You now know how to collect text input from users, but there are many occasions where you would rather give users a finite set of choices than have them enter the data in a text component. Using a set of buttons or a list of items tells your users what choices they have. (It also saves you the trouble of error checking.) In this section, you will learn how to program checkboxes, radio buttons, lists of choices, and sliders.
1. Checkboxes
If you want to collect just a “yes” or “no” input, use a checkbox component. Checkboxes automatically come with labels that identify them. The user can check the box by clicking inside it and turn off the checkmark by clicking inside the box again. Pressing the space bar when the focus is in the checkbox also toggles the checkmark.
Figure 11.12 shows a simple program with two checkboxes, one for turning the italic attribute of a font on or off, and the other for boldface. Note that the second checkbox has focus, as indicated by the rectangle around the label. Each time the user clicks one of the checkboxes, the screen is refreshed, using the new font attributes.
Checkboxes need a label next to them to identify their purpose. Give the label text in the constructor:
bold = new JCheckBox(“Botd”);
Use the setSelected method to turn a checkbox on or off. For example:
bold.setSelected(true);
The isSelected method then retrieves the current state of each checkbox. It is false if unchecked, true if checked.
When the user clicks on a checkbox, this triggers an action event. As always, you attach an action listener to the checkbox. In our program, the two checkboxes share the same action listener.
ActionListener listener = . . .;
bold.addActionListener(listener);
italic.addActionListener(listener);
The listener queries the state of the bold and italic checkboxes and sets the font of the panel to plain, bold, italic, or both bold and italic.
ActionListener listener = event -> {
int mode = 0;
if (bold.isSelected()) mode += Font.BOLD;
if (italic.isSelected()) mode += Font.ITALIC;
label.setFont(new Font(Font.SERIF, mode, FONTSIZE));
};
Listing 11.2 is the program listing for the checkbox example.
2. Radio Buttons
In the previous example, the user could check either, both, or neither of the two checkboxes. In many cases, we want the user to check only one of several boxes. When another box is checked, the previous box is automatically unchecked. Such a group of boxes is often called a radio button group because the buttons work like the station selector buttons on a radio. When you push in one button, the previously depressed button pops out. Figure 11.13 shows a typical example. We allow the user to select a font size from among the choices—Small, Medium, Large, or Extra large—but, of course, we will allow selecting only one size at a time.
Implementing radio button groups is easy in Swing. You construct one object of type ButtonGroup for every group of buttons. Then, you add objects of type JRadioButton to the button group. The button group object is responsible for turning off the previously set button when a new button is clicked.
var group = new ButtonGroup();
var smallButton = new JRadioButton(MSmall”, false);
group.add(smallButton);
var mediumButton = new JRadioButton(“Medium”, true);
…
group.add(mediumButton);
The second argument of the constructor is true for the button that should be checked initially and false for all others. Note that the button group controls only the behavior of the buttons; if you want to group the buttons for layout purposes, you also need to add them to a container such as a JPanel.
If you look again at Figures 11.12 and 11.13, you will note that the appearance of the radio buttons is different from that of checkboxes. Checkboxes are square and contain a checkmark when selected. Radio buttons are round and contain a dot when selected.
The event notification mechanism for radio buttons is the same as for any other buttons. When the user checks a radio button, the button generates an action event. In our example program, we define an action listener that sets the font size to a particular value:
ActionListener listener = event ->
label.setFont(new Font(“Serif”, Font.PLAIN, size));
Compare this listener setup to that of the checkbox example. Each radio button gets a different listener object. Each listener object knows exactly what it needs to do—set the font size to a particular value. With checkboxes, we used a different approach: Both checkboxes have the same action listener that calls a method looking at the current state of both checkboxes.
Could we follow the same approach here? We could have a single listener that computes the size as follows:
if (smallButton.isSelected()) size = 8;
else if (mediumButton.isSelected()) size = 12;
…
However, we prefer to use separate action listener objects because they tie the size values more closely to the buttons.
Listing 11.3 is the complete program for font size selection that puts a set of radio buttons to work.
3. Borders
If you have multiple groups of radio buttons in a window, you will want to visually indicate which buttons are grouped. Swing provides a set of useful borders for this purpose. You can apply a border to any component that extends JComponent. The most common usage is to place a border around a panel and fill that panel with other user interface elements, such as radio buttons.
You can choose from quite a few borders, but you need to follow the same steps for all of them.
- Call a static method of the BorderFactory to create a border. You can choose among the following styles (see Figure 11.14):
-
- Lowered bevel
- Raised bevel
- Etched
- Line
- Matte
- Empty (just to create some blank space around the component)
- If you like, add a title to your border by passing your border to BorderFactory .createTittedBorder.
- If you really want to go all out, combine several borders with a call to BorderFactory.createCompoundBorder.
- Add the resulting border to your component by calling the setBorder method of the JComponent class.
For example, here is how you add an etched border with a title to a panel:
Border etched = BorderFactory.createEtchedBorder();
Border titled = BorderFactory.createTitledBorder(etched, “A Title”);
panel.setBorder(titled);
Different borders have different options for setting border widths and colors; see the API notes for details. True border enthusiasts will appreciate that there is also a SoftBevetBorder class for beveled borders with softened corners and that a LineBorder can have rounded corners as well. You can construct these borders only by using one of the class constructors—there is no BorderFactory method for them.
