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Subsections
JButton
JLabel
JCheckBox
JTextField and JTextArea
JComboBox
JSlider

Section 6.6

Basic Components

In preceding sections, you've seen how to use a graphics context to draw on the screen and how to handle mouse events and keyboard events. In one sense, that's all there is to GUI programming. If you're willing to program all the drawing and handle all the mouse and keyboard events, you have nothing more to learn. However, you would either be doing a lot more work than you need to do, or you would be limiting yourself to very simple user interfaces. A typical user interface uses standard GUI components such as buttons, scroll bars, text-input boxes, and menus. These components have already been written for you, so you don't have to duplicate the work involved in developing them. They know how to draw themselves, and they can handle the details of processing the mouse and keyboard events that concern them.

Consider one of the simplest user interface components, a push button. The button has a border, and it displays some text. This text can be changed. Sometimes the button is disabled, so that clicking on it doesn't have any effect. When it is disabled, its appearance changes. When the user clicks on the push button, the button changes appearance while the mouse button is pressed and changes back when the mouse button is released. In fact, it's more complicated than that. If the user moves the mouse outside the push button before releasing the mouse button, the button changes to its regular appearance. To implement this, it is necessary to respond to mouse exit or mouse drag events. Furthermore, on many platforms, a button can receive the input focus. The button changes appearance when it has the focus. If the button has the focus and the user presses the space bar, the button is triggered. This means that the button must respond to keyboard and focus events as well.

Fortunately, you don't have to program any of this, provided you use an object belonging to the standard class javax.swing.JButton. A JButton object draws itself and processes mouse, keyboard, and focus events on its own. You only hear from the Button when the user triggers it by clicking on it or pressing the space bar while the button has the input focus. When this happens, the JButton object creates an event object belonging to the class java.awt.event.ActionEvent. The event object is sent to any registered listeners to tell them that the button has been pushed. Your program gets only the information it needs -- the fact that a button was pushed.

The standard components that are defined as part of the Swing graphical user interface API are defined by subclasses of the class JComponent, which is itself a subclass of Component. (Note that this includes the JPanel class that we have already been working with extensively.) Many useful methods are defined in the Component and JComponent classes and so can be used with any Swing component. We begin by looking at a few of these methods. Suppose that comp is a variable that refers to some JComponent. Then the following methods can be used:

Note that using any component is a multi-step process. The component object must be created with a constructor. It must be added to a container. In many cases, a listener must be registered to respond to events from the component. And in some cases, a reference to the component must be saved in an instance variable so that the component can be manipulated by the program after it has been created. In this section, we will look at a few of the basic standard components that are available in Swing. In the next section we will consider the problem of laying out components in containers.

6.6.1  JButton

An object of class JButton is a push button that the user can click to trigger some action. You've already seen buttons used Section 6.1 and Section 6.2, but we consider them in much more detail here. To use any component effectively, there are several aspects of the corresponding class that you should be familiar with. For JButton, as an example, I list these aspects explicitely:

Of course, JButtons also have all the general Component methods, such as setEnabled() and setFont(). The setEnabled() and setText() methods of a button are particularly useful for giving the user information about what is going on in the program. A disabled button is better than a button that gives an obnoxious error message such as "Sorry, you can't click on me now!"

6.6.2  JLabel

JLabel is certainly the simplest type of component. An object of type JLabel exists just to display a line of text. The text cannot be edited by the user, although it can be changed by your program. The constructor for a JLabel specifies the text to be displayed:

JLabel message = new JLabel("Hello World!");

There is another constructor that specifies where in the label the text is located, if there is extra space. The possible alignments are given by the constants JLabel.LEFT, JLabel.CENTER, and JLabel.RIGHT. For example,

JLabel message = new JLabel("Hello World!", JLabel.CENTER);

creates a label whose text is centered in the available space. You can change the text displayed in a label by calling the label's setText() method:

message.setText("Goodby World!");

Since the JLabel class is a subclass of JComponent, you can use methods such as setForeground() with labels. If you want the background color to have any effect, you should call setOpaque(true) on the label, since otherwise the JLabel might not fill in its background. For example:

JLabel message = new JLabel("Hello World!", JLabel.CENTER);
message.setForeground(Color.red);   // Display red text...
message.setBackground(Color.black); //    on a black background...
message.setFont(new Font("Serif",Font.BOLD,18));  // in a big bold font.
message.setOpaque(true);  // Make sure background is filled in.

