the combo box.
  `class`
  `BasicComboPopup.ListDataHandler`
  As of 1.4, this class is now obsolete, doesn't do anything, and is only included for backwards API compatibility.
  `protected class`
  `BasicComboPopup.ListMouseHandler`
  This listener hides the popup when the mouse is released in the list.
  `protected class`
  `BasicComboPopup.ListMouseMotionHandler`
  This listener changes the selected item as you move the mouse over the list.
  `protected class`
  `BasicComboPopup.ListSelectionHandler`
  As of Java 2 platform v 1.4, this class is now obsolete, doesn't do anything, and is only included for backwards API compatibility.
  `protected class`
  `BasicComboPopup.PropertyChangeHandler`
  This listener watches for bound properties that have changed in the combo box.

  ### Nested classes/interfaces declared in class javax.swing.JPopupMenu

  `JPopupMenu.AccessibleJPopupMenu, JPopupMenu.Separator`

  ### Nested classes/interfaces declared in class javax.swing.JComponent

  `JComponent.AccessibleJComponent`

  ### Nested classes/interfaces declared in class java.awt.Container

  `Container.AccessibleAWTContainer`

  ### Nested classes/interfaces declared in class java.awt.Component

  `Component.AccessibleAWTComponent, Component.BaselineResizeBehavior, Component.BltBufferStrategy, Component.FlipBufferStrategy`

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `protected Timer`
  `autoscrollTimer`
  This protected field is implementation specific.
  `protected JComboBox<Object>`
  `comboBox`
  The instance of `JComboBox`.
  `protected boolean`
  `hasEntered`
  `true` if the mouse cursor is in the popup.
  `protected boolean`
  `isAutoScrolling`
  If `true` the auto-scrolling is enabled.
  `protected ItemListener`
  `itemListener`
  This protected field is implementation specific.
  `protected KeyListener`
  `keyListener`
  This protected field is implementation specific.
  `protected JList<Object>`
  `list`
  This protected field is implementation specific.
  `protected ListDataListener`
  `listDataListener`
  This protected field is implementation specific.
  `protected MouseListener`
  `listMouseListener`
  This protected field is implementation specific.
  `protected MouseMotionListener`
  `listMouseMotionListener`
  This protected field is implementation specific.
  `protected ListSelectionListener`
  `listSelectionListener`
  This protected field is implementation specific.
  `protected MouseListener`
  `mouseListener`
  This protected field is implementation specific.
  `protected MouseMotionListener`
  `mouseMotionListener`
  This protected field is implementation specific.
  `protected PropertyChangeListener`
  `propertyChangeListener`
  This protected field is implementation specific.
  `protected static final int`
  `SCROLL_DOWN`
  The direction of scrolling down.
  `protected static final int`
  `SCROLL_UP`
  The direction of scrolling up.
  `protected int`
  `scrollDirection`
  The direction of scrolling.
  `protected JScrollPane`
  `scroller`
  This protected field is implementation specific.
  `protected boolean`
  `valueIsAdjusting`
  As of Java 2 platform v1.4 this previously undocumented field is no longer used.

  ### Fields declared in class javax.swing.JComponent

  `listenerList, TOOL_TIP_TEXT_KEY, ui, UNDEFINED_CONDITION, WHEN_ANCESTOR_OF_FOCUSED_COMPONENT, WHEN_FOCUSED, WHEN_IN_FOCUSED_WINDOW`

