# Class Checkbox

java.lang.Object
java.awt.Component
java.awt.Checkbox

All Implemented Interfaces:
  `ImageObserver, ItemSelectable, MenuContainer, Serializable, Accessible`

public class Checkbox extends Component implements ItemSelectable, Accessible
A check box is a graphical component that can be in either an "on" (`true`) or "off" (`false`) state. Clicking on a check box changes its state from "on" to "off," or from "off" to "on."

The following code example creates a set of check boxes in a grid layout:

>     setLayout(new GridLayout(3, 1));
>     add(new Checkbox("one", null, true));
>     add(new Checkbox("two"));
>     add(new Checkbox("three"));

This image depicts the check boxes and grid layout created by this code example:

The button labeled `one` is in the "on" state, and the other two are in the "off" state. In this example, which uses the `GridLayout` class, the states of the three check boxes are set independently.

Alternatively, several check boxes can be grouped together under the control of a single object, using the `CheckboxGroup` class. In a check box group, at most one button can be in the "on" state at any given time. Clicking on a check box to turn it on forces any other check box in the same group that is on into the "off" state.

Since:
  1.0
See Also:
  - `GridLayout`
  - `CheckboxGroup`
  - Serialized Form

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `protected class`
  `Checkbox.AccessibleAWTCheckbox`
  This class implements accessibility support for the `Checkbox` class.

  ### Nested classes/interfaces declared in class Component

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

- ## Field Summary

  ### Fields declared in class Component

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

  ### Fields declared in interface ImageObserver

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

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `Checkbox()`
  Creates a check box with an empty string for its label.
  `Checkbox(String label)`
  Creates a check box with the specified label.
  `Checkbox(String label, boolean state)`
  Creates a check box with the specified label and sets the specified state.
  `Checkbox(String label, boolean state, CheckboxGroup group)`
  Constructs a Checkbox with the specified label, set to the specified state, and in the specified check box group.
  `Checkbox(String label, CheckboxGroup group, boolean state)`
  Creates a check box with the specified label, in the specified check box group, and set to the specified state.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `addItemListener(ItemListener l)`
  Adds the specified item listener to receive item events from this check box.
  `void`
  `addNotify()`
  Creates the peer of the Checkbox.
  `AccessibleContext`
  `getAccessibleContext()`
  Gets the AccessibleContext associated with this Checkbox.
  `CheckboxGroup`
  `getCheckboxGroup()`
  Determines this check box's group.
  `ItemListener[]`
  `getItemListeners()`
  Returns an array of all the item listeners registered on this checkbox.
  `String`
  `getLabel()`
  Gets the label of this check box.
  `<T extends EventListener>
  T[]`
  `getListeners(Class<T> listenerType)`
  Returns an array of all the objects currently registered as `FooListener`s upon this `Checkbox`.
  `Object[]`
  `getSelectedObjects()`
  Returns an array (length 1) containing the checkbox label or null if the checkbox is not selected.
  `boolean`
  `getState()`
  Determines whether this check box is in the "on" or "off" state.
  `protected String`
  `paramString()`
  Returns a string representing the state of this `Checkbox`.
  `protected void`
  `processEvent(AWTEvent e)`
  Processes events on this check box.
  `protected void`
  `processItemEvent(ItemEvent e)`
  Processes item events occurring on this check box by dispatching them to any registered `ItemListener` objects.
  `void`
  `removeItemListener(ItemListener l)`
  Removes the specified item listener so that the item listener no longer receives item events from this check box.
  `void`
  `setCheckboxGroup(CheckboxGroup g)`
  Sets this check box's group to the specified check box group.
  `void`
  `setLabel(String label)`
  Sets this check box's label to be the string argument.
  `void`
  `setState(boolean state)`
  Sets the state of this check box to the specified state.

