Displaying Text and Images in a Window

Our next example introduces a framework for building GUI applications. This framework uses several concepts that you will see in many of our GUI applications. This is our first example in which the application appears in its own window. Most windows you will create are an instance of class JFrame or a subclass of JFrame. JFrame provides the basic attributes and behaviors of a windowa title bar at the top of the window, and buttons to minimize, maximize and close the window. Since an application's GUI is typically specific to the application, most of our examples will consist of two classesa subclass of JFrame that helps us demonstrate new GUI concepts and an application class in which main creates and displays the application's primary window.

Labeling GUI Components

A typical GUI consists of many components. In a large GUI, it can be difficult to identify the purpose of every component unless the GUI designer provides text instructions or information stating the purpose of each component. Such text is known as a label and is created with class JLabela subclass of JComponent. A JLabel displays a single line of read-only text, an image, or both text and an image. Applications rarely change a label's contents after creating it.

Look-and-Feel Observation 11.5

The application of Fig. 11.6 and Fig. 11.7 demonstrates several JLabel features and presents the framework we use in most of our GUI examples. We did not highlight the code in this example since most of it is new. [Note: There are many more features for each GUI component than we can cover in our examples. To learn the complete details of each GUI component, visit its page in the online documentation. For class JLabel, visit java.sun.com/j2se/5.0/docs/api/javax/swing/JLabel.html.]

Figure 11.6. JLabels with text and icons.

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Figure 11.7. Test class for LabelFrame.

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Class LabelFrame (Fig. 11.6) is a subclass of JFrame. We will use an instance of class LabelFrame to display a window containing three JLabels. Lines 38 import the classes used in class LabelFrame. The class extends JFrame to inherit the features of a window. Lines 1214 declare the three JLabel instance variables, each of which is instantiated in the LabelFrame constructor (lines 1741). Typically, the JFrame subclass's constructor builds the GUI that is displayed in the window when the application executes. Line 19 invokes superclass JFrame's constructor with the argument "Testing JLabel".JFrame's constructor uses this String to specify the text in the window's title bar.

Specifying the Layout

When building a GUI, each GUI component must be attached to a container, such as a window created with a JFrame. Also, you typically must decide where to position each GUI component. This is known as specifying the layout of the GUI components. As you will learn at the end of this chapter and in Chapter 22, Java provides several layout managers that can help you position components.

Many integrated development environments provide GUI design tools in which you can specify the exact size and location of a component in a visual manner by using the mouse, then the IDE will generate the GUI code for you. Though such IDEs can greatly simplify GUI creation, they are each different in capability.

To ensure that the code in this book can be used with any IDE, we did not use an IDE to create our GUI code. For this reason, we take advantage of Java's various layout managers in our GUI examples. One such layout manager is FlowLayout in which GUI components are placed on a container from left to right in the order in which they are attached to the container by the program. When there is no more room to fit components left to right, components continue to display left to right on the next line. If the container is resized, a FlowLayout reflows (i.e., rearranges) the components to accommodate the new width of the container, possibly with fewer or more rows of GUI components. Line 20 specifies that the layout of the LabelFrame should be a FlowLayout. Method setLayout is inherited into class LabelFrame indirectly from class Container. The argument to the method must be an object of a class that implements the LayoutManager interface, such as an object of class FlowLayout. In this case, we create a new FlowLayout object and pass its reference as the argument to setLayout.

Creating and Attaching label1

Now that we have specified the layout for the window, we can begin creating and attaching GUI components to the window. Line 23 creates a JLabel object and passes the String "Label with text" to the constructor. The JLabel displays this text when the JLabel appears on the screen as part of the application's GUI. Line 24 uses method setToolTipText (inherited by JLabel from JComponent) to specify the tool tip that is displayed when the user positions the mouse cursor over the JLabel in the GUI. You can see a sample tool tip in the second screen capture of Fig. 11.7. When you execute this application, try positioning the mouse over each JLabel to see its tool tip. Line 25 attaches label1 to the LabelFrame by passing label1 to the add method, which is inherited indirectly from class Container.

