JRadioButton

The JRadioButton class allows the selection of one option in a group of mutually exclusive options; that is, where there are two or more possible options but only one option should ever be chosen. To use JRadioButton objects you need to assign them to a ButtonGroup object which manages the mutual exclusion for you: 

// Define the options
JRadioButton redOption = new JRadioButton("Red");
JRadioButton greenOption = new JRadioButton("Green");
JRadioButton blueOption = new JRadioButton("Blue");

// Assign the options to a group
ButtonGroup group = new ButtonGroup();
group.add(redOption);
group.add(greenOption);
group.add(blueOption);

// Show the options in a grid
JPanel colourPanel = new JPanel(new GridLayout(0, 1));
colourPanel.add(redOption);
colourPanel.add(greenOption);
colourPanel.add(blueOption);
        
// Add the grid to the main panel
panel.add(colourPanel);
  • The first section instantiates three JRadioButton objects, corresponding to the colours red, green and blue
  • The second section adds each of the radio button objects to a ButtonGroup object, which will serve to enforce the fact that only one colour should ever be selected
  • The third section places the radio buttons below each other in a one-column grid[1]
  • The fourth section adds the grid to the outer panel

Running the above should result in this:

[1] The number of rows is set to zero, indicating that any number of rows could be added.
JRadioButton
JRadioButton

If you click in any of the selectable circles, then that item will be selected and the previously selected option (if any) will be de-selected. It is generally a good idea, unless it is entirely optional to select any option, to programmatically set one of the options as a default using the setSelected() method: 

blueOption.setSelected(true);

You can use isSelected() to determine if a radio button is selected: 

boolean redWanted = redOption.isSelected();

There is no straightforward way of determining which radio button within a group has been selected without checking each in turn.  

It is important to note the difference between a group of JRadioButton objects and a group of JCheckBox objects. Whereas radio buttons indicate a mutually exclusive set of options, if you have a series of check boxes then they each operate independently, and any combination of check boxes may be selected or de-selected. You therefore don't require a ButtonGroup object for your check boxes.

As an example, suppose you want the user to select the size and toppings required on a pizza:

The size (small, medium or large) should be selectable with a group of radio buttons because a single pizza can only be one size.

The toppings (chicken, pork, onions, mushrooms, etc.) should be selectable with a group of check boxes because you can have any combination.

Next lesson: 14.8 JComboBox