  ### Methods declared in class Component

  `action, add, addComponentListener, addFocusListener, addHierarchyBoundsListener, addHierarchyListener, addInputMethodListener, addKeyListener, addMouseListener, addMouseMotionListener, addMouseWheelListener, addPropertyChangeListener, addPropertyChangeListener, applyComponentOrientation, areFocusTraversalKeysSet, bounds, checkImage, checkImage, coalesceEvents, contains, contains, createImage, createImage, createVolatileImage, createVolatileImage, deliverEvent, disable, disableEvents, dispatchEvent, doLayout, enable, enable, enableEvents, enableInputMethods, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, firePropertyChange, getAlignmentX, getAlignmentY, getBackground, getBaseline, getBaselineResizeBehavior, getBounds, getBounds, getColorModel, getComponentAt, getComponentAt, getComponentListeners, getComponentOrientation, getCursor, getDropTarget, getFocusCycleRootAncestor, getFocusListeners, getFocusTraversalKeys, getFocusTraversalKeysEnabled, getFont, getFontMetrics, getForeground, getGraphics, getGraphicsConfiguration, getHeight, getHierarchyBoundsListeners, getHierarchyListeners, getIgnoreRepaint, getInputContext, getInputMethodListeners, getInputMethodRequests, getKeyListeners, getLocale, getLocation, getLocation, getLocationOnScreen, getMaximumSize, getMinimumSize, getMouseListeners, getMouseMotionListeners, getMousePosition, getMouseWheelListeners, getName, getParent, getPreferredSize, getPropertyChangeListeners, getPropertyChangeListeners, getSize, getSize, getToolkit, getTreeLock, getWidth, getX, getY, gotFocus, handleEvent, hasFocus, hide, imageUpdate, inside, invalidate, isBackgroundSet, isCursorSet, isDisplayable, isDoubleBuffered, isEnabled, isFocusable, isFocusCycleRoot, isFocusOwner, isFocusTraversable, isFontSet, isForegroundSet, isLightweight, isMaximumSizeSet, isMinimumSizeSet, isOpaque, isPreferredSizeSet, isShowing, isValid, isVisible, keyDown, keyUp, layout, list, list, list, list, list, locate, location, lostFocus, minimumSize, mouseDown, mouseDrag, mouseEnter, mouseExit, mouseMove, mouseUp, move, nextFocus, paint, paintAll, postEvent, preferredSize, prepareImage, prepareImage, print, printAll, processComponentEvent, processFocusEvent, processHierarchyBoundsEvent, processHierarchyEvent, processInputMethodEvent, processKeyEvent, processMouseEvent, processMouseMotionEvent, processMouseWheelEvent, remove, removeComponentListener, removeFocusListener, removeHierarchyBoundsListener, removeHierarchyListener, removeInputMethodListener, removeKeyListener, removeMouseListener, removeMouseMotionListener, removeMouseWheelListener, removeNotify, removePropertyChangeListener, removePropertyChangeListener, repaint, repaint, repaint, repaint, requestFocus, requestFocus, requestFocus, requestFocus, requestFocusInWindow, requestFocusInWindow, requestFocusInWindow, reshape, resize, resize, revalidate, setBackground, setBounds, setBounds, setComponentOrientation, setCursor, setDropTarget, setEnabled, setFocusable, setFocusTraversalKeys, setFocusTraversalKeysEnabled, setFont, setForeground, setIgnoreRepaint, setLocale, setLocation, setLocation, setMaximumSize, setMinimumSize, setMixingCutoutShape, setName, setPreferredSize, setSize, setSize, setVisible, show, show, size, toString, transferFocus, transferFocusBackward, transferFocusUpCycle, update, validate`

  ### Methods declared in class Object

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

- ## Constructor Details

  - ### Checkbox

    public Checkbox() throws HeadlessException
    Creates a check box with an empty string for its label. The state of this check box is set to "off," and it is not part of any check box group.
    Throws:
      `HeadlessException` - if GraphicsEnvironment.isHeadless() returns true
    See Also:
      - `GraphicsEnvironment.isHeadless()`

  - ### Checkbox

    public Checkbox(String label) throws HeadlessException
    Creates a check box with the specified label. The state of this check box is set to "off," and it is not part of any check box group.
    Parameters:
      `label` - a string label for this check box, or `null` for no label.
    Throws:
      `HeadlessException` - if `GraphicsEnvironment.isHeadless` returns `true`
    See Also:
      - `GraphicsEnvironment.isHeadless()`

  - ### Checkbox

    public Checkbox(String label, boolean state) throws HeadlessException
    Creates a check box with the specified label and sets the specified state. This check box is not part of any check box group.
    Parameters:
      `label` - a string label for this check box, or `null` for no label
      `state` - the initial state of this check box
    Throws:
      `HeadlessException` - if `GraphicsEnvironment.isHeadless` returns `true`
    See Also:
      - `GraphicsEnvironment.isHeadless()`

