The JFC Swing Tutorial: A Guide to Constructing GUIs (2nd Edition)

2017-07-07 02:10:07

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Caret events occur when the caret ”the cursor indicating the insertion point ”in a text component moves or when the selection in a text component changes (see Figure 1). The text component's document can initiate caret events when it inserts or removes text, for example.

Figure 1. You can see a caret in use in `TextComponentDemo` ”it's the vertical line at the end of the fourth line.

You can attach a caret listener to an instance of any JTextComponent subclass with the addCaretListener method.

Note: An alternate way of detecting caret changes is to attach a listener directly to the caret object itself rather than to the text component that manages the caret. A caret fires change events (not caret events), so you would need to write a change listener rather than a caret listener.

Here's the caret event-handling code from an application called TextComponentDemo .

//where initialization occurs CaretListenerLabel caretListenerLabel = new CaretListenerLabel("Caret Status"); ... textPane.addActionListener(caretListenerLabel); ... protected class CaretListenerLabel extends JLabel implements CaretListener { ... public void caretUpdate(CaretEvent e) { //Get the location in the text int dot = e.getDot(); int mark = e.getMark(); ... } }

Note: The caretUpdate method is not guaranteed to be called in the event-dispatching thread. To use any methods inside of caretUpdate that update the GUI, special handling is required to ensure that they are executed on the event-dispatching thread. You can do this by wrapping the code inside a Runnable and calling SwingUtilities.invokeLater on that Runnable .

You can find the full source code for the program and instructions for compiling and running it in Using Text Components (page 60) in Chapter 3. For a discussion about the caret listener aspect of the program see Listening for Caret and Selection Changes (page 74).

The Caret Listener API

The CaretListener interface has just one method (see Table 3), so it has no corresponding adapter class. Table 4 lists the methods in the CaretEvent class. Also see the Caret-Listener API documentation at: http://java.sun.com/j2se/1.4.2/docs/api/javax/swing/event/CaretListener.html. The CaretEvent API documentation is online at: http://java.sun.com/j2se/1.4.2/docs/api/javax/swing/event/CaretEvent.html.

Table 3. The CaretListener Interface

Method	Purpose
`caretUpdate(CaretEvent)`	Called when the caret in the listened-to component moves or when the selection in the listened-to component changes.

Table 4. The CaretEvent Class

Method	Purpose
`int getDot()`	Return the current location of the caret. If text is selected, the caret marks one end of the selection.
`int getMark()`	Return the other end of the selection. If nothing is selected, the value returned by this method is equal to the value returned by `getDot` . Note that the dot is not guaranteed to be less than the mark.
Object getSource() ( in java.util.EventObject)	Return the object that fired the event.

Examples That Use Caret Listeners

The following examples use caret listeners.

Example	Where Described	Notes
`TextComponentDemo`	Listening for Caret and Selection Changes (page 74)	Uses a "listener label" to display caret and selection status.