3. Swing Components

3. Swing Components
	Chapter 10. Graphic User Interface

3.1. Rules for Using Components

3.1.1. How to Add a Component to a Container

For any Component object except a JFrame to display itself onscreen, you must first add it to a Container object.

The Container class defines three methods for adding components:

add(Component comp) method simply requires that you specify the component to add.
add(Component comp, int ind) method lets you add an argument specifying the integer position at which the component should be added. This argument is also used to determine the layout manager-dependent position at which the component should be added.
add(String pos, Component comp) method has as first argument a layout manager-dependent string that specifies the component's position to the layout manager.

	Note
	Adding a component to a container removes the component from the container it used to be in (if any).

3.1.2. What the Component Class Provides

Basic drawing support
Component provides the paint(), update(), and repaint() methods, which enable components to draw themselves on screen.
Event handling
Component defines the general-purpose handleEvent() method and a group of methods such as action() that handle specific event types.
Appearance control: font
Component provides methods to get and set the current font, and to get information about the current font.
Appearance control: color
Component provides methods to get and set the foreground and background colors.
Image handling
Component provides the basis for displaying images. Most containers can display images.
Onscreen size and position control
Component provides methods that get or set (subject to layout manager oversight) the component's current size and location.

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3.3. Labels

The JLabel class provides an easy way of putting unselectable text in your program's GUI. Labels are aligned to the left of their drawing area, by default.

Figure 10.10. Labels

3.3.1. Code Example

ImageIcon icon = new ImageIcon("images/middle.gif");

JLabel lbl1 = new JLabel("Image and Text", JLabel.CENTER);
lbl1.setIcon(icon);

// Set the position of the text, relative to the icon:
lbl1.setVerticalTextPosition(JLabel.BOTTOM);
lbl1.setHorizontalTextPosition(JLabel.CENTER);

JLabel lbl2 = new JLabel("Text-Only Label");

JLabel lbl3 = new JLabel(icon);
lbl3.setHorizontalAlignment(JLabel.CENTER);

Checkboxes are two-state buttons that can be either "selected" or "deselected" . When the user clicks a checkbox, the checkbox state changes and generates an action event. The JCheckBox class implements the Swing version of checkboxes.

Figure 10.11. Checkbox

3.4.1. Code Example

cb1 = new JCheckBox(); // Default state is "deselected" (false)
cb1.setText("Checkbox 1");

cb2 = new JCheckBox("Checkbox 2");

cb3 = new JCheckBox("Checkbox 3");
cb3.setSelected(true); // Set state to "selected" (true)

// Register a listener for the checkboxes
cb1.addItemListener(this);
cb2.addItemListener(this);
cb3.addItemListener(this);

...

public void itemStateChanged(ItemEvent e) {
    Object src = e.getItemSelectable();
    if (src == cb1) {
        if (e.getStateChange() == ItemEvent.SELECTED)
            System.out.println("Checkbox 1 selected.");
        else System.out.println("Checkbox 1 unselected.");
    } else if (src == cb2) {
        ...
    } else if (src == cb3) {
        ...
    }
}

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3.5. Radio Buttons

Radio buttons are similar to checkboxes with the difference that you can group them to ensure that only one checkbox is selected at a time. In Swing, each JRadioButton is added to a ButtonGroup so the group behaves as a set of radio buttons.

Figure 10.12. Radiobutton

3.5.1. Code Example

rb1 = new JRadioButton("RadioButton 1");

rb2 = new JRadioButton("RadioButton 2");
rb2.setSelected(true);

rb3 = new JRadioButton("RadioButton 3");

// Group the radio buttons
ButtonGroup grp = new ButtonGroup();
grp.add(rb1);
grp.add(rb2);
grp.add(rb3);

// Register a listener for the radio buttons
rb1.addActionListener(this);
rb2.addActionListener(this);
rb3.addActionListener(this);

...

public void actionPerformed(ActionEvent e) {
    Object src = e.getSource();
    if (src == rb1) {
        System.out.println("RadioButton 1 selected!");
    } else if (src == rb2) {
        ...
    } else if (src == rb3) {
        ...
    }
}

3.6. Combo Boxes

The JComboBox class provides a menu-like list of choices, accessed by a distinctive button. The user presses the button to bring up a "menu", and then chooses one of the items.