  - ### Checkbox

    public Checkbox(String label, boolean state, CheckboxGroup group) throws HeadlessException
    Constructs a Checkbox with the specified label, set to the specified state, and in the specified check box group.
    Parameters:
      `label` - a string label for this check box, or `null` for no label.
      `state` - the initial state of this check box.
      `group` - a check box group for this check box, or `null` for no group.
    Throws:
      `HeadlessException` - if `GraphicsEnvironment.isHeadless` returns `true`
    Since:
      1.1
    See Also:
      - `GraphicsEnvironment.isHeadless()`

  - ### Checkbox

    public Checkbox(String label, CheckboxGroup group, boolean state) throws HeadlessException
    Creates a check box with the specified label, in the specified check box group, and set to the specified state.
    Parameters:
      `label` - a string label for this check box, or `null` for no label.
      `group` - a check box group for this check box, or `null` for no group.
      `state` - the initial state of this check box.
    Throws:
      `HeadlessException` - if `GraphicsEnvironment.isHeadless` returns `true`
    Since:
      1.1
    See Also:
      - `GraphicsEnvironment.isHeadless()`

- ## Method Details

  - ### addNotify

    public void addNotify()
    Creates the peer of the Checkbox. The peer allows you to change the look of the Checkbox without changing its functionality.
    Overrides:
      `addNotify` in class `Component`
    See Also:
      - `Component.getToolkit()`

  - ### getLabel

    public String getLabel()
    Gets the label of this check box.
    Returns:
      the label of this check box, or `null` if this check box has no label.
    See Also:
      - `setLabel(String)`

  - ### setLabel

    public void setLabel(String label)
    Sets this check box's label to be the string argument.
    Parameters:
      `label` - a string to set as the new label, or `null` for no label.
    See Also:
      - `getLabel()`

  - ### getState

    public boolean getState()
    Determines whether this check box is in the "on" or "off" state. The boolean value `true` indicates the "on" state, and `false` indicates the "off" state.
    Returns:
      the state of this check box, as a boolean value
    See Also:
      - `setState(boolean)`

  - ### setState

    public void setState(boolean state)
    Sets the state of this check box to the specified state. The boolean value `true` indicates the "on" state, and `false` indicates the "off" state.

    Note that this method should be primarily used to initialize the state of the checkbox. Programmatically setting the state of the checkbox will not trigger an `ItemEvent`. The only way to trigger an `ItemEvent` is by user interaction.

    Parameters:
      `state` - the boolean state of the check box
    See Also:
      - `getState()`

  - ### getSelectedObjects

    public Object[] getSelectedObjects()
    Returns an array (length 1) containing the checkbox label or null if the checkbox is not selected.
    Specified by:
      `getSelectedObjects` in interface `ItemSelectable`
    Returns:
      the list of selected objects, or `null`
    See Also:
      - `ItemSelectable`

  - ### getCheckboxGroup

    public CheckboxGroup getCheckboxGroup()
    Determines this check box's group.
    Returns:
      this check box's group, or `null` if the check box is not part of a check box group.
    See Also:
      - `setCheckboxGroup(CheckboxGroup)`

  - ### setCheckboxGroup

    public void setCheckboxGroup(CheckboxGroup g)
    Sets this check box's group to the specified check box group. If this check box is already in a different check box group, it is first taken out of that group.

    If the state of this check box is `true` and the new group already has a check box selected, this check box's state is changed to `false`. If the state of this check box is `true` and the new group has no check box selected, this check box becomes the selected checkbox for the new group and its state is `true`.

    Parameters:
      `g` - the new check box group, or `null` to remove this check box from any check box group
    See Also:
      - `getCheckboxGroup()`

  - ### addItemListener

    public void addItemListener(ItemListener l)
    Adds the specified item listener to receive item events from this check box. Item events are sent to listeners in response to user input, but not in response to calls to setState(). If l is null, no exception is thrown and no action is performed.

    Refer to AWT Threading Issues for details on AWT's threading model.

