[Chapter 9] 9.3 Checkbox

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| Chapter 9 Pick Me |

## 9.3 Checkbox

The `Checkbox` is a general purpose way to record a `true` or `false` state. When several checkboxes are associated in a `CheckboxGroup` (CheckboxGroup), only one can be selected at a time; selecting each `Checkbox` causes the previous selection to become deselected. The `CheckboxGroup` is Java's way of offering the interface element known as radio buttons or a radio box. When you create a `Checkbox`, you decide whether to place it into a `CheckboxGroup` by setting the proper argument in its constructor.

Every `Checkbox` has both a label and a state, although the label could be empty. You can change the label based on the state of the `Checkbox`. Figure 9.4 shows what several `Checkbox` components might look like. The two on the left are independent, while the five on the right are in a `CheckboxGroup`. Note that the appearance of a `Checkbox` varies quite a bit from platform to platform. However, the appearance of a `CheckboxGroup` is always different from the appearance of an ungrouped `Checkbox`, and the appearance of a checked `Checkbox` is different from an unchecked `Checkbox`.

#### Figure 9.4: Two separate checkboxes and a CheckboxGroup

### Checkbox Methods

Constructors

public Checkbox ()

  This constructor for `Checkbox` creates a new instance with no label or grouping. The initial state of the item is `false`. A checkbox doesn't necessarily need a label; however, a checkbox without a label might be confusing, unless it is being used as a column in a table or a spreadsheet.

public Checkbox (String label)

  The second constructor creates a new `Checkbox` with a label of `label` and no grouping. The initial state of the item is `false`. If you want a simple yes/no choice and plan to make no the default, use this constructor. If the `Checkbox` will be in a group or you want its initial value to be `true`, use the next constructor.

public Checkbox (String label, boolean state)

  This constructor allows you to specify the `Checkbox`'s initial state. With it you create a `Checkbox` with a label of `label` and an initial state of `state`.

public Checkbox (String label, boolean state, CheckboxGroup group)
public Checkbox (String label, CheckboxGroup group, boolean state)

  The final constructor for `Checkbox` is the most flexible. With this constructor you create a `Checkbox` with a label of `label`, a `CheckboxGroup` of `group`, and an initial state of `state`. If `group` is `null`, the `Checkbox` is independent.

  In Java 1.0, you created an independent `Checkbox` with an initial value of `true` by using `null` as the group:

      Checkbox cb = new Checkbox ("Help", null, true)

  The shape of the `Checkbox` reflects whether it's in a `CheckboxGroup` or independent. On Microsoft Windows, grouped checkboxes are represented as circles. On a UNIX system, they are diamonds. On both systems, independent checkboxes are squares.

Label

public String getLabel ()

  The `getLabel()` method retrieves the current label on the `Checkbox` and returns it as a `String` object.

public synchronized void setLabel (String label)

  The `setLabel()` method changes the label of the `Checkbox` to `label`. If the new label is a different size than the old one, you have to `validate()` the container after the change to ensure the entire label will be seen.

State

A state of `true` means the `Checkbox` is selected. A state of `false` means that the `Checkbox` is not selected.

public boolean getState ()

  The `getState()` method retrieves the current state of the `Checkbox` and returns it as a boolean.

public void setState (boolean state)

  The `setState()` method changes the state of the `Checkbox` to `state`. If the `Checkbox` is in a `CheckboxGroup` and `state` is true, the other items in the group become false.

ItemSelectable method

public Objects[] getSelectedObjects ()

  The `getSelectedObjects()` method returns the `Checkbox` label as a one-element `Object` array if it is currently selected, or `null` if the `Checkbox` is not selected. Because this method is part of the `ItemSelectable` interface, you can use it to look at the selected items in a `Choice`, `List`, or `Checkbox`.

