Windows: Additional Notes

Windows Additional Notes

In this section, we discuss several important JFrame issues. A JFrame is a window with a title bar and a border. Class JFrame is a subclass of java.awt.Frame (which is a subclass of java.awt.Window). As such, JFrame is one of the few Swing GUI components that is not a lightweight GUI component. When you display a window from a Java program, the window is provided by the local platform's windowing toolkit, and therefore the window will look like every other window displayed on that platform. When a Java application executes on a Macintosh and displays a window, the window's title bar and borders will look like those of other Macintosh applications. When a Java application executes on a Microsoft Windows system and displays a window, the window's title bar and borders will look like those of other Microsoft Windows applications. And when a Java application executes on a Unix platform and displays a window, the window's title bar and borders will look like other Unix applications on that platform.

Class JFrame supports three operations when the user closes the window. By default, a window is hidden (i.e., removed from the screen) when the user closes it. This can be controlled with JFrame method setDefaultCloseOperation. Interface WindowConstants (package javax.swing), which class JFrame implements, declares three constants for use with this methodDISPOSE_ON_CLOSE, DO_NOTHING_ON_CLOSE and HIDE_ON_CLOSE (the default). Some platforms allow only a limited number of windows to be displayed on the screen. Thus, a window is a valuable resource that should be given back to the system when it is no longer needed. Class Window (an indirect superclass of JFrame) declares method dispose for this purpose. When a Window is no longer needed in an application, you should explicitly dispose of it. This can be done by calling the Window's dispose method or by calling method setDefaultCloseOperation with the argument WindowConstants.DISPOSE_ON_CLOSE. Terminating an application will return window resources to the system. Setting the default close operation to DO_NOTHING_ON_CLOSE indicates that the program will determine what to do when the user indicates that the window should be closed.

Good Programming Practice 22.1

Windows are an expensive system resource. Return them to the system when they are no longer needed.

By default, a window is not displayed on the screen until the program invokes the window's setVisible method (inherited from class java.awt.Component) with a TRue argument. A window's size should be set with a call to method setSize (inherited from class java.awt.Component). The position of a window when it appears on the screen is specified with method setLocation (inherited from class java.awt.Component).

Common Programming Error 22.1

Forgetting to call method setVisible on a window is a runtime logic errorthe window is not displayed.

Common Programming Error 22.2

Forgetting to call the setSize method on a window is a runtime logic erroronly the title bar appears.

When the user manipulates the window, this action generates window events. Event listeners are registered for window events with Window method addWindowListener. Interface WindowListener provides seven window-event-handling methodswindow-Activated (called when the user makes a window the active window), windowClosed (called after the window is closed), windowClosing (called when the user initiates closing of the window), windowDeactivated (called when the user makes another window the active window), windowDeiconified (called when the user restores a window from being minimized), windowIconified (called when the user minimizes a window) and windowOpened (called when a program first displays a window on the screen).