Chapter: Java The Complete Reference : Introducing GUI Programming with Swing : Exploring Swing

The Swing Buttons - Swing

Swing defines four types of buttons: JButton, JToggleButton, JCheckBox, and JRadioButton. All are subclasses of the AbstractButton class, which extends JComponent. Thus, all buttons share a set of common traits.

The Swing Buttons

Swing defines four types of buttons: JButton, JToggleButton, JCheckBox, and JRadioButton. All are subclasses of the AbstractButton class, which extends JComponent. Thus, all buttons share a set of common traits.

AbstractButton contains many methods that allow you to control the behavior of buttons. For example, you can define different icons that are displayed for the button when it is disabled, pressed, or selected. Another icon can be used as a rollover icon, which is displayed when the mouse is positioned over a button. The following methods set these icons:

void setDisabledIcon(Icon di) void setPressedIcon(Icon pi) void setSelectedIcon(Icon si) void setRolloverIcon(Icon ri)

Here, di, pi, si, and ri are the icons to be used for the indicated purpose.

The text associated with a button can be read and written via the following methods:

String getText( )

void setText(String str)

Here, str is the text to be associated with the button.

The model used by all buttons is defined by the ButtonModel interface. A button generates an action event when it is pressed. Other events are possible. Each of the concrete button classes is examined next.

JButton

The JButton class provides the functionality of a push button. You have already seen a simple form of it in the preceding chapter. JButton allows an icon, a string, or both to be associated with the push button. Three of its constructors are shown here:

JButton(Icon icon) JButton(String str) JButton(String str, Icon icon)

Here, str and icon are the string and icon used for the button.

When the button is pressed, an ActionEvent is generated. Using the ActionEvent object passed to the actionPerformed( ) method of the registered ActionListener, you can obtain the action command string associated with the button. By default, this is the string displayed inside the button. However, you can set the action command by calling setActionCommand( ) on the button. You can obtain the action command by calling getActionCommand( ) on the event object. It is declared like this:

String getActionCommand( )

The action command identifies the button. Thus, when using two or more buttons within the same application, the action command gives you an easy way to determine which button was pressed.

In the preceding chapter, you saw an example of a text-based button. The following demonstrates an icon-based button. It displays four push buttons and a label. Each button displays an icon that represents a timepiece. When a button is pressed, the name of that timepiece is displayed in the label.

// Demonstrate an icon-based JButton.

import java.awt.*;

import java.awt.event.*; import javax.swing.*; /*

public class JButtonDemo extends JApplet implements ActionListener {

JLabel jlab;

public void init() { try {

SwingUtilities.invokeAndWait( new Runnable() {

public void run() { makeGUI();

}

);

} catch (Exception exc) {

System.out.println("Can't create because of " + exc);

}

private void makeGUI() {

Change to flow layout. setLayout(new FlowLayout());

Add buttons to content pane.

ImageIcon hourglass = new ImageIcon("hourglass.png"); JButton jb = new JButton(hourglass); jb.setActionCommand("Hourglass"); jb.addActionListener(this);

add(jb);

ImageIcon analog = new ImageIcon("analog.png"); jb = new JButton(analog); jb.setActionCommand("Analog Clock"); jb.addActionListener(this);

add(jb);

ImageIcon digital = new ImageIcon("digital.png"); jb = new JButton(digital); jb.setActionCommand("Digital Clock"); jb.addActionListener(this);

add(jb);

ImageIcon stopwatch = new ImageIcon("stopwatch.png"); jb = new JButton(stopwatch); jb.setActionCommand("Stopwatch"); jb.addActionListener(this);

add(jb);

// Create and add the label to content pane.

jlab = new JLabel("Choose a Timepiece"); add(jlab);

}

// Handle button events.

public void actionPerformed(ActionEvent ae) { jlab.setText("You selected " + ae.getActionCommand());

}

Output from the button example is shown here:

JToggleButton

A useful variation on the push button is called a toggle button. A toggle button looks just like a push button, but it acts differently because it has two states: pushed and released. That is, when you press a toggle button, it stays pressed rather than popping back up as a regular push button does. When you press the toggle button a second time, it releases (pops up). Therefore, each time a toggle button is pushed, it toggles between its two states.

