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How to Make Frames (Main Windows) - Java Tutorial 5.0 英文版

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Trail: Creating a GUI with JFC/Swing
Lesson: Using Swing Components

How to Make Frames (Main Windows)

A frame, implemented as an instance of the JFrame class, is a window that typically has decorations such as a border, a title, and buttons for closing and iconifying the window. Applications with a GUI typically use at least one frame. Applets sometimes use frames, as well.

To make a window that's dependent on another window — disappearing when the other window is iconified, for example — use a dialog instead of a frame. To make a window that appears within another window, use an internal frame.

Creating and Showing Frames

Here's a picture of the extremely plain window created by an example called FrameDemo. You can find the source code in FrameDemo.java. You can run FrameDemo using Java Web Start.

The following code, taken from FrameDemo, is a typical example of the code used to create and set up a frame.

//1. Optional: Specify who draws the window decorations. 
JFrame.setDefaultLookAndFeelDecorated(true);

//2. Create the frame.
JFrame frame = new JFrame("FrameDemo");

//3. Optional: What happens when the frame closes?
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

//4. Create components and put them in the frame.
//...create emptyLabel...
frame.getContentPane().add(emptyLabel, BorderLayout.CENTER);

//5. Size the frame.
frame.pack();

//6. Show it.
frame.setVisible(true);

Here are some details about the code:

1. Calling setDefaultLookAndFeelDecorated(true) requests that any subsequently created frames have window decorations provided by the look and feel, and not by the window system. For details, see Specifying Window Decorations.

2. The next line of code creates a frame using a constructor that lets you set the frame's title. The other frequently used JFrame constructor is the no-argument constructor.

3. Next the code specifies what should happen when the user closes the frame. The EXIT_ON_CLOSE operation, not surprisingly, makes the program exit when the user closes the frame. This behavior is appropriate for this program because the program has only one frame, and closing the frame makes the program useless. See Responding to Window-Closing Events for more information.

4. The next bit of code adds a blank label to the frame's content pane. If you're not already familiar with content panes and how to add components to them, please read Adding Components to the Content Pane.

   For frames that have menus, you'd typically add the menu bar to the frame here using the setJMenuBar method. See How to Use Menus for details.

5. The pack method sizes the frame so that all its contents are at or above their preferred sizes. An alternative to pack is to establish a frame's size explicitly by calling setSize or setBounds (which also sets the frame's location). In general, using pack is preferable to calling setSize, since pack leaves the frame's layout manager in charge of the frame's size, and layout managers are good at adjusting to platform dependencies and other factors that affect component size.

   This example doesn't set the frame's location, but it's easy to do so using either the setLocationRelativeTo or setLocation method. For example, the following code centers a frame onscreen:

   frame.setLocationRelativeTo(null);
   

6. Calling setVisible(true) makes the frame appear onscreen. Sometimes you might see the show method used instead. The two usages are equivalent, but we use setVisible(true) for consistency's sake.

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Note:  The pack and setVisible methods realize the frame and the components it contains. Once a component has been realized, any code that modifies or inspects it should be executed on the event-dispatching thread. For more information, refer to How to Use Threads.

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Specifying Window Decorations

By default, window decorations are supplied by the native window system. However, you can request that the look and feel provide the decorations for a frame. You can even specify that the frame have no window decorations at all, a feature that can be used on its own, or to provide your own decorations, or with full-screen exclusive mode.

Besides specifying who provides the window decorations, you can also specify which icon is used the represent the window. Exactly how this icon is used depends on the window system or look and feel that provides the window decorations. If the window system supports minimization, then the icon is used to represent the minimized window. Most window systems or look and feels also display the icon in the window decorations. A typical icon size is 16x16 pixels, but some window systems use other sizes.

The following snapshots show three frames that are identical except for their window decorations. As you can tell by the appearance of the button in each frame, all three use the Java look and feel. However, only the first and third frames use window decorations provided by the Java look and feel. The second uses decorations provided by the window system, which happens to be Microsoft Windows but could as easily be any other system running the 1.4 version of the Java platform. The third frame uses Java look and feel window decorations, but has a custom icon.

