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Checkbox.javaAPI DocJava SE 6 API27752Tue Jun 10 00:25:12 BST 2008java.awt

Checkbox

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 following context describes the graphic.

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.

version
1.88 07/11/06
author
Sami Shaio
see
java.awt.GridLayout
see
java.awt.CheckboxGroup
since
JDK1.0

Fields Summary
String
label
The label of the Checkbox. This field can be null.
boolean
state
The state of the Checkbox.
CheckboxGroup
group
The check box group. This field can be null indicating that the checkbox is not a group checkbox.
transient ItemListener
itemListener
private static final String
base
private static int
nameCounter
private static final long
serialVersionUID
private int
checkboxSerializedDataVersion
Constructors Summary
public Checkbox()
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.

exception
HeadlessException if GraphicsEnvironment.isHeadless() returns true
see
java.awt.GraphicsEnvironment#isHeadless

        this("", false, null);
    
public Checkbox(String label)
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.

param
label a string label for this check box, or null for no label.
exception
HeadlessException if GraphicsEnvironment.isHeadless returns true
see
java.awt.GraphicsEnvironment#isHeadless

	this(label, false, null);
    
public Checkbox(String label, boolean state)
Creates a check box with the specified label and sets the specified state. This check box is not part of any check box group.

param
label a string label for this check box, or null for no label
param
state the initial state of this check box
exception
HeadlessException if GraphicsEnvironment.isHeadless returns true
see
java.awt.GraphicsEnvironment#isHeadless

        this(label, state, null);
    
public 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.

param
label a string label for this check box, or null for no label.
param
state the initial state of this check box.
param
group a check box group for this check box, or null for no group.
exception
HeadlessException if GraphicsEnvironment.isHeadless returns true
see
java.awt.GraphicsEnvironment#isHeadless
since
JDK1.1

        GraphicsEnvironment.checkHeadless();
	this.label = label;
	this.state = state;
	this.group = group;
	if (state && (group != null)) {
	    group.setSelectedCheckbox(this);
	}
    
public 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.

param
label a string label for this check box, or null for no label.
param
group a check box group for this check box, or null for no group.
param
state the initial state of this check box.
exception
HeadlessException if GraphicsEnvironment.isHeadless returns true
see
java.awt.GraphicsEnvironment#isHeadless
since
JDK1.1

    	this(label, state, group);
    
Methods Summary
public synchronized voidaddItemListener(java.awt.event.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.

param
l the item listener
see
#removeItemListener
see
#getItemListeners
see
#setState
see
java.awt.event.ItemEvent
see
java.awt.event.ItemListener
since
JDK1.1

	if (l == null) {
	    return;
	}
        itemListener = AWTEventMulticaster.add(itemListener, l);
        newEventsOnly = true;
    
public voidaddNotify()
Creates the peer of the Checkbox. The peer allows you to change the look of the Checkbox without changing its functionality.

see
java.awt.Toolkit#createCheckbox(java.awt.Checkbox)
see
java.awt.Component#getToolkit()

        synchronized (getTreeLock()) {
	    if (peer == null) 
	        peer = getToolkit().createCheckbox(this);
	    super.addNotify();
	}
    
java.lang.StringconstructComponentName()
Constructs a name for this component. Called by getName when the name is null.

return
a name for this component

        synchronized (getClass()) {
	    return base + nameCounter++;
	}
    
booleaneventEnabled(java.awt.AWTEvent e)

        if (e.id == ItemEvent.ITEM_STATE_CHANGED) {
            if ((eventMask & AWTEvent.ITEM_EVENT_MASK) != 0 ||
                itemListener != null) {
                return true;
            }
            return false;
        }
        return super.eventEnabled(e);
    
public javax.accessibility.AccessibleContextgetAccessibleContext()
Gets the AccessibleContext associated with this Checkbox. For checkboxes, the AccessibleContext takes the form of an AccessibleAWTCheckbox. A new AccessibleAWTCheckbox is created if necessary.

return
an AccessibleAWTCheckbox that serves as the AccessibleContext of this Checkbox
since
1.3

        if (accessibleContext == null) {
            accessibleContext = new AccessibleAWTCheckbox();
        }
        return accessibleContext;
    
public java.awt.CheckboxGroupgetCheckboxGroup()
Determines this check box's group.

return
this check box's group, or null if the check box is not part of a check box group.
see
#setCheckboxGroup(CheckboxGroup)

	return group;
    
public synchronized java.awt.event.ItemListener[]getItemListeners()
Returns an array of all the item listeners registered on this checkbox.

return
all of this checkbox's ItemListeners or an empty array if no item listeners are currently registered
see
#addItemListener
see
#removeItemListener
see
java.awt.event.ItemEvent
see
java.awt.event.ItemListener
since
1.4

        return (ItemListener[]) (getListeners(ItemListener.class));
    
public java.lang.StringgetLabel()
Gets the label of this check box.

return
the label of this check box, or null if this check box has no label.
see
#setLabel(String)

	return label;
    
public T[]getListeners(java.lang.Class listenerType)
Returns an array of all the objects currently registered as FooListeners upon this Checkbox. FooListeners 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.

param
listenerType the type of listeners requested; this parameter should specify an interface that descends from java.util.EventListener
return
an array of all objects registered as FooListeners on this checkbox, or an empty array if no such listeners have been added
exception
ClassCastException if listenerType doesn't specify a class or interface that implements java.util.EventListener
see
#getItemListeners
since
1.3

	EventListener l = null; 
	if  (listenerType == ItemListener.class) { 
	    l = itemListener;
	} else {
	    return super.getListeners(listenerType);
	}
	return AWTEventMulticaster.getListeners(l, listenerType);
    
public java.lang.Object[]getSelectedObjects()
Returns an array (length 1) containing the checkbox label or null if the checkbox is not selected.