CheckboxGroup

This section lists methods that you issue to `Checkbox` to affect its relationship to a `CheckboxGroup`. Methods provided by the `CheckboxGroup` itself can be found later in this chapter.

public CheckboxGroup getCheckboxGroup ()

  The `getCheckboxGroup()` method returns the current `CheckboxGroup` for the `Checkbox`. If the `Checkbox` is not in a group, this method returns `null`.

public void setCheckboxGroup (CheckboxGroup group)

  The `setCheckboxGroup()` method allows you to insert a `Checkbox` into a different `CheckboxGroup`. To make the `Checkbox` independent, pass a `group` argument of `null`. The method sets every `Checkbox` in the original `CheckboxGroup` to `false` (`cb.getCheckboxGroup().setCurrent(null)`), then the `Checkbox` is added to the new group without changing any values in the new group.

  `Checkbox` components take on a different shape when they are in a `CheckboxGroup`. If the checkbox was originally not in a `CheckboxGroup`, the shape of the checkbox does not change automatically when you put it in one with `setCheckboxGroup()`. (This also holds when you remove a `Checkbox` from a `CheckboxGroup` and make it independent or vice versa.) In order for the `Checkbox` to look right once added to `group`, you need to destroy and create (`removeNotify()` and `addNotify()`, respectively) the `Checkbox` peer to correct the shape. Also, it is possible to get multiple true `Checkbox` components in `group` this way, since the new `CheckboxGroup`'s current selection does not get adjusted. To avoid this problem, make sure it is grouped properly the first time, or be sure to clear the selections with a call to `getCheckboxGroup().setCurrent(null)`.

Miscellaneous methods

public synchronized void addNotify ()

  The `addNotify()` method will create the `Checkbox` peer in the appropriate shape. If you override this method, call `super.addNotify()` first, then add your customizations for the new class. You will then be able to do everything you need with the information about the newly created peer.

protected String paramString ()

  When you call the `toString()` method of `Checkbox`, the default `toString()` method of `Component` is called. This in turn calls `paramString()` which builds up the string to display. At the `Checkbox` level, the label (if non-null) and the state of the item are appended. Assuming the Dialog `Checkbox` in Figure 9.4 was selected, the results would be:

    java.awt.Checkbox[85,34,344x32,label=Dialog,state=true]

### Checkbox Events

The primary event for a `Checkbox` occurs when the user selects it. With the 1.0 event model, this generates an `ACTION_EVENT` and triggers the `action()` method. Once the `Checkbox` has the input focus, the various keyboard events can be generated, but they do not serve any useful purpose because the `Checkbox` doesn't change. The sole key of value for a `Checkbox` is the spacebar. This may generate the `ACTION_EVENT` after `KEY_PRESS` and `KEY_RELEASE`; thus the sequence of method calls would be `keyDown()`, `keyUp()`, and then `action()`.

With the version 1.1 event model, you register an `ItemListener` with the method `addItemListener()`. Then when the user selects the `Checkbox`, the method `ItemListener.itemStateChanged()` is called through the protected `Checkbox.processItemEvent()` method. Key, mouse, and focus listeners are registered through the `Component` methods of `addKeyListener()`, `addMouseListener()`, and `addFocusListener()`, respectively. Action

public boolean action (Event e, Object o)

  The `action()` method for a `Checkbox` is called when the user selects it. `e` is the `Event` instance for the specific event, while `o` is the opposite of the old state of the toggle. If the `Checkbox` was `true` when it was selected, `o` will be `false`. Likewise, if it was `false`, `o` will be `true`. This incantation sounds unnecessarily complex, and for a single `Checkbox`, it is: `o` is just the new state of the `Checkbox`. The following code uses `action()` with a single `Checkbox`.

      public boolean action (Event e, Object o) {
          if (e.target instanceof Checkbox) {
              System.out.println ("Checkbox is now " + o);
          }
          return false;
      }

  On the other hand, if the `Checkbox` is in a `CheckboxGroup`, `o` is still the opposite of the old state of the toggle, which may or may not be the new state of the `Checkbox`. If the `Checkbox` is initially `false`, `o` will be `true`, and the `Checkbox`'s new state will be `true`. However, if the `Checkbox` is initially `true`, selecting the `Checkbox` doesn't change anything because one `Checkbox` in the group must always be `true`. In this case, `o` is `false` (the opposite of the old state), though the `Checkbox`'s state remains `true`.