  ### Fields declared in class java.awt.Component

  `accessibleContext, BOTTOM_ALIGNMENT, CENTER_ALIGNMENT, LEFT_ALIGNMENT, RIGHT_ALIGNMENT, TOP_ALIGNMENT`

  ### Fields declared in interface java.awt.image.ImageObserver

  `ABORT, ALLBITS, ERROR, FRAMEBITS, HEIGHT, PROPERTIES, SOMEBITS, WIDTH`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `BasicComboPopup(JComboBox<Object> combo)`
  Constructs a new instance of `BasicComboPopup`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected void`
  `autoScrollDown()`
  This protected method is implementation specific and should be private.
  `protected void`
  `autoScrollUp()`
  This protected method is implementation specific and should be private.
  `protected Rectangle`
  `computePopupBounds(int px, int py, int pw, int ph)`
  Calculate the placement and size of the popup portion of the combo box based on the combo box location and the enclosing screen bounds.
  `protected void`
  `configureList()`
  Configures the list which is used to hold the combo box items in the popup.
  `protected void`
  `configurePopup()`
  Configures the popup portion of the combo box.
  `protected void`
  `configureScroller()`
  Configures the scrollable portion which holds the list within the combo box popup.
  `protected MouseEvent`
  `convertMouseEvent(MouseEvent e)`
  Converts mouse event.
  `protected ItemListener`
  `createItemListener()`
  Creates an `ItemListener` which will be added to the combo box.
  `protected KeyListener`
  `createKeyListener()`
  Creates the key listener that will be added to the combo box.
  `protected JList<Object>`
  `createList()`
  Creates the JList used in the popup to display the items in the combo box model.
  `protected ListDataListener`
  `createListDataListener()`
  Creates a list data listener which will be added to the `ComboBoxModel`.
  `protected MouseListener`
  `createListMouseListener()`
  Creates a mouse listener that watches for mouse events in the popup's list.
  `protected MouseMotionListener`
  `createListMouseMotionListener()`
  Creates a mouse motion listener that watches for mouse motion events in the popup's list.
  `protected ListSelectionListener`
  `createListSelectionListener()`
  Creates a list selection listener that watches for selection changes in the popup's list.
  `protected MouseListener`
  `createMouseListener()`
  Creates a listener that will watch for mouse-press and release events on the combo box.
  `protected MouseMotionListener`
  `createMouseMotionListener()`
  Creates the mouse motion listener which will be added to the combo box.
  `protected PropertyChangeListener`
  `createPropertyChangeListener()`
  Creates a `PropertyChangeListener` which will be added to the combo box.
  `protected JScrollPane`
  `createScroller()`
  Creates the scroll pane which houses the scrollable list.
  `protected void`
  `delegateFocus(MouseEvent e)`
  This is a utility method that helps event handlers figure out where to send the focus when the popup is brought up.
  `AccessibleContext`
  `getAccessibleContext()`
  Gets the AccessibleContext associated with this BasicComboPopup.
  `KeyListener`
  `getKeyListener()`
  Implementation of ComboPopup.getKeyListener().
  `JList<Object>`
  `getList()`
  Implementation of ComboPopup.getList().
  `MouseListener`
  `getMouseListener()`
  Implementation of ComboPopup.getMouseListener().
  `MouseMotionListener`
  `getMouseMotionListener()`
  Implementation of ComboPopup.getMouseMotionListener().
  `protected int`
  `getPopupHeightForRowCount(int maxRowCount)`
  Retrieves the height of the popup based on the current ListCellRenderer and the maximum row count.
  `void`
  `hide()`
  Implementation of ComboPopup.hide().
  `protected void`
  `installComboBoxListeners()`
  This method adds the necessary listeners to the JComboBox.
  `protected void`
  `installComboBoxModelListeners(ComboBoxModel<?> model)`
  Installs the listeners on the combo box model.
  `protected void`
  `installKeyboardActions()`
  Registers keyboard actions.
  `protected void`
  `installListListeners()`
  Adds the listeners to the list control.
  `boolean`
  `isFocusTraversable()`
  Overridden to unconditionally return false.
  `void`
  `show()`
  Implementation of ComboPopup.show().
  `protected void`
  `startAutoScrolling(int direction)`
  This protected method is implementation specific and should be private.
  `protected void`
  `stopAutoScrolling()`
  This protected method is implementation specific and should be private.
  `protected void`
  `togglePopup()`
  Makes the popup visible if it is hidden and makes it hidden if it is visible.
  `protected void`
  `uninstallComboBoxModelListeners(ComboBoxModel<?> model)`
  Removes the listeners from the combo box model
  `void`
  `uninstallingUI()`
  Called when the UI is uninstalling.
  `protected void`
  `uninstallKeyboardActions()`
  Unregisters keyboard actions.
  `protected void`
  `updateListBoxSelectionForEvent(MouseEvent anEvent, boolean shouldScroll)`
  A utility method used by the event listeners.