see
ItemSelectable

        if (state) {
            Object[] items = new Object[1];
            items[0] = label;
            return items;
        }
        return null;
    
public booleangetState()
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.

return
the state of this check box, as a boolean value
see
#setState

	return state;
    
private static native voidinitIDs()
Initialize JNI field and method ids

protected java.lang.StringparamString()
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.

return
the parameter string of this check box

	String str = super.paramString();
	String label = this.label;
	if (label != null) {
	    str += ",label=" + label;
	}
	return str + ",state=" + state;
    
protected voidprocessEvent(java.awt.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.

param
e the event
see
java.awt.event.ItemEvent
see
#processItemEvent
since
JDK1.1

        if (e instanceof ItemEvent) {
            processItemEvent((ItemEvent)e);
            return;
        }
	super.processEvent(e);
    
protected voidprocessItemEvent(java.awt.event.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.

param
e the item event
see
java.awt.event.ItemEvent
see
java.awt.event.ItemListener
see
#addItemListener
see
java.awt.Component#enableEvents
since
JDK1.1

        ItemListener listener = itemListener;
        if (listener != null) {
            listener.itemStateChanged(e);
        }
    
private voidreadObject(java.io.ObjectInputStream s)
Reads the ObjectInputStream and if it isn't null adds a listener to receive item events fired by the Checkbox. Unrecognized keys or values will be ignored.

param
s the ObjectInputStream to read
exception
HeadlessException if GraphicsEnvironment.isHeadless returns true
serial
see
#removeItemListener(ItemListener)
see
#addItemListener(ItemListener)
see
java.awt.GraphicsEnvironment#isHeadless
see
#writeObject(ObjectOutputStream)

      GraphicsEnvironment.checkHeadless();
      s.defaultReadObject();

      Object keyOrNull;
      while(null != (keyOrNull = s.readObject())) {
	String key = ((String)keyOrNull).intern();

	if (itemListenerK == key)
	  addItemListener((ItemListener)(s.readObject()));

	else // skip value for unrecognized key
	  s.readObject();
      }
    
public synchronized voidremoveItemListener(java.awt.event.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.

param
l the item listener
see
#addItemListener
see
#getItemListeners
see
java.awt.event.ItemEvent
see
java.awt.event.ItemListener
since
JDK1.1

	if (l == null) {
	    return;
	}
        itemListener = AWTEventMulticaster.remove(itemListener, l);
    
public voidsetCheckboxGroup(java.awt.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.

param
g the new check box group, or null to remove this check box from any check box group
see
#getCheckboxGroup

        CheckboxGroup oldGroup;
        boolean oldState;

        /* Do nothing if this check box has already belonged
         * to the check box group g.
         */
        if (this.group == g) {
            return;
        }

	synchronized (this) {
            oldGroup = this.group;
            oldState = getState();

	    this.group = g;
	    CheckboxPeer peer = (CheckboxPeer)this.peer;
	    if (peer != null) {
		peer.setCheckboxGroup(g);
	    }
	    if (this.group != null && getState()) {
	        if (this.group.getSelectedCheckbox() != null) {
	            setState(false);
	        } else {
	            this.group.setSelectedCheckbox(this);
	        }
	    }
	}

        /* Locking check box below could cause deadlock with
         * CheckboxGroup's setSelectedCheckbox method.
         *
         * Fix for 4726853 by kdm@sparc.spb.su
         * Here we should check if this check box was selected
         * in the previous group and set selected check box to
         * null for that group if so.
         */
        if (oldGroup != null && oldState) {
            oldGroup.setSelectedCheckbox(null);
        }
    
public voidsetLabel(java.lang.String label)
Sets this check box's label to be the string argument.

param
label a string to set as the new label, or null for no label.
see
#getLabel

        boolean testvalid = false;

	synchronized (this) {
	    if (label != this.label && (this.label == null ||
					!this.label.equals(label))) {
	        this.label = label;
		CheckboxPeer peer = (CheckboxPeer)this.peer;
		if (peer != null) {
		    peer.setLabel(label);
		}
		testvalid = true;
	    }
	}
	    
	// This could change the preferred size of the Component.
	if (testvalid && valid) {
	    invalidate();
	}
    
public voidsetState(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.

param
state the boolean state of the check box
see
#getState

	/* Cannot hold check box lock when calling group.setSelectedCheckbox. */
    	CheckboxGroup group = this.group;
	if (group != null) {
	    if (state) {
		group.setSelectedCheckbox(this);
	    } else if (group.getSelectedCheckbox() == this) {
		state = true;
	    }
	}
	setStateInternal(state);
    
voidsetStateInternal(boolean state)
Helper function for setState and CheckboxGroup.setSelectedCheckbox Should remain package-private.


                  
       
	this.state = state;
	CheckboxPeer peer = (CheckboxPeer)this.peer;
	if (peer != null) {
	    peer.setState(state);
	}
    
private voidwriteObject(java.io.ObjectOutputStream s)
Writes default serializable fields to stream. Writes a list of serializable ItemListeners as optional data. The non-serializable ItemListeners are detected and no attempt is made to serialize them.

param
s the ObjectOutputStream to write
serialData
null terminated sequence of 0 or more pairs; the pair consists of a String and an Object; the String indicates the type of object and is one of the following: itemListenerK indicating an ItemListener object
see
AWTEventMulticaster#save(ObjectOutputStream, String, EventListener)
see
java.awt.Component#itemListenerK
see
#readObject(ObjectInputStream)


                                                                                                      
       
       
    
      s.defaultWriteObject();

      AWTEventMulticaster.save(s, itemListenerK, itemListener);
      s.writeObject(null);