Figure 10.13. Combo box

3.6.1. Code Example

String[] str = { "Bird", "Cat", "Dog", "Rabbit", "Pig" };

// Create the combo box, select item at index 4
// Indices start at 0, so 4 specifies the pig

JComboBox cb = new JComboBox(str);
cb.setSelectedIndex(4);
cb.addActionListener(this);

...

public void actionPerformed(ActionEvent e) {
    JComboBox cb = (JComboBox) e.getSource();
    System.out.println("User chose " + ((String) cb.getSelectedItem()));
}

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3.7. Lists

The JList class provides an list of selectable items. These items can be composed of text, images, or both.

Figure 10.14. List

3.7.1. Code Example

// Build the lists, which allows multiple selections
String[] elements = { "uno", "dos", "tres", "cuatro", "cinco", ... };
spanish = new JList(elements);
spanish.addListSelectionListener(this);
spanish.setSelectionMode(ListSelectionModel.MULTIPLE_INTERVAL_SELECTION);

italian = new JList(new DefaultListModel());
italian.addListSelectionListener(this);
italian.setSelectionMode(ListSelectionModel.MULTIPLE_INTERVAL_SELECTION);

// Add data using the list model
DefaultListModel model = (DefaultListModel) italian.getModel();
model.addElement("uno");
model.addElement("due");
model.addElement("tre");
model.addElement("quattro");
model.addElement("cinque");

. . .

// Implementing the listener for the selection events
public void valueChanged(ListSelectionEvent e) {
        
    if (e.getValueIsAdjusting()) {
        if (e.getSource() == spanish) {
            int[] indices = spanish.getSelectedIndices();
            Object[] values = spanish.getSelectedValues();
            for (int i = 0; i < indices.length; i++) {
                String item = (String) values[i];
                System.out.println("Select event occurred on item #" +
                       indices[i] + " (\"" + item + "\") in Spanish.");
            }
        } 
        else {            
            int[] indices = italian.getSelectedIndices();
            DefaultListModel model = (DefaultListModel) italian.getModel();
            for (int i = 0; i < indices.length; i++) {
                String item = (String) model.getElementAt(indices[i]);
                System.out.println("Select event occurred on item #" +
                       indices[i] + " (\"" + item + "\") in Italian.");
            }
        }            
    }        
}

3.8. Scroll Panes

A JScrollPane manages a single child component, displaying as much of the component as space permits. By default, a scroll pane's scrollbars are visible only when they're needed.

Figure 10.15. ScrollPane

3.8.1. Scrollbar parameters

HORIZONTAL_SCROLLBAR_AS_NEEDED, VERTICAL_SCROOLBAR_AS_NEEDED
The default value. Show each scrollbar only when it's needed.
HORIZONTAL_SCROLLBAR_ALWAYS, VERTICAL_SCROLLBAR_ALWAYS
Always show the scrollbars.
HORIZONTAL_SCROLLBAR_NEVER, VERTICAL_SCROLLBAR_NEVER
Never show scrollbars. You might use this option if you don't want the user to directly control what part of the child component is shown.

3.8.2. Code Example

JScrollPane scroll = new JScrollPane(aComponent, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, 
                                                 JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);

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3.9. Text Components

The JTextComponent derived classes display selectable text and, optionally, allow the user to edit the text. They can also support styled text and password fields.

Figure 10.16. JTextComponent hierarchy

3.9.1. TextField and TextArea

From JTextComponent this classes inherit methods that allow them to set and get the current selection, enable and disable editing, get the currently selected text (or all the text), and set the text.

Figure 10.17. JTextField and JTextArea components

3.9.2. Code Example

// Where instance variables are defined:
private JTextField textField;
private JTextArea textArea;

public void init() {

    textField = new JTextField(20);
    textField.addActionListener(this);

    textArea = new JTextArea(5, 20);
    textArea.setEditable(false);

    ...// Add the two components to the panel. 
}

public void actionPerformed(ActionEvent evt) {
    String text = textField.getText();
    textArea.append(text + "\n");
    textField.selectAll();
}

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