    Specified by:
      `addItemListener` in interface `ItemSelectable`
    Parameters:
      `l` - the item listener
    Since:
      1.1
    See Also:
      - `removeItemListener(ItemListener)`
      - `getItemListeners()`
      - `setState(boolean)`
      - `ItemEvent`
      - `ItemListener`

  - ### removeItemListener

    public void removeItemListener(ItemListener l)
    Removes the specified item listener so that the item listener no longer receives item events from this check box. If l is null, no exception is thrown and no action is performed.

    Refer to AWT Threading Issues for details on AWT's threading model.

    Specified by:
      `removeItemListener` in interface `ItemSelectable`
    Parameters:
      `l` - the item listener
    Since:
      1.1
    See Also:
      - `addItemListener(ItemListener)`
      - `getItemListeners()`
      - `ItemEvent`
      - `ItemListener`

  - ### getItemListeners

    public ItemListener[] getItemListeners()
    Returns an array of all the item listeners registered on this checkbox.
    Returns:
      all of this checkbox's `ItemListener`s or an empty array if no item listeners are currently registered
    Since:
      1.4
    See Also:
      - `addItemListener(ItemListener)`
      - `removeItemListener(ItemListener)`
      - `ItemEvent`
      - `ItemListener`

  - ### getListeners

    public <T extends EventListener> T[] getListeners(Class<T> listenerType)
    Returns an array of all the objects currently registered as `FooListener`s upon this `Checkbox`. `FooListener`s are registered using the `addFooListener` method.

    You can specify the `listenerType` argument with a class literal, such as `FooListener.class`. For example, you can query a `Checkbox c` for its item listeners with the following code:

        ItemListener[] ils = (ItemListener[])(c.getListeners(ItemListener.class));

    If no such listeners exist, this method returns an empty array.
    Overrides:
      `getListeners` in class `Component`
    Type Parameters:
      `T` - the type of the listeners
    Parameters:
      `listenerType` - the type of listeners requested; this parameter should specify an interface that descends from `java.util.EventListener`
    Returns:
      an array of all objects registered as `FooListener`s on this checkbox, or an empty array if no such listeners have been added
    Throws:
      `ClassCastException` - if `listenerType` doesn't specify a class or interface that implements `java.util.EventListener`
    Since:
      1.3
    See Also:
      - `getItemListeners()`

  - ### processEvent

    protected void processEvent(AWTEvent e)
    Processes events on this check box. If the event is an instance of `ItemEvent`, this method invokes the `processItemEvent` method. Otherwise, it calls its superclass's `processEvent` method.

    Note that if the event parameter is `null` the behavior is unspecified and may result in an exception.

    Overrides:
      `processEvent` in class `Component`
    Parameters:
      `e` - the event
    Since:
      1.1
    See Also:
      - `ItemEvent`
      - `processItemEvent(ItemEvent)`

  - ### processItemEvent

    protected void processItemEvent(ItemEvent e)
    Processes item events occurring on this check box by dispatching them to any registered `ItemListener` objects.

    This method is not called unless item events are enabled for this component. Item events are enabled when one of the following occurs:

    - An `ItemListener` object is registered via `addItemListener`.
    - Item events are enabled via `enableEvents`.

    Note that if the event parameter is `null` the behavior is unspecified and may result in an exception.

    Parameters:
      `e` - the item event
    Since:
      1.1
    See Also:
      - `ItemEvent`
      - `ItemListener`
      - `addItemListener(ItemListener)`
      - `Component.enableEvents(long)`

  - ### paramString

    protected String paramString()
    Returns a string representing the state of this `Checkbox`. This method is intended to be used only for debugging purposes, and the content and format of the returned string may vary between implementations. The returned string may be empty but may not be `null`.
    Overrides:
      `paramString` in class `Component`
    Returns:
      the parameter string of this check box

  - ### getAccessibleContext

    public AccessibleContext getAccessibleContext()
    Gets the AccessibleContext associated with this Checkbox. For checkboxes, the AccessibleContext takes the form of an AccessibleAWTCheckbox. A new AccessibleAWTCheckbox is created if necessary.
    Specified by:
      `getAccessibleContext` in interface `Accessible`
    Overrides:
      `getAccessibleContext` in class `Component`
    Returns:
      an AccessibleAWTCheckbox that serves as the AccessibleContext of this Checkbox
    Since:
      1.3