  ### Methods declared in class javax.swing.JPopupMenu

  `add, add, add, addMenuKeyListener, addPopupMenuListener, addSeparator, createActionChangeListener, createActionComponent, firePopupMenuCanceled, firePopupMenuWillBecomeInvisible, firePopupMenuWillBecomeVisible, getComponent, getComponentAtIndex, getComponentIndex, getDefaultLightWeightPopupEnabled, getInvoker, getLabel, getMargin, getMenuKeyListeners, getPopupMenuListeners, getSelectionModel, getSubElements, getUI, getUIClassID, insert, insert, isBorderPainted, isLightWeightPopupEnabled, isPopupTrigger, isVisible, menuSelectionChanged, pack, paintBorder, paramString, processKeyEvent, processKeyEvent, processMouseEvent, remove, removeMenuKeyListener, removePopupMenuListener, setBorderPainted, setDefaultLightWeightPopupEnabled, setInvoker, setLabel, setLightWeightPopupEnabled, setLocation, setPopupSize, setPopupSize, setSelected, setSelectionModel, setUI, setVisible, show, updateUI`

  ### Methods declared in class javax.swing.JComponent

  `addAncestorListener, addNotify, addVetoableChangeListener, computeVisibleRect, contains, createToolTip, disable, enable, firePropertyChange, firePropertyChange, fireVetoableChange, getActionForKeyStroke, getActionMap, getAlignmentX, getAlignmentY, getAncestorListeners, getAutoscrolls, getBaseline, getBaselineResizeBehavior, getBorder, getBounds, getClientProperty, getComponentGraphics, getComponentPopupMenu, getConditionForKeyStroke, getDebugGraphicsOptions, getDefaultLocale, getFontMetrics, getGraphics, getHeight, getInheritsPopupMenu, getInputMap, getInputMap, getInputVerifier, getInsets, getInsets, getListeners, getLocation, getMaximumSize, getMinimumSize, getNextFocusableComponent, getPopupLocation, getPreferredSize, getRegisteredKeyStrokes, getRootPane, getSize, getToolTipLocation, getToolTipText, getToolTipText, getTopLevelAncestor, getTransferHandler, getVerifyInputWhenFocusTarget, getVetoableChangeListeners, getVisibleRect, getWidth, getX, getY, grabFocus, isDoubleBuffered, isLightweightComponent, isManagingFocus, isOpaque, isOptimizedDrawingEnabled, isPaintingForPrint, isPaintingOrigin, isPaintingTile, isRequestFocusEnabled, isValidateRoot, paint, paintChildren, paintComponent, paintImmediately, paintImmediately, print, printAll, printBorder, printChildren, printComponent, processComponentKeyEvent, processKeyBinding, processMouseEvent, processMouseMotionEvent, putClientProperty, registerKeyboardAction, registerKeyboardAction, removeAncestorListener, removeNotify, removeVetoableChangeListener, repaint, repaint, requestDefaultFocus, requestFocus, requestFocus, requestFocusInWindow, requestFocusInWindow, resetKeyboardActions, reshape, revalidate, scrollRectToVisible, setActionMap, setAlignmentX, setAlignmentY, setAutoscrolls, setBackground, setBorder, setComponentPopupMenu, setDebugGraphicsOptions, setDefaultLocale, setDoubleBuffered, setEnabled, setFocusTraversalKeys, setFont, setForeground, setInheritsPopupMenu, setInputMap, setInputVerifier, setMaximumSize, setMinimumSize, setNextFocusableComponent, setOpaque, setPreferredSize, setRequestFocusEnabled, setToolTipText, setTransferHandler, setUI, setVerifyInputWhenFocusTarget, unregisterKeyboardAction, update`