  Therefore, if you're working with a `CheckboxGroup` and need to do something once when the selection changes, perform your action only when `o` is true. To find out which `Checkbox` was actually chosen, you need to call the `getLabel()` method for the target of event `e`. (It would be nice if `o` gave us the label of the `Checkbox` that was selected, but it doesn't.) An example of this follows:

      public boolean action (Event e, Object o) {
          if (e.target instanceof Checkbox) {
              System.out.println (((Checkbox)(e.target)).getLabel() +
                  " was selected.");
              if (new Boolean (o.toString()).booleanValue()) {
                  System.out.println ("New option chosen");
              } else {
                  System.out.println ("Use re-selected option");
              }
          }
          return false;
      }

One other unfortunate twist of `CheckboxGroup`: it would be nice if there was some easy way to find out about checkboxes that change state without selection--for example, if you could find out which `Checkbox` was deselected when a new `Checkbox` was selected. Unfortunately, you can't, except by keeping track of the state of all your checkboxes at all times. When a `Checkbox` state becomes `false` because another `Checkbox` was selected, no additional event is generated, in either Java 1.0 or 1.1. Keyboard

`Checkbox`es are able to capture keyboard-related events once the `Checkbox` has the input focus, which happens when it is selected. If you can find a use for this, you can use `keyDown()` and `keyUp()`. For most interface designs I can think of, `action()` is sufficient. A possible use for keyboard events is to jump to other `Checkbox` options in a `CheckboxGroup`, but I think that is more apt to confuse users than help.

public boolean keyDown (Event e, int key)

  The `keyDown()` method is called whenever the user presses a key while the `Checkbox` has the input focus. `e` is the `Event` instance for the specific event, while `key` is the integer representation of the character pressed. The identifier for the event (`e.id`) for `keyDown()` could be either `KEY_PRESS` for a regular key or `KEY_ACTION` for an action-oriented key (i.e., arrow or function key). There is no visible indication that the user has pressed a key over the checkbox.

public boolean keyUp (Event e, int key)

  The `keyUp()` method is called whenever the user releases a key while the `Checkbox` has the input focus. `e` is the `Event` instance for the specific event, while `key` is the integer representation of the character pressed. The identifier for the event (`e.id`) for `keyUp()` could be either `KEY_RELEASE` for a regular key or `KEY_ACTION_RELEASE` for an action-oriented key (i.e., arrow or function key). `keyUp()` may be used to determine how long `key` has been pressed.

Mouse

Ordinarily, the `Checkbox` component does not reliably trigger any mouse events. Focus

Ordinarily, the `Checkbox` component does not reliably trigger any focus events. Listeners and 1.1 event handling

With the 1.1 event model, you register listeners, and they are told when the event happens.

public void addItemListener(ItemListener listener)

  The `addItemListener()` method registers `listener` as an object interested in being notified when an `ItemEvent` passes through the `EventQueue` with this `Checkbox` as its target. Then, the `listener.itemStateChanged()` method will be called. Multiple listeners can be registered.

public void removeItemListener(ItemListener listener)

  The `removeItemListener()` method removes `listener` as a interested listener. If listener is not registered, nothing happens.

protected void processEvent(AWTEvent e)

  The `processEvent()` method receives every `AWTEvent` with this `Checkbox` as its target. `processEvent()` then passes it along to any listeners for processing. When you subclass `Checkbox`, overriding `processEvent()` allows you to process all events yourself, before sending them to any listeners. In a way, overriding `processEvent()` is like overriding `handleEvent()` using the 1.0 event model.

  If you override `processEvent()`, remember to call `super.processEvent(e)` last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call `enableEvents()` (inherited from `Component`) to ensure that events are delivered even in the absence of registered listeners.

protected void processItemEvent(ItemEvent e)

  The `processItemEvent()` method receives every `ItemEvent` with this `Checkbox` as its target. `processItemEvent()` then passes it along to any listeners for processing. When you subclass `Checkbox`, overriding `processItemEvent()` allows you to process all events yourself, before sending them to any listeners. In a way, overriding `processItemEvent()` is like overriding `action()` using the 1.0 event model.

  If you override `processItemEvent()`, remember to call the method `super.processItemEvent(e)` last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call `enableEvents()` (inherited from `Component`) to ensure that events are delivered even in the absence of registered listeners.

Lists |  | CheckboxGroup