6.6.3  JCheckBox

A JCheckBox is a component that has two states: selected or unselected. The user can change the state of a check box by clicking on it. The state of a checkbox is represented by a boolean value that is true if the box is selected and false if the box is unselected. A checkbox has a label, which is specified when the box is constructed:

JCheckBox showTime = new JCheckBox("Show Current Time");

Usually, it's the user who sets the state of a JCheckBox, but you can also set the state in your program. The current state of a checkbox is set using its setSelected(boolean) method. For example, if you want the checkbox showTime to be checked, you would say "showTime.setSelected(true)". To uncheck the box, say "showTime.setSelected(false)". You can determine the current state of a checkbox by calling its isSelected() method, which returns a boolean value.

In many cases, you don't need to worry about events from checkboxes. Your program can just check the state whenever it needs to know it by calling the isSelected() method. However, a checkbox does generate an event when its state is changed by the user, and you can detect this event and respond to it if you want something to happen at the moment the state changes. When the state of a checkbox is changed by the user, it generates an event of type ActionEvent. If you want something to happen when the user changes the state, you must register an ActionListener with the checkbox by calling its addActionListener() method. (Note that if you change the state by calling the setSelected() method, no ActionEvent is generated. However, there is another method in the JCheckBox class, doClick(), which simulates a user click on the checkbox and does generate an ActionEvent.)

When handling an ActionEvent, you can call evt.getSource() in the actionPerformed() method to find out which object generated the event. (Of course, if you are only listening for events from one component, you don't even have to do this.) The returned value is of type Object, but you can type-cast it to another type if you want. Once you know the object that generated the event, you can ask the object to tell you its current state. For example, if you know that the event had to come from one of two checkboxes, cb1 or cb2, then your actionPerformed() method might look like this:

          public void actionPerformed(ActionEvent evt) {
             Object source = evt.getSource();
             if (source == cb1) {
                boolean newState = ((JCheckBox)cb1).isSelected();
                ... // respond to the change of state
             }
             else if (source == cb2) {
                boolean newState = ((JCheckBox)cb2).isSelected();
                ... // respond to the change of state
             }
          }

Alternatively, you can use evt.getActionCommand() to retrieve the action command associated with the source. For a JCheckBox, the action command is, by default, the label of the checkbox.

6.6.4  JTextField and JTextArea

The JTextField and JTextArea classes represent components that contain text that can be edited by the user. A JTextField holds a single line of text, while a JTextArea can hold multiple lines. It is also possible to set a JTextField or JTextArea to be read-only so that the user can read the text that it contains but cannot edit the text. Both classes are subclasses of an abstract class, JTextComponent, which defines their common properties.

JTextField and JTextArea have many methods in common. The instance method setText(), which takes a parameter of type String, can be used to change the text that is displayed in an input component. The contents of the component can be retrieved by calling its getText() instance method, which returns a value of type String. If you want to stop the user from modifying the text, you can call setEditable(false). Call the same method with a parameter of true to make the input component user-editable again.

The user can only type into a text component when it has the input focus. The user can give the input focus to a text component by clicking it with the mouse, but sometimes it is useful to give the input focus to a text field programmatically. You can do this by calling its requestFocus() method. For example, when I discover an error in the user's input, I usually call requestFocus() on the text field that contains the error. This helps the user see where the error occurred and let's the user start typing the correction immediately.

By default, there is no space between the text in a text component and the edge of the component, which usually doesn't look very good. You can use the setMargin() method of the component to add some blank space between the edge of the component and the text. This method takes a parameter of type java.awt.Insets which contains four integer instance variables that specify the margins on the top, left, bottom, and right edge of the component. For example,

textComponent.setMargin( new Insets(5,5,5,5) );

adds a five-pixel margin between the text in textComponent and each edge of the component.

