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5.2 Labels

Having covered the features of the Component class, we can now look at some of the simplest components. The first component introduced here is a Label. A label is a Component that displays a single line of static text.[3] It is useful for putting a title or message next to another component. The text can be centered or justified to the left or right. Labels react to all events they receive. However, they do not get any events from their peers.

[3] Java in A Nutshell (from O'Reilly & Associates) includes a multiline Label component.

Label Methods

Constants

There are three alignment specifiers for labels. The alignment tells the Label where to position its text within the space allotted. Setting an alignment for a Label might not do anything noticeable if the LayoutManager being used does not resize the Label to give it more space. With FlowLayout, the alignment is barely noticeable. See Chapter 7, Layouts, for more information.

public final static int LEFT

LEFT is the constant for left alignment. If no alignment is specified in the constructor, left alignment is the default.

public final static int CENTER

CENTER is the constant for center alignment.

public final static int RIGHT

RIGHT is the constant for right alignment.

Constructors

public Label ()

This constructor creates an empty Label. By default, the label's text is left justified.

public Label (String label)

This constructor creates a Label whose initial text is label. By default, the label's text is left justified.

public Label (String label, int alignment)

This constructor creates a Label whose initial text is label. The alignment of the label is alignment. If alignment is invalid (not LEFT, RIGHT, or CENTER), the constructor throws the run-time exception IllegalArgumentException.

Text

public String getText ()

The getText() method returns the current value of Label.

public void setText (String label)

The setText() method changes the text of the Label to label. If the new label is a different size from the old one, you should revalidate the display to ensure the label's entire contents will be seen.

Alignment

public int getAlignment ()

The getAlignment() method returns the current alignment of the Label.

public void setAlignment (int alignment)

The setAlignment() method changes the alignment of the Label to alignment. If alignment is invalid (not LEFT, RIGHT, or CENTER), setAlignment() throws the run-time exception IllegalArgumentException. Figure 5.2 shows all three alignments.

Miscellaneous methods

public synchronized void addNotify ()

The addNotify() method creates the Label peer. If you override this method, first call super.addNotify(), then put in your customizations. Then you will be able to do everything you need with the information about the newly created peer.

protected String paramString ()

The paramString() method overrides Component's paramString() method. It is a protected method that calls the overridden paramString() to build a String from the different parameters of the Component. When the method paramString() is called for a Label, the alignment and label's text are added. Thus, for the Label created by the constructor new Label (`ZapfDingbats`, Label.RIGHT), the results displayed from a call to toString() would be:

java.awt.Label[0,0,0x0,invalid,align=right,label=ZapfDingbats]

Label Events

The Label component can react to any event it receives, though the Label peer normally does not send any. However, there is nothing to stop you from posting an event yourself.


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