Window decorations provided by the look and feel	Window decorations provided by the window system	Custom icon; window decorations provided by the look and feel

Here is an example of creating a frame with a custom icon and with window decorations provided by the look and feel:

//Ask for window decorations provided by the look and feel.
JFrame.setDefaultLookAndFeelDecorated(true);

//Create the frame.
JFrame frame = new JFrame("A window");

//Set the frame's icon to an image loaded from a file.
frame.setIconImage(new ImageIcon(imgURL).getImage());

As the preceding code snippet implies, you must invoke the setDefaultLookAndFeelDecorated method before creating the frame whose decorations you wish to affect. The value you set with setDefaultLookAndFeelDecorated is used for all subsequently created JFrames. You can switch back to using window system decorations by invoking JFrame.setDefaultLookAndFeelDecorated(false). Some look and feels might not support window decorations; in this case, the window system decorations are used.

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Version note:  The setDefaultLookAndFeelDecorated method was added in 1.4. Previously, the decorations on a frame were always provided by the window system.

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The full source code for the application that creates the frames pictured above is in FrameDemo2.java. Besides showing how to choose window decorations, FrameDemo2 also shows how to disable all window decorations and gives an example of positioning windows. It includes two methods that create the Image objects used as icons — one is loaded from a file, and the other is painted from scratch.

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Try this: 

1. Run FrameDemo2 using Java^TM Web Start. Or, to compile and run the example yourself, consult the example index.
2. Bring up two windows, both with look-and-feel-provided decorations, but with different icons.
   The Java look and feel displays the icons in its window decorations. Depending on your window system, the icon may be used elsewhere to represent the window, especially when the window is minimized.
3. Bring up one or more windows with window system decorations.
   See if your window system treats these icons differently.
4. Bring up one or more windows with no window decorations.
   Play with the various types of windows to see how the window decorations, window system, and frame icons interact.

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Responding to Window-Closing Events

By default, when the user closes a frame onscreen, the frame is hidden. Although invisible, the frame still exists and the program can make it visible again. If you want different behavior, then you need to either register a window listener that handles window-closing events, or you need to specify default close behavior using the setDefaultCloseOperation method. You can even do both.

The argument to setDefaultCloseOperation must be one of the following values, the first three of which are defined in the WindowConstants interface (implemented by JFrame, JInternalPane, and JDialog):

DO_NOTHING_ON_CLOSE

Don't do anything when the user requests that the window close. Instead, the program should probably use a window listener that performs some other action in its windowClosing method.

HIDE_ON_CLOSE (the default for JDialog and JFrame)

Hide the window when the user closes it. This removes the window from the screen but leaves it displayable.

DISPOSE_ON_CLOSE (the default for JInternalFrame)

Hide and dispose of the window when the user closes it. This removes the window from the screen and frees up any resources used by it.

EXIT_ON_CLOSE (defined in the JFrame class)

Exit the application, using System.exit(0). This is recommended for applications only. If used within an applet, a SecurityException may be thrown. Introduced in 1.3.

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Version note:  As of 1.4, DISPOSE_ON_CLOSE can have results similar to EXIT_ON_CLOSE if only one window is onscreen. More precisely, when the last displayable window within the Java virtual machine (VM) is disposed of, the VM may terminate if it is 1.4 or a compatible version. In earlier versions such as 1.3 the VM remains running even when all windows have been disposed of. See AWT Threading Issues for details.

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The default close operation is executed after the any window listeners handle the window-closing event. So, for example, assume that you specify that the default close operation is to dispose of a frame. You also implement a window listener that tests whether the frame is the last one visible and, if so, saves some data and exits the application. Under these conditions, when the user closes a frame, the window listener will be called first. If it doesn't exit the application, then the default close operation — disposing of the frame — will then be performed.

For more information about handling window-closing events, see How to Write Window Listeners. Besides handling window-closing events, window listeners can also react to other window state changes, such as iconification and activation.

The Frame API

The following tables list the commonly used JFrame constructors and methods. Other methods you might want to call are defined by the java.awt.Frame, java.awt.Window, and java.awt.Component classes, from which JFrame descends.

Because each JFrame object has a root pane, frames have support for interposing input and painting behavior in front of the frame's children, placing children on different "layers", and for Swing menu bars. These topics are introduced in Using Top-Level Containers and explained in detail in How to Use Root Panes.