The JTextField class has a constructor

public JTextField(int columns)

where columns is an integer that specifies the number of characters that should be visible in the text field. This is used to determine the preferred width of the text field. (Because characters can be of different sizes and because the preferred width is not always respected, the actual number of characters visible in the text field might not be equal to columns.) You don't have to specify the number of columns; for example, you might use the text field in a context where it will expand to fill whatever space is available. In that case, you can use the constructor JTextField(), with no parameters. You can also use the following constructors, which specify the initial contents of the text field:

public JTextField(String contents);
public JTextField(String contents, int columns);

The constructors for a JTextArea are

public JTextArea()
public JTextArea(int rows, int columns)
public JTextArea(String contents)
public JTextArea(String contents, int rows, int columns)

The parameter rows specifies how many lines of text should be visible in the text area. This determines the preferred height of the text area, just as columns determines the preferred width. However, the text area can actually contain any number of lines; the text area can be scrolled to reveal lines that are not currently visible. It is common to use a JTextArea as the CENTER component of a BorderLayout. In that case, it isn't useful to specify the number of lines and columns, since the TextArea will expand to fill all the space available in the center area of the container.

The JTextArea class adds a few useful methods to those inherited from JTextComponent. For example, the instance method append(moreText), where moreText is of type String, adds the specified text at the end of the current content of the text area. (When using append() or setText() to add text to a JTextArea, line breaks can be inserted in the text by using the newline character, '\n'.) And setLineWrap(wrap), where wrap is of type boolean, tells what should happen when a line of text is too long to be displayed in the text area. If wrap is true, then any line that is too long will be "wrapped" onto the next line; if wrap is false, the line will simply extend outside the text area, and the user will have to scroll the text area horizontally to see the entire line. The default value of wrap is false.

Since it might be necessary to scroll a text area to see all the text that it contains, you might expect a text area to come with scroll bars. Unfortunately, this does not happen automatically. To get scroll bars for a text area, you have to put the JTextArea inside another component, called a JScrollPane. This can be done as follows:

JTextArea inputArea = new JTextArea();
JScrollPane scroller = new JScrollPane( inputArea );

The scroll pane provides scroll bars that can be used to scroll the text in the text area. The scroll bars will appear only when needed, that is when the size of the text exceeds the size of the text area. Note that when you want to put the text area into a container, you should add the scroll pane, not the text area itself, to the container.

Here is an applet that simply displays a text area, with a rather large font, in a scrollpane. You should check out how the scroll bars change as you type in additional lines of text:

When the user is typing in a JTextField and presses return, an ActionEvent is generated. If you want to respond to such events, you can register an ActionListener with the text field, using the text field's addActionListener() method. (Since a JTextArea can contain multiple lines of text, pressing return in a text area does not generate an event; is simply begins a new line of text.)

JTextField has a subclass, JPasswordField, which is identical except that it does not reveal the text that it contains. The characters in a JPasswordField are all displayed as asterisks (or some other fixed character). A password field is, obviously, designed to let the user enter a password without showing that password on the screen.

Text components are actually quite complex, and I have covered only their most basic properties here. I will return to the topic of text components in Chapter 12.

6.6.5  JComboBox

The JComboBox class provides a way to let the user select one option from a list of options. The options are presented as a kind of pop-up menu, and only the currently selected option is visible on the screen.

When a JComboBox object is first constructed, it initially contains no items. An item is added to the bottom of the menu by calling the combo box's instance method, addItem(str), where str is the string that will be displayed in the menu.

For example, the following code will create an object of type JComboBox that contains the options Red, Blue, Green, and Black:

JComboBox colorChoice = new JComboBox();
colorChoice.addItem("Red");
colorChoice.addItem("Blue");
colorChoice.addItem("Green");
colorChoice.addItem("Black");

You can call the getSelectedIndex() method of a JComboBox to find out which item is currently selected. This method returns an integer that gives the position of the selected item in the list, where the items are numbered starting from zero. Alternatively, you can call getSelectedItem() to get the selected item itself. (This method returns a value of type Object, since a JComboBox can actually hold other types of objects besides strings.) You can change the selection by calling the method setSelectedIndex(n), where n is an integer giving the position of the item that you want to select.

