[Chapter 9] 9.4 CheckboxGroup

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| Chapter 9 Pick Me |

## 9.4 CheckboxGroup

The `CheckboxGroup` lets multiple checkboxes work together to provide a mutually exclusion choice (at most one `Checkbox` can be selected at a time). Because the `CheckboxGroup` is neither a `Component` nor a `Container`, you should normally put all the `Checkbox` components associated with a `CheckboxGroup` in their own `Panel` (or other `Container`). The `LayoutManager` of the `Panel` should be `GridLayout (0, 1)` if you want them in one column. Figure 9.5 shows both a good way and bad way of positioning a set of `Checkbox` items in a `CheckboxGroup`. The image on the left is preferred because the user can sense that the items are grouped; the image on the right suggests three levels of different checkboxes and can therefore surprise the user when checkboxes are deselected.

#### Figure 9.5: Straightforward and confusing layouts of Checkbox components

### CheckboxGroup Methods

Constructors

public CheckboxGroup ()

  This constructor creates an instance of `CheckboxGroup`.

Miscellaneous methods

public int getSelectedCheckbox ()
public Checkbox getCurrent ()

  The `getSelectedCheckbox()` method returns the `Checkbox` within the `CheckboxGroup` whose value is `true`. If no item is selected, `null` is returned.

  `getCurrent()` is the Java 1.0 name for this method.

public synchronized void setSelectedCheckbox (Checkbox checkbox)
public synchronized void setCurrent (Checkbox checkbox)

  The `setSelectedCheckbox()` method makes `checkbox` the currently selected `Checkbox` within the `CheckboxGroup`. If `checkbox`is `null`, the method deselects all the items in the `CheckboxGroup`. If `checkbox` is not within the `CheckboxGroup`, nothing happens.

  `setCurrent()` is the Java 1.0 name for this method.

public String toString ()

  The `toString()` method of `CheckboxGroup` creates a `String` representation of the current choice (as returned by `getSelectedCheckbox()`). Using the "straightforward" layout in Figure 9.5 as an example, the results would be:

      java.awt.CheckboxGroup[current=java.awt.Checkbox[0,31,85x21,
          label=Helvetica,state=true]]

  If there is no currently selected item, the results within the square brackets would be `current=null`.

Checkbox |  | ItemSelectable