  ### Methods declared in class java.awt.Container

  `add, add, add, add, add, addContainerListener, addImpl, addPropertyChangeListener, addPropertyChangeListener, applyComponentOrientation, areFocusTraversalKeysSet, countComponents, deliverEvent, doLayout, findComponentAt, findComponentAt, getComponent, getComponentAt, getComponentAt, getComponentCount, getComponents, getComponentZOrder, getContainerListeners, getFocusTraversalKeys, getFocusTraversalPolicy, getLayout, getMousePosition, insets, invalidate, isAncestorOf, isFocusCycleRoot, isFocusCycleRoot, isFocusTraversalPolicyProvider, isFocusTraversalPolicySet, layout, list, list, locate, minimumSize, paintComponents, preferredSize, printComponents, processContainerEvent, processEvent, remove, removeAll, removeContainerListener, setComponentZOrder, setFocusCycleRoot, setFocusTraversalPolicy, setFocusTraversalPolicyProvider, setLayout, transferFocusDownCycle, validate, validateTree`

  ### Methods declared in class java.awt.Component

  `action, add, addComponentListener, addFocusListener, addHierarchyBoundsListener, addHierarchyListener, addInputMethodListener, addKeyListener, addMouseListener, addMouseMotionListener, addMouseWheelListener, bounds, checkImage, checkImage, coalesceEvents, contains, createImage, createImage, createVolatileImage, createVolatileImage, disableEvents, dispatchEvent, enable, enableEvents, enableInputMethods, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, getBackground, getBounds, getColorModel, getComponentListeners, getComponentOrientation, getCursor, getDropTarget, getFocusCycleRootAncestor, getFocusListeners, getFocusTraversalKeysEnabled, getFont, getForeground, getGraphicsConfiguration, getHierarchyBoundsListeners, getHierarchyListeners, getIgnoreRepaint, getInputContext, getInputMethodListeners, getInputMethodRequests, getKeyListeners, getLocale, getLocation, getLocationOnScreen, getMouseListeners, getMouseMotionListeners, getMousePosition, getMouseWheelListeners, getName, getParent, getPropertyChangeListeners, getPropertyChangeListeners, getSize, getToolkit, getTreeLock, gotFocus, handleEvent, hasFocus, imageUpdate, inside, isBackgroundSet, isCursorSet, isDisplayable, isEnabled, isFocusable, isFocusOwner, isFontSet, isForegroundSet, isLightweight, isMaximumSizeSet, isMinimumSizeSet, isPreferredSizeSet, isShowing, isValid, keyDown, keyUp, list, list, list, location, lostFocus, mouseDown, mouseDrag, mouseEnter, mouseExit, mouseMove, mouseUp, move, nextFocus, paintAll, postEvent, prepareImage, prepareImage, processComponentEvent, processFocusEvent, processHierarchyBoundsEvent, processHierarchyEvent, processInputMethodEvent, processMouseWheelEvent, remove, removeComponentListener, removeFocusListener, removeHierarchyBoundsListener, removeHierarchyListener, removeInputMethodListener, removeKeyListener, removeMouseListener, removeMouseMotionListener, removeMouseWheelListener, removePropertyChangeListener, removePropertyChangeListener, repaint, repaint, repaint, requestFocus, requestFocus, requestFocusInWindow, resize, resize, setBounds, setBounds, setComponentOrientation, setCursor, setDropTarget, setFocusable, setFocusTraversalKeysEnabled, setIgnoreRepaint, setLocale, setLocation, setMixingCutoutShape, setName, setSize, setSize, show, size, toString, transferFocus, transferFocusBackward, transferFocusUpCycle`

  ### Methods declared in class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

  ### Methods declared in interface javax.swing.plaf.basic.ComboPopup

  `isVisible`

- ## Field Details

  - ### comboBox

    protected JComboBox<Object> comboBox
    The instance of `JComboBox`.

  - ### list

    protected JList<Object> list
    This protected field is implementation specific. Do not access directly or override. Use the accessor methods instead.
    See Also:
      - `getList()`
      - `createList()`

  - ### scroller

    protected JScrollPane scroller
    This protected field is implementation specific. Do not access directly or override. Use the create method instead
    See Also:
      - `createScroller()`

  - ### valueIsAdjusting

    protected boolean valueIsAdjusting
    As of Java 2 platform v1.4 this previously undocumented field is no longer used.