The most common way to use a JComboBox is to call its getSelectedIndex() method when you have a need to know which item is currently selected. However, like other components that we have seen, JComboBox components generate ActionEvents when the user selects an item. You can register an ActionListener with the JComboBox if you want to respond to such events as they occur.

JComboBoxes have a nifty feature, which is probably not all that useful in practice. You can make a JComboBox "editable" by calling its method setEditable(true). If you do this, the user can edit the selection by clicking on the JComboBox and typing. This allows the user to make a selection that is not in the pre-configured list that you provide. (The "Combo" in the name "JComboBox" refers to the fact that it's a kind of combination of menu and text-input box.) If the user has edited the selection in this way, then the getSelectedIndex() method will return the value -1, and getSelectedItem() will return the string that the user typed. An ActionEvent is triggered if the user presses return while typing in the JComboBox.

6.6.6  JSlider

A JSlider provides a way for the user to select an integer value from a range of possible values. The user does this by dragging a "knob" along a bar. A slider can, optionally, be decorated with tick marks and with labels. This demonstration applet shows three sliders with different decorations and with different ranges of values:

Here, the second slider is decorated with ticks, and the third one is decorated with labels. It's possible for a single slider to have both types of decorations.

The most commonly used constructor for JSliders specifies the start and end of the range of values for the slider and its initial value when it first appears on the screen:

public JSlider(int minimum, int maximum, int value)

If the parameters are omitted, the values 0, 100, and 50 are used. By default, a slider is horizontal, but you can make it vertical by calling its method setOrientation(JSlider.VERTICAL). The current value of a JSlider can be read at any time with its getValue() method, which returns a value of type int. If you want to change the value, you can do so with the method setValue(n), which takes a parameter of type int.

If you want to respond immediately when the user changes the value of a slider, you can register a listener with the slider. JSliders, unlike other components we have seen, do not generate ActionEvents. Instead, they generate events of type ChangeEvent. ChangeEvent and related classes are defined in the package javax.swing.event rather than java.awt.event, so if you want to use ChangeEvents, you should import javax.swing.event.* at the beginning of your program. You must also define some object to implement the ChangeListener interface, and you must register the change listener with the slider by calling its addChangeListener() method. A ChangeListener must provide a definition for the method:

public void stateChanged(ChangeEvent evt)

This method will be called whenever the value of the slider changes. (Note that it will also be called when you change the value with the setValue() method, as well as when the user changes the value.) In the stateChanged() method, you can call evt.getSource() to find out which object generated the event.

Using tick marks on a slider is a two-step process: Specify the interval between the tick marks, and tell the slider that the tick marks should be displayed. There are actually two types of tick marks, "major" tick marks and "minor" tick marks. You can have one or the other or both. Major tick marks are a bit longer than minor tick marks. The method setMinorTickSpacing(i) indicates that there should be a minor tick mark every i units along the slider. The parameter is an integer. (The spacing is in terms of values on the slider, not pixels.) For the major tick marks, there is a similar command, setMajorTickSpacing(i). Calling these methods is not enough to make the tick marks appear. You also have to call setPaintTicks(true). For example, the second slider in the above applet was created and configured using the commands:

slider2 = new JSlider();  // (Uses default min, max, and value.)
slider2.addChangeListener(this);
slider2.setMajorTickSpacing(25);
slider2.setMinorTickSpacing(5);
slider2.setPaintTicks(true);

Labels on a slider are handled similarly. You have to specify the labels and tell the slider to paint them. Specifying labels is a tricky business, but the JSlider class has a method to simplify it. You can create a set of labels and add them to a slider named sldr with the command:

sldr.setLabelTable( sldr.createStandardLabels(i) );

where i is an integer giving the spacing between the labels. To arrange for the labels to be displayed, call setPaintLabels(true). For example, the third slider in the above applet was created and configured with the commands:

slider3 = new JSlider(2000,2100,2006);
slider3.addChangeListener(this);
slider3.setLabelTable( slider3.createStandardLabels(50) );
slider3.setPaintLabels(true);
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