4. Combo Boxes
If you have more than a handful of alternatives, radio buttons are not a good choice because they take up too much screen space. Instead, you can use a combo box. When the user clicks on this component, a list of choices drops down, and the user can then select one of them (see Figure 11.15).
If the drop-down list box is set to be editable, you can edit the current selection as if it were a text field. For that reason, this component is called a combo box—it combines the flexibility of a text field with a set of predefined choices. The JComboBox class provides a combo box component.
As of Java 7, the JComboBox class is a generic class. For example, a JComboBox<String> holds objects of type String, and a JComboBox<Integer> holds integers.
Call the setEditabte method to make the combo box editable. Note that editing affects only the selected item. It does not change the list of choices in any way.
You can obtain the current selection, which may have been edited if the combo box is editable, by calling the getSelectedItem method. However, for an editable combo box, that item may have any type, depending on the editor that takes the user edits and turns the result into an object. (See Volume II, Chapter 6 for a discussion of editors.) If your combo box isn’t editable, you are better off calling
combo.getItemAt(combo.getSelectedIndex())
which gives you the selected item with the correct type.
In the example program, the user can choose a font style from a list of styles (Serif, SansSerif, Monospaced, etc.). The user can also type in another font.
Add the choice items with the addItem method. In our program, addItem is called only in the constructor, but you can call it any time.
var faceCombo = new JComboBox<String>();
faceCombo.addItem(“Serif”);
faceCombo.addItem(“SansSerif”);
…
This method adds the string to the end of the list. You can add new items anywhere in the list with the insertItemAt method:
faceCombo.insertItemAt(“Monospaced”, 0); // add at the beginning
You can add items of any type—the combo box invokes each item’s toString method to display it.
If you need to remove items at runtime, use the removeItem or removeItemAt method, depending on whether you supply the item to be removed or its position.
faceCombo.removeItem(“Monospaced”);
faceCombo.removeItemAt(0); // remove first item
The removeAllItems method removes all items at once.
When the user selects an item from a combo box, the combo box generates an action event. To find out which item was selected, call getSource on the event parameter to get a reference to the combo box that sent the event.
Then call the getSelectedItem method to retrieve the currently selected item. You will need to cast the returned value to the appropriate type, usually String.
ActionListener listener = event ->
label.setFont(new Font(
faceCombo.getItemAt(faceCombo.getSelectedIndex()),
Font.PLAIN,
DEFAULT_SIZE));
Listing 11.4 shows the complete program.
5. Sliders
Combo boxes let users choose from a discrete set of values. Sliders offer a choice from a continuum of values—for example, any number between 1 and 100.
The most common way of constructing a slider is as follows:
var slider = new JSlider(min, max, initiatVatue);
If you omit the minimum, maximum, and initial values, they are initialized with 0, 100, and 50, respectively.
Or if you want the slider to be vertical, use the following constructor call:
var slider = new JSlider(SwingConstants.VERTICAL, min, max, initialValue);
These constructors create a plain slider, such as the top slider in Figure 11.16. You will see presently how to add decorations to a slider.
As the user slides the slider bar, the value of the slider moves between the minimum and the maximum values. When the value changes, a ChangeEvent is sent to all change listeners. To be notified of the change, call the addChangeListener method and install an object that implements the functional ChangeListener interface. In the callback, retrieve the slider value:
ChangeListener listener = event ->
{
JStider slider = (JSlider) event.getSource();
int value = slider.getValue();
…
};
You can embellish the slider by showing ticks. For example, in the sample program, the second slider uses the following settings:
slider.setMajorTickSpacing(20);
slider.setMinorTickSpacing(5);
The slider is decorated with large tick marks every 20 units and small tick marks every 5 units. The units refer to slider values, not pixels.
These instructions only set the units for the tick marks. To actually have the tick marks appear, call
slider.setPaintTicks(true);
The major and minor tick marks are independent. For example, you can set major tick marks every 20 units and minor tick marks every 7 units, but that will give you a very messy scale.
You can force the slider to snap to ticks. Whenever the user has finished dragging a slider in snap mode, it is immediately moved to the closest tick. You activate this mode with the call
slider.setSnapToTicks(true);
You can display tick mark labels for the major tick marks by calling
slider.setPaintLabels(true);
For example, with a slider ranging from 0 to 100 and major tick spacing of 20, the ticks are labeled 0, 20, 40, 60, 80, and 100.
You can also supply other tick mark labels, such as strings or icons (see Figure 11.16). The process is a bit convoluted. You need to fill a hash table with keys of type Integer and values of type Component. You then call the setLabetTabte method. The components are placed under the tick marks. Usually, JLabet objects are used. Here is how you can label ticks as A, B, C, D, E, and F:
var tabetTabte = new Hashtabte<Integer, Component>();
tabetTable.put(0, new JLabet(“A”));
labetTabte.put(20, new JLabet(“B”));
…
tabetTabte.put(100, new JLabet(“F”));
stider.setLabelTable(labelTabte);
Listing 11.5 also shows a slider with icons as tick labels.
The fourth slider in Figure 11.16 has no track. To suppress the “track” in which the slider moves, call
stider.setPaintTrack(fatse);
The fifth slider has its direction reversed by a call to
stider.setInverted(true);
The example program in Listing 11.5 shows all these visual effects with a collection of sliders. Each slider has a change event listener installed that places the current slider value into the text field at the bottom of the frame.
Source: Horstmann Cay S. (2019), Core Java. Volume I – Fundamentals, Pearson; 11th edition.