  - ### mouseMotionListener

    protected MouseMotionListener mouseMotionListener
    This protected field is implementation specific. Do not access directly or override. Use the accessor or create methods instead.
    See Also:
      - `getMouseMotionListener()`
      - `createMouseMotionListener()`

  - ### mouseListener

    protected MouseListener mouseListener
    This protected field is implementation specific. Do not access directly or override. Use the accessor or create methods instead.
    See Also:
      - `getMouseListener()`
      - `createMouseListener()`

  - ### keyListener

    protected KeyListener keyListener
    This protected field is implementation specific. Do not access directly or override. Use the accessor or create methods instead.
    See Also:
      - `getKeyListener()`
      - `createKeyListener()`

  - ### listSelectionListener

    protected ListSelectionListener listSelectionListener
    This protected field is implementation specific. Do not access directly or override. Use the create method instead.
    See Also:
      - `createListSelectionListener()`

  - ### listMouseListener

    protected MouseListener listMouseListener
    This protected field is implementation specific. Do not access directly or override. Use the create method instead.
    See Also:
      - `createListMouseListener()`

  - ### listMouseMotionListener

    protected MouseMotionListener listMouseMotionListener
    This protected field is implementation specific. Do not access directly or override. Use the create method instead
    See Also:
      - `createListMouseMotionListener()`

  - ### propertyChangeListener

    protected PropertyChangeListener propertyChangeListener
    This protected field is implementation specific. Do not access directly or override. Use the create method instead
    See Also:
      - `createPropertyChangeListener()`

  - ### listDataListener

    protected ListDataListener listDataListener
    This protected field is implementation specific. Do not access directly or override. Use the create method instead
    See Also:
      - `createListDataListener()`

  - ### itemListener

    protected ItemListener itemListener
    This protected field is implementation specific. Do not access directly or override. Use the create method instead
    See Also:
      - `createItemListener()`

  - ### autoscrollTimer

    protected Timer autoscrollTimer
    This protected field is implementation specific. Do not access directly or override.

  - ### hasEntered

    protected boolean hasEntered
    `true` if the mouse cursor is in the popup.

  - ### isAutoScrolling

    protected boolean isAutoScrolling
    If `true` the auto-scrolling is enabled.

  - ### scrollDirection

    protected int scrollDirection
    The direction of scrolling.

  - ### SCROLL_UP

    protected static final int SCROLL_UP
    The direction of scrolling up.
    See Also:
      - Constant Field Values

  - ### SCROLL_DOWN

    protected static final int SCROLL_DOWN
    The direction of scrolling down.
    See Also:
      - Constant Field Values

- ## Constructor Details

  - ### BasicComboPopup

    public BasicComboPopup(JComboBox<Object> combo)
    Constructs a new instance of `BasicComboPopup`.
    Parameters:
      `combo` - an instance of `JComboBox`

- ## Method Details

  - ### show

    public void show()
    Implementation of ComboPopup.show().
    Specified by:
      `show` in interface `ComboPopup`
    Overrides:
      `show` in class `Component`

  - ### hide

    public void hide()
    Implementation of ComboPopup.hide().
    Specified by:
      `hide` in interface `ComboPopup`
    Overrides:
      `hide` in class `JComponent`

  - ### getList

    public JList<Object> getList()
    Implementation of ComboPopup.getList().
    Specified by:
      `getList` in interface `ComboPopup`
    Returns:
      the list that is being used to draw the items in the combo box

  - ### getMouseListener

    public MouseListener getMouseListener()
    Implementation of ComboPopup.getMouseListener().
    Specified by:
      `getMouseListener` in interface `ComboPopup`
    Returns:
      a `MouseListener` or null
    See Also:
      - `ComboPopup.getMouseListener()`

  - ### getMouseMotionListener

    public MouseMotionListener getMouseMotionListener()
    Implementation of ComboPopup.getMouseMotionListener().
    Specified by:
      `getMouseMotionListener` in interface `ComboPopup`
    Returns:
      a `MouseMotionListener` or null
    See Also:
      - `ComboPopup.getMouseMotionListener()`

