How to Use Labels - Java Tutorial 5.0 英文版

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How to Use Labels

With the JLabel class, you can display unselectable text and images. If you need to create a component that displays a string or an image (or both), you can do so by using or extending JLabel. If the component is interactive and has state, consider using a button instead of a label.

By specifying HTML codes in a label's text, you can make the label have multiple lines, multiple fonts, multiple colors, and so on. If the label uses just a single color or font, you can avoid the overhead of HTML processing by using the setForeground or setFont method instead. See Using HTML in Swing Components for details.

Here's a picture of an application that displays three labels. The window is divided into three rows of equal height; the label in each row is as wide as possible.

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Try this: 

1. Run LabelDemo using Java^TM Web Start. Or, to compile and run the example yourself, consult the example index.
2. Resize the window so you can see how the labels' contents are placed within the labels' drawing area.
   All the label contents have the default vertical alignment — the label contents are centered vertically in the label's drawing area. The top label, which contains both image and text, is specified to have horizontal center alignment. The second label, which contains just text, has the left (leading) alignment that is the default for text-only labels in left-to-right languages. The third label, which contains just an image, has horizontal center alignment, which is the default for image-only labels.

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Below is the code from LabelDemo.java that creates the labels in the previous example.

ImageIcon icon = createImageIcon("images/middle.gif");
. . .
label1 = new JLabel("Image and Text",
                    icon,
                    JLabel.CENTER);
//Set the position of the text, relative to the icon:
label1.setVerticalTextPosition(JLabel.BOTTOM);
label1.setHorizontalTextPosition(JLabel.CENTER);

label2 = new JLabel("Text-Only Label");
label3 = new JLabel(icon);

The code for the createImageIcon method is similar to that used throughout this tutorial. You can find it explained in How to Use Icons.

Often, a label describes another component. When this is true, you can improve your program's accessibility by using the setLabelFor method to identify the component the label describes. For example:

amountLabel.setLabelFor(amountField);

The preceding code, taken from the FormattedTextFieldDemo example discussed in How to Use Formatted Text Fields, lets assistive technologies know that the label (amountLabel) provides information about the formatted text field (amountField). For more information about assistive technologies, see How to Support Assistive Technologies.

The Label API

The following tables list the commonly used JLabel constructors and methods. Other methods you're likely to call are defined by the Component and JComponent classes. They include setFont, setForeground, setBorder, setOpaque, and setBackground. See The JComponent Class for details. The API for using labels falls into three categories:

• Setting or Getting the Label's Contents
• Fine Tuning the Label's Appearance
• Supporting Accessibility

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Note: In the following API, don't confuse label alignment with X and Y alignment. X and Y alignment are used by layout managers and can affect the way any component — not just a label — is sized or positioned. Label alignment, on the other hand, has no effect on a label's size or position. It simply determines where, inside the label's painting area, the label's contents are positioned. In the usual case, the label's painting area is exactly the size needed to paint the label, and thus label alignment is irrelevant. For more information about X and Y alignment, see How to Use BoxLayout.

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Setting or Getting the Label's Contents

Method or Constructor	Purpose
JLabel(Icon) JLabel(Icon, int) JLabel(String) JLabel(String, Icon, int) JLabel(String, int) JLabel()	Create a JLabel instance, initializing it to have the specified text/image/alignment. The int argument specifies the horizontal alignment of the label's contents within its drawing area. The horizontal alignment must be one of the following constants defined in the SwingConstants interface (which JLabel implements): LEFT, CENTER, RIGHT, LEADING, or TRAILING. For ease of localization, we strongly encourage you to use LEADING and TRAILING, rather than LEFT and RIGHT.
void setText(String) String getText()	Set or get the text displayed by the label. You can use HTML tags to format the text, as described in Using HTML in Swing Components
void setIcon(Icon) Icon getIcon()	Set or get the image displayed by the label.
void setDisplayedMnemonic(char) char getDisplayedMnemonic()	Set or get the letter that should look like a keyboard alternative. This is handy when a label describes a component (such as a text field) that has a keyboard alternative but can't display it. If the labelFor property is also set (using setLabelFor), then when the user activates the mnemonic, the keyboard focus is transferred to the component specified by the labelFor property.
void setDisplayedMnemonicIndex(int) int getDisplayedMnemonicIndex()	Set or get a hint as to which character in the text should be decorated to represent the mnemonic. This is handy when you have two instances of the same character and wish to decorate the second instance. For example, setDisplayedMnemonicIndex(5) decorates the character that's at position 5 (that is, the 6th character in the text). Not all look and feels may support this feature. Introduced in 1.4.
void setDisabledIcon(Icon) Icon getDisabledIcon()	Set or get the image displayed by the label when it's disabled. If you don't specify a disabled image, then the look-and-feel creates one by manipulating the default image.

Fine Tuning the Label's Appearance

Method	Purpose
void setHorizontalAlignment(int) void setVerticalAlignment(int) int getHorizontalAlignment() int getVerticalAlignment()	Set or get where in the label its contents should be placed. The SwingConstants interface defines five possible values for horizontal alignment: LEFT, CENTER (the default for image-only labels), RIGHT, LEADING (the default for text-only labels), TRAILING. For vertical alignment: TOP, CENTER (the default), and BOTTOM.
void setHorizontalTextPosition(int) void setVerticalTextPosition(int) int getHorizontalTextPosition() int getVerticalTextPosition()	Set or get where the button's text should be placed, relative to the button's image. The SwingConstants interface defines five possible values for horizontal position: LEADING, LEFT, CENTER, RIGHT, and TRAILING (the default). For vertical position: TOP, CENTER (the default), and BOTTOM.
void setIconTextGap(int) int getIconTextGap()	Set or get the number of pixels between the label's text and its image.

Supporting Accessibility

Method	Purpose
void setLabelFor(Component) Component getLabelFor()	Set or get which component the label describes.

Examples that Use Labels

The following table lists some of the many examples that use labels.

Example	Where Described	Notes
LabelDemo	This section	Shows how to specify horizontal and vertical alignment, as well as aligning a label's text and image.
HtmlDemo	Using HTML in Swing Components	Lets you experiment with specifying HTML text for a label.
BoxAlignmentDemo	Fixing Alignment Problems	Demonstrates a possible alignment problem when using a label in a vertical box layout. Shows how to solve the problem.
DialogDemo	How to Use Dialogs	Uses a changeable label to display instructions and provide feedback.
SplitPaneDemo	How to Use Split Panes and How to Use Lists.	Displays an image using a label inside of a scroll pane.
SliderDemo2	How to Use Sliders	Uses JLabel to provide labels for a slider.
TableDialogEditDemo	How to Use Tables	Implements a label subclass, ColorRenderer, to display colors in table cells.
FormattedTextFieldDemo	How to Use Formatted Text Fields	Has four rows, each containing a label and the formatted text field it describes.
TextComponentDemo	Text Component Features	TextComponentDemo has an inner class (CaretListenerLabel) that extends JLabel to provide a label that listens for events, updating itself based on the events.
ColorChooserDemo	How to Use Color Choosers	Uses an opaque label to display the currently chosen color against a fixed-color background.

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