GUI Components: Part 1 - Java: How to Program Early Objects - page 485

The vertical-position constants in SwingConstants are TOP , CENTER and BOTTOM (Fig. 12.8).
Line 39 (Fig. 12.6) sets the tool-tip text for label3 . Line 40 adds label3 to the JFrame .

Constant Description Constant Description

Horizontal-position constants Vertical-position constants

LEFT

Place text on the left

TOP

Place text at the top

CENTER

Place text in the center

CENTER

Place text in the center

RIGHT

Place text on the right

BOTTOM

Place text at the bottom

Fig. 12.8 | Positioning constants ( static members of interface SwingConstants ).

Creating and Displaying a LabelFrame Window
Class LabelTest (Fig. 12.7) 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 ap-
plication to terminate . Line 10 invokes LabelFrame 's setDefaultCloseOperation meth-
od (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 in pixels . 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.

12.6 Text Fields and an Introduction to Event Handling
with Nested Classes
Normally, a user interacts with an application's GUI to indicate the tasks that the appli-
cation should perform. For example, when you write an e-mail in an e-mail application,
clicking the Send button tells the application to send the e-mail to the specified e-mail ad-
dresses. GUIs are event driven . When the user interacts with a GUI component, the in-
teraction—known as an event —drives the program to perform a task. Some common user
interactions that cause an application to perform a task include clicking a button, typing in
a text field, selecting an item from a menu, closing a window and moving the mouse. The
code that performs a task in response to an event is called an event handler , and the process
of responding to events is known as event handling .
Let's consider two other GUI components that can generate events— JTextField s
and JPasswordField s (package javax.swing ). Class JTextField extends class JTextCom-
ponent (package javax.swing.text ), which provides many features common to Swing's
text-based components. Class JPasswordField extends JTextField and adds methods
that are specific to processing passwords. Each of these components is a single-line area in
which the user can enter text via the keyboard. Applications can also display text in a
JTextField (see the output of Fig. 12.10). A JPasswordField shows that characters are
being typed as the user enters them, but hides the actual characters with an echo character ,
assuming that they represent a password that should remain known only to the user.