  - ### getKeyListener

    public KeyListener getKeyListener()
    Implementation of ComboPopup.getKeyListener().
    Specified by:
      `getKeyListener` in interface `ComboPopup`
    Returns:
      a `KeyListener` or null
    See Also:
      - `ComboPopup.getKeyListener()`

  - ### uninstallingUI

    public void uninstallingUI()
    Called when the UI is uninstalling. Since this popup isn't in the component tree, it won't get it's uninstallUI() called. It removes the listeners that were added in addComboBoxListeners().
    Specified by:
      `uninstallingUI` in interface `ComboPopup`

  - ### uninstallComboBoxModelListeners

    protected void uninstallComboBoxModelListeners(ComboBoxModel<?> model)
    Removes the listeners from the combo box model
    Parameters:
      `model` - The combo box model to install listeners
    See Also:
      - `installComboBoxModelListeners(javax.swing.ComboBoxModel<?>)`

  - ### uninstallKeyboardActions

    protected void uninstallKeyboardActions()
    Unregisters keyboard actions.

  - ### createMouseListener

    protected MouseListener createMouseListener()
    Creates a listener that will watch for mouse-press and release events on the combo box. Warning: When overriding this method, make sure to maintain the existing behavior.
    Returns:
      a `MouseListener` which will be added to the combo box or null

  - ### createMouseMotionListener

    protected MouseMotionListener createMouseMotionListener()
    Creates the mouse motion listener which will be added to the combo box. Warning: When overriding this method, make sure to maintain the existing behavior.
    Returns:
      a `MouseMotionListener` which will be added to the combo box or null

  - ### createKeyListener

    protected KeyListener createKeyListener()
    Creates the key listener that will be added to the combo box. If this method returns null then it will not be added to the combo box.
    Returns:
      a `KeyListener` or null

  - ### createListSelectionListener

    protected ListSelectionListener createListSelectionListener()
    Creates a list selection listener that watches for selection changes in the popup's list. If this method returns null then it will not be added to the popup list.
    Returns:
      an instance of a `ListSelectionListener` or null

  - ### createListDataListener

    protected ListDataListener createListDataListener()
    Creates a list data listener which will be added to the `ComboBoxModel`. If this method returns null then it will not be added to the combo box model.
    Returns:
      an instance of a `ListDataListener` or null

  - ### createListMouseListener

    protected MouseListener createListMouseListener()
    Creates a mouse listener that watches for mouse events in the popup's list. If this method returns null then it will not be added to the combo box.
    Returns:
      an instance of a `MouseListener` or null

  - ### createListMouseMotionListener

    protected MouseMotionListener createListMouseMotionListener()
    Creates a mouse motion listener that watches for mouse motion events in the popup's list. If this method returns null then it will not be added to the combo box.
    Returns:
      an instance of a `MouseMotionListener` or null

  - ### createPropertyChangeListener

    protected PropertyChangeListener createPropertyChangeListener()
    Creates a `PropertyChangeListener` which will be added to the combo box. If this method returns null then it will not be added to the combo box.
    Returns:
      an instance of a `PropertyChangeListener` or null

  - ### createItemListener

    protected ItemListener createItemListener()
    Creates an `ItemListener` which will be added to the combo box. If this method returns null then it will not be added to the combo box.

    Subclasses may override this method to return instances of their own ItemEvent handlers.

    Returns:
      an instance of an `ItemListener` or null

  - ### createList

    protected JList<Object> createList()
    Creates the JList used in the popup to display the items in the combo box model. This method is called when the UI class is created.
    Returns:
      a `JList` used to display the combo box items

  - ### configureList

    protected void configureList()
    Configures the list which is used to hold the combo box items in the popup. This method is called when the UI class is created.
    See Also:
      - `createList()`

  - ### installListListeners

    protected void installListListeners()
    Adds the listeners to the list control.