Common Programming Error 11.1

Look-and-Feel Observation 11.6

Creating and Attaching label2

Icons are a popular way to enhance the look-and-feel of an application and are also commonly used to indicate functionality. For examples, most of today's VCRs and DVD players use the same icon to play a tape or DVD. Several Swing components can display images. An icon is normally specified with an Icon argument to a constructor or to the component's setIcon method. An Icon is an object of any class that implements interface Icon (package javax.swing). One such class is ImageIcon (package javax.swing), which supports several image formats, including Graphics Interchange Format (GIF), Portable Network Graphics (PNG) and Joint Photographic Experts Group (JPEG). File names for each of these types end with .gif, .png or .jpg (or .jpeg), respectively. We discuss images in more detail in Chapter 21, Multimedia: Applets and Applications.

Line 28 declares an ImageIcon object. The file bug1.gif contains the image to load and store in the ImageIcon object. (This image is included in the directory for this example on the CD that accompanies this book) The ImageIcon object is assigned to Icon reference bug. Remember, class ImageIcon implements interface Icon; an ImageIcon is an Icon.

In line 28, the expression getClass().getResource( "bug1.gif" ) invokes method getClass (inherited from class Object) to retrieve a reference to the Class object that represents the LabelFrame class declaration. That reference is then used to invoke Class method getResource, which returns the location of the image as a URL. The ImageIcon constructor uses the URL to locate the image then loads it into memory. As we discussed in Chapter 1, the JVM loads class declarations into memory, using a class loader. The class loader knows where each class it loads is located on disk. Method geTResource uses the Class object's class loader to determine the location of a resource, such as an image file. In this example, the image file is stored in the same location as the LabelFrame.class file. The techniques described here enable an application to load image files from locations that are relative to LabelFrame's.class file on disk.

A JLabel can display an Icon. Lines 2930 use another JLabel constructor to create a JLabel that displays the text "Label with text and icon" and the Icon bug created in line 28. The last constructor argument indicates that the label's contents are left justified, or left aligned (i.e., the icon and text are at the left side of the label's area on the screen). Interface SwingConstants (package javax.swing) declares a set of common integer constants (such as SwingConstants.LEFT) that are used with many Swing components. By default, the text appears to the right of the image when a label contains both text and an image. Note that the horizontal and vertical alignments of a JLabel can be set with methods setHorizontalAlignment and setVerticalAlignment, respectively. Line 31 specifies the tool-tip text for label2, and line 32 adds label2 to the JFrame.

Creating and Attaching label3

Class JLabel provides many methods to change a label's appearance after the label has been instantiated. Line 34 creates a JLabel and invokes its no-argument constructor. Such a label initially has no text or Icon. Line 35 uses JLabel method setText to set the text displayed on the label. The corresponding method getText retrieves the current text displayed on a label. Line 36 uses JLabel method setIcon to specify the Icon to display on the label. The corresponding method getIcon retrieves the current Icon displayed on a label. Lines 3738 use JLabel methods setHorizontalTextPosition and setVerticalTextPosition to specify the text position in the label. In this case, the text will be centered horizontally and will appear at the bottom of the label. Thus, the Icon will appear above the text. The horizontal-position constants in SwingConstants are LEFT, CENTER and RIGHT (Fig. 11.8). The vertical-position constants in SwingConstants are TOP, CENTER and BOTTOM (Fig. 11.8). Line 39 sets the tool-tip text for label3. Line 40 adds label3 to the JFrame.

Creating and Displaying a LabelFrame Window

Class LabelTest (Fig. 11.7) creates an object of class LabelFrame (line 9), then specifies the default close operation for the window. By default, closing a window simply hides the window. However, when the user closes the LabelFrame window, we would like the application to terminate. Line 10 invokes LabelFrame's setDefaultCloseOperation method (inherited from class JFrame) with constant JFrame.EXIT_ON_CLOSE as the argument to indicate that the program should terminate when the window is closed by the user. This line is important. Without it the application will not terminate when the user closes the window. Next, line 11 invokes LabelFrame's setSize method to specify the width and height of the window. Finally, line 12 invokes LabelFrame's setVisible method with the argument true to display the window on the screen. Try resizing the window to see how the FlowLayout changes the JLabel positions as the window width changes.