Toggle buttons are objects of the JToggleButton class. JToggleButton implements AbstractButton. In addition to creating standard toggle buttons, JToggleButton is a superclass for two other Swing components that also represent two-state controls. These are JCheckBox and JRadioButton, which are described later in this chapter. Thus, JToggleButton defines the basic functionality of all two-state components.

JToggleButton defines several constructors. The one used by the example in this section is shown here:

JToggleButton(String str)

This creates a toggle button that contains the text passed in str. By default, the button is in the off position. Other constructors enable you to create toggle buttons that contain images, or images and text.

JToggleButton uses a model defined by a nested class called JToggleButton.Toggle-ButtonModel. Normally, you won’t need to interact directly with the model to use a standard toggle button.

Like JButton, JToggleButton generates an action event each time it is pressed. Unlike JButton, however, JToggleButton also generates an item event. This event is used by those components that support the concept of selection. When a JToggleButton is pressed in, it is selected. When it is popped out, it is deselected.

To handle item events, you must implement the ItemListener interface. Recall from Chapter 24, that each time an item event is generated, it is passed to the itemStateChanged( ) method defined by ItemListener. Inside itemStateChanged( ), the getItem( ) method can be called on the ItemEvent object to obtain a reference to the JToggleButton instance that generated the event. It is shown here:

Object getItem( )

A reference to the button is returned. You will need to cast this reference to JToggleButton. The easiest way to determine a toggle button’s state is by calling the isSelected( ) method

(inherited from AbstractButton) on the button that generated the event. It is shown here: boolean isSelected( )

It returns true if the button is selected and false otherwise.

Here is an example that uses a toggle button. Notice how the item listener works. It simply calls isSelected( ) to determine the button’s state.

// Demonstrate JToggleButton.

import java.awt.*;

import java.awt.event.*; import javax.swing.*; /*

public class JToggleButtonDemo extends JApplet {

JLabel jlab;

JToggleButton jtbn;

public void init() { try {

SwingUtilities.invokeAndWait( new Runnable() {

public void run() { makeGUI();

}

);

} catch (Exception exc) {

System.out.println("Can't create because of " + exc);

}

private void makeGUI() {

//Change to flow layout.

setLayout(new FlowLayout());

//Create a label.

jlab = new JLabel("Button is off.");

// Make a toggle button.

jtbn = new JToggleButton("On/Off");

// Add an item listener for the toggle button.

jtbn.addItemListener(new ItemListener() {

public void itemStateChanged(ItemEvent ie) { if(jtbn.isSelected())

jlab.setText("Button is on."); else

jlab.setText("Button is off.");

}

});

// Add the toggle button and label to the content pane.

add(jtbn);

add(jlab);

}

The output from the toggle button example is shown here:

Check Boxes

The JCheckBox class provides the functionality of a check box. Its immediate superclass is JToggleButton, which provides support for two-state buttons, as just described. JCheckBox defines several constructors. The one used here is

JCheckBox(String str)

It creates a check box that has the text specified by str as a label. Other constructors let you specify the initial selection state of the button and specify an icon.

When the user selects or deselects a check box, an ItemEvent is generated. You can obtain a reference to the JCheckBox that generated the event by calling getItem( ) on the

ItemEvent passed to the itemStateChanged( ) method defined by ItemListener. The easiest way to determine the selected state of a check box is to call isSelected( ) on the JCheckBox instance.

The following example illustrates check boxes. It displays four check boxes and a label. When the user clicks a check box, an ItemEvent is generated. Inside the itemStateChanged( ) method, getItem( ) is called to obtain a reference to the JCheckBox object that generated the event. Next, a call to isSelected( ) determines if the box was selected or cleared. The getText( ) method gets the text for that check box and uses it to set the text inside the label.

// Demonstrate JCheckbox.

import java.awt.*; import java.awt.event.*; import javax.swing.*;

public class JCheckBoxDemo extends JApplet implements ItemListener {

JLabel jlab;

public void init() { try {

SwingUtilities.invokeAndWait( new Runnable() {

public void run() { makeGUI();

}

);

} catch (Exception exc) {

System.out.println("Can't create because of " + exc);

}

private void makeGUI() {

//Change to flow layout.

setLayout(new FlowLayout());

//Add check boxes to the content pane.