  - ### createScroller

    protected JScrollPane createScroller()
    Creates the scroll pane which houses the scrollable list.
    Returns:
      the scroll pane which houses the scrollable list

  - ### configureScroller

    protected void configureScroller()
    Configures the scrollable portion which holds the list within the combo box popup. This method is called when the UI class is created.

  - ### configurePopup

    protected void configurePopup()
    Configures the popup portion of the combo box. This method is called when the UI class is created.

  - ### installComboBoxListeners

    protected void installComboBoxListeners()
    This method adds the necessary listeners to the JComboBox.

  - ### installComboBoxModelListeners

    protected void installComboBoxModelListeners(ComboBoxModel<?> model)
    Installs the listeners on the combo box model. Any listeners installed on the combo box model should be removed in `uninstallComboBoxModelListeners`.
    Parameters:
      `model` - The combo box model to install listeners
    See Also:
      - `uninstallComboBoxModelListeners(javax.swing.ComboBoxModel<?>)`

  - ### installKeyboardActions

    protected void installKeyboardActions()
    Registers keyboard actions.

  - ### isFocusTraversable

    public boolean isFocusTraversable()
    Overridden to unconditionally return false.
    Overrides:
      `isFocusTraversable` in class `Component`
    Returns:
      `true` if this `Component` is focusable; `false` otherwise
    See Also:
      - `Component.setFocusable(boolean)`

  - ### startAutoScrolling

    protected void startAutoScrolling(int direction)
    This protected method is implementation specific and should be private. do not call or override.
    Parameters:
      `direction` - the direction of scrolling

  - ### stopAutoScrolling

    protected void stopAutoScrolling()
    This protected method is implementation specific and should be private. do not call or override.

  - ### autoScrollUp

    protected void autoScrollUp()
    This protected method is implementation specific and should be private. do not call or override.

  - ### autoScrollDown

    protected void autoScrollDown()
    This protected method is implementation specific and should be private. do not call or override.

  - ### getAccessibleContext

    public AccessibleContext getAccessibleContext()
    Gets the AccessibleContext associated with this BasicComboPopup. The AccessibleContext will have its parent set to the ComboBox.
    Specified by:
      `getAccessibleContext` in interface `Accessible`
    Overrides:
      `getAccessibleContext` in class `JPopupMenu`
    Returns:
      an AccessibleContext for the BasicComboPopup
    Since:
      1.5

  - ### delegateFocus

    protected void delegateFocus(MouseEvent e)
    This is a utility method that helps event handlers figure out where to send the focus when the popup is brought up. The standard implementation delegates the focus to the editor (if the combo box is editable) or to the JComboBox if it is not editable.
    Parameters:
      `e` - a mouse event

  - ### togglePopup

    protected void togglePopup()
    Makes the popup visible if it is hidden and makes it hidden if it is visible.

  - ### convertMouseEvent

    protected MouseEvent convertMouseEvent(MouseEvent e)
    Converts mouse event.
    Parameters:
      `e` - a mouse event
    Returns:
      converted mouse event

  - ### getPopupHeightForRowCount

    protected int getPopupHeightForRowCount(int maxRowCount)
    Retrieves the height of the popup based on the current ListCellRenderer and the maximum row count.
    Parameters:
      `maxRowCount` - the row count
    Returns:
      the height of the popup

  - ### computePopupBounds

    protected Rectangle computePopupBounds(int px, int py, int pw, int ph)
    Calculate the placement and size of the popup portion of the combo box based on the combo box location and the enclosing screen bounds. If no transformations are required, then the returned rectangle will have the same values as the parameters.
    Parameters:
      `px` - starting x location
      `py` - starting y location
      `pw` - starting width
      `ph` - starting height
    Returns:
      a rectangle which represents the placement and size of the popup

  - ### updateListBoxSelectionForEvent

    protected void updateListBoxSelectionForEvent(MouseEvent anEvent, boolean shouldScroll)
    A utility method used by the event listeners. Given a mouse event, it changes the list selection to the list item below the mouse.
    Parameters:
      `anEvent` - a mouse event
      `shouldScroll` - if `true` list should be scrolled.