The API for using frames falls into these categories:

• Creating and Setting Up a Frame
• Setting the Window Size and Location
• Methods Related to the Root Pane

Creating and Setting Up a Frame

Method or Constructor	Purpose
JFrame() JFrame(String)	Create a frame that is initially invisible. The String argument provides a title for the frame. To make the frame visible, invoke setVisible(true) on it.
void setDefaultCloseOperation(int) int getDefaultCloseOperation()	Set or get the operation that occurs when the user pushes the close button on this frame. Possible choices are: DO_NOTHING_ON_CLOSE HIDE_ON_CLOSE DISPOSE_ON_CLOSE EXIT_ON_CLOSE The first three constants are defined in the WindowConstants interface, which JFrame implements. The EXIT_ON_CLOSE constant is defined in the JFrame class, and was introduced in 1.3.
void setIconImage(Image) Image getIconImage() (in Frame)	Set or get the icon that represents the frame. Note that the argument is a java.awt.Image object, not a javax.swing.ImageIcon (or any other javax.swing.Icon implementation).
void setTitle(String) String getTitle() (in Frame)	Set or get the frame's title.
void setUndecorated(boolean) boolean isUndecorated() (in Frame)	Set or get whether the window system should provide decorations for this frame. Works only if the frame is not yet displayable (hasn't been packed or shown). Typically used with full-screen exclusive mode or to enable custom window decorations. Introduced in 1.4.
static void setDefaultLookAndFeelDecorated(boolean) static boolean isDefaultLookAndFeelDecorated()	Determine whether subsequently created JFrames should have their Window decorations (such as borders, widgets for closing the window, title) provided by the current look and feel. Note that this is only a hint, as some look and feels may not support this feature. Introduced in 1.4.

Setting the Window Size and Location

Method	Purpose
void pack() (in Window)	Size the window so that all its contents are at or above their preferred sizes.
void setSize(int, int) void setSize(Dimension) Dimension getSize() (in Component)	Set or get the total size of the window. The integer arguments to setSize specify the width and height, respectively.
void setBounds(int, int, int, int) void setBounds(Rectangle) Rectangle getBounds() (in Component)	Set or get the size and position of the window. For the integer version of setBounds, the window's upper left corner is at the x, y location specified by the first two arguments, and has the width and height specified by the last two arguments.
void setLocation(int, int) Point getLocation() (in Component)	Set or get the location of the upper left corner of the window. The parameters are the x and y values, respectively.
void setLocationRelativeTo(Component) (in Window)	Position the window so that it's centered over the specified component. If the argument is null, the window is centered onscreen. To properly center the window, you should invoke this method after the window's size has been set.

Methods Related to the Root Pane

Method	Purpose
void setContentPane(Container) Container getContentPane()	Set or get the frame's content pane. The content pane contains the frame's visible GUI components and should be opaque.
JRootPane createRootPane() void setRootPane(JRootPane) JRootPane getRootPane()	Create, set, or get the frame's root pane. The root pane manages the interior of the frame including the content pane, the glass pane, and so on.
void setJMenuBar(JMenuBar) JMenuBar getJMenuBar()	Set or get the frame's menu bar to manage a set of menus for the frame.
void setGlassPane(Component) Component getGlassPane()	Set or get the frame's glass pane. You can use the glass pane to intercept mouse events or paint on top of your program's GUI.
void setLayeredPane(JLayeredPane) JLayeredPane getLayeredPane()	Set or get the frame's layered pane. You can use the frame's layered pane to put components on top of or behind other components.

Examples that Use Frames

All of the standalone applications in this trail use JFrame. The following table lists a few and tells you where each is discussed.

Example	Where Described	Notes
FrameDemo	The Example Explained	Displays a basic frame with one component.
FrameDemo2	Specifying Window Decorations	Lets you create frames with various window decorations.
Framework	—	A study in creating and destroying windows, in implementing a menu bar, and in exiting an application.
LayeredPaneDemo	How to Use Layered Panes	Illustrates how to use a layered pane (but not the frame's layered pane).
GlassPaneDemo	The Glass Pane	Illustrates the use of a frame's glass pane.
MenuDemo	How to Use Menus	Shows how to put a JMenuBar in a JFrame.

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