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

• public CheckboxGroup ()
• This constructor creates an instance of CheckboxGroup.

• 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 checkboxis 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.