JCheckBox cb = new JCheckBox("C"); cb.addItemListener(this);

add(cb);

cb = new JCheckBox("C++"); cb.addItemListener(this); add(cb);

cb = new JCheckBox("Java"); cb.addItemListener(this); add(cb);

cb = new JCheckBox("Perl"); cb.addItemListener(this); add(cb);

// Create the label and add it to the content pane.

jlab = new JLabel("Select languages");

add(jlab);

}

// Handle item events for the check boxes.

public void itemStateChanged(ItemEvent ie) {

JCheckBox cb = (JCheckBox)ie.getItem();

if(cb.isSelected())

jlab.setText(cb.getText() + " is selected"); else

jlab.setText(cb.getText() + " is cleared");

}

Output from this example is shown here:

Radio Buttons

Radio buttons are a group of mutually exclusive buttons, in which only one button can be selected at any one time. They are supported by the JRadioButton class, which extends JToggleButton. JRadioButton provides several constructors. The one used in the example is shown here:

JRadioButton(String str)

Here, str is the label for the button. Other constructors let you specify the initial selection state of the button and specify an icon.

In order for their mutually exclusive nature to be activated, radio buttons must be configured into a group. Only one of the buttons in the group can be selected at any time. For example, if a user presses a radio button that is in a group, any previously selected button in that group is automatically deselected. A button group is created by the ButtonGroup class. Its default constructor is invoked for this purpose. Elements are

then added to the button group via the following method: void add(AbstractButton ab)

Here, ab is a reference to the button to be added to the group.

A JRadioButton generates action events, item events, and change events each time the button selection changes. Most often, it is the action event that is handled, which means that you will normally implement the ActionListener interface. Recall that the only method defined by ActionListener is actionPerformed( ). Inside this method, you can use a number of different ways to determine which button was selected. First, you can check its action command by calling getActionCommand( ). By default, the action command is the same as the button label, but you can set the action command to something else by calling setActionCommand( ) on the radio button. Second, you can call getSource( ) on the ActionEvent object and check that reference against the buttons. Third, you can check each radio button to find out which one is currently selected by calling isSelected( ) on each button. Finally, each button could use its own action event handler implemented as either an anonymous inner class or a lambda expression. Remember, each time an action event occurs, it means that the button being selected has changed and that one and only one button will be selected.

The following example illustrates how to use radio buttons. Three radio buttons are created. The buttons are then added to a button group. As explained, this is necessary to cause their mutually exclusive behavior. Pressing a radio button generates an action event, which is handled by actionPerformed( ). Within that handler, the getActionCommand( ) method gets the text that is associated with the radio button and uses it to set the text within a label.

// Demonstrate JRadioButton

import java.awt.*;

import java.awt.event.*; import javax.swing.*; /*

public class JRadioButtonDemo extends JApplet implements ActionListener {

JLabel jlab;

public void init() { try {

SwingUtilities.invokeAndWait( new Runnable() {

public void run() { makeGUI();

}

);

} catch (Exception exc) {

System.out.println("Can't create because of " + exc);

}

private void makeGUI() {

//Change to flow layout.

setLayout(new FlowLayout());

//Create radio buttons and add them to content pane.

JRadioButton b1 = new JRadioButton("A");

b1.addActionListener(this);

add(b1);

JRadioButton b2 = new JRadioButton("B"); b2.addActionListener(this);

add(b2);

JRadioButton b3 = new JRadioButton("C"); b3.addActionListener(this);

add(b3);

//Define a button group.

ButtonGroup bg = new ButtonGroup(); bg.add(b1);

bg.add(b2);

bg.add(b3);

//Create a label and add it to the content pane.

jlab = new JLabel("Select One");

add(jlab);

}

// Handle button selection.

public void actionPerformed(ActionEvent ae) { jlab.setText("You selected " + ae.getActionCommand());

}

Output from the radio button example is shown here:

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Java The Complete Reference : Introducing GUI Programming with Swing : Exploring Swing : The Swing Buttons - Swing |