Java Tutorial/Swing Event/FocusListener

Содержание

1 FocusListener and FocusEvent
2 How to Write a Focus Listener
3 The event IDs defined in the FocusEvent class
4 The FocusListener Interface: respond to a component gaining or losing the focus
5 Use Focus Events in Swing
6 Validate a value on the lostFocus event
7 Validating a JTextField When Permanently Losing the Focus

FocusListener and FocusEvent

   <source lang="java">

/*

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*/

/*

* FocusEventDemo.java
*
*/

import java.awt.Container; import java.awt.Dimension; import java.awt.GridBagConstraints; import java.awt.GridBagLayout; import java.awt.Insets; import java.awt.event.FocusEvent; import java.awt.event.FocusListener; import java.util.Vector; import javax.swing.BorderFactory; import javax.swing.JButton; import javax.swing.JComboBox; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JList; import javax.swing.JPanel; import javax.swing.JScrollPane; import javax.swing.JTextArea; import javax.swing.JTextField; import javax.swing.UIManager; import javax.swing.UnsupportedLookAndFeelException; public class FocusEventDemo extends JFrame implements FocusListener {

 final static String newline = "\n";
 JTextArea display;
 public FocusEventDemo(String name) {
   super(name);
 }
 public void addComponentsToPane(final Container pane) {
   GridBagLayout gridbag = new GridBagLayout();
   pane.setLayout(gridbag);
   GridBagConstraints c = new GridBagConstraints();
   c.fill = GridBagConstraints.HORIZONTAL;
   c.weightx = 1.0; // Make column as wide as possible.
   JTextField textField = new JTextField("A TextField");
   textField.setMargin(new Insets(0, 2, 0, 2));
   textField.addFocusListener(this);
   gridbag.setConstraints(textField, c);
   add(textField);
   c.weightx = 0.1; // Widen every other column a bit, when possible.
   c.fill = GridBagConstraints.NONE;
   JLabel label = new JLabel("A Label");
   label.setBorder(BorderFactory.createEmptyBorder(0, 5, 0, 5));
   label.addFocusListener(this);
   gridbag.setConstraints(label, c);
   add(label);
   String comboPrefix = "ComboBox Item #";
   final int numItems = 15;
   Vector<String> vector = new Vector<String>(numItems);
   for (int i = 0; i < numItems; i++) {
     vector.addElement(comboPrefix + i);
   }
   JComboBox comboBox = new JComboBox(vector);
   comboBox.addFocusListener(this);
   gridbag.setConstraints(comboBox, c);
   add(comboBox);
   c.gridwidth = GridBagConstraints.REMAINDER;
   JButton button = new JButton("A Button");
   button.addFocusListener(this);
   gridbag.setConstraints(button, c);
   add(button);
   c.weightx = 0.0;
   c.weighty = 0.1;
   c.fill = GridBagConstraints.BOTH;
   String listPrefix = "List Item #";
   Vector<String> listVector = new Vector<String>(numItems);
   for (int i = 0; i < numItems; i++) {
     listVector.addElement(listPrefix + i);
   }
   JList list = new JList(listVector);
   list.setSelectedIndex(1); // It"s easier to see the focus change
   // if an item is selected.
   list.addFocusListener(this);
   JScrollPane listScrollPane = new JScrollPane(list);
   gridbag.setConstraints(listScrollPane, c);
   add(listScrollPane);
   c.weighty = 1.0; // Make this row as tall as possible.
   c.gridheight = GridBagConstraints.REMAINDER;
   // Set up the area that reports focus-gained and focus-lost events.
   display = new JTextArea();
   display.setEditable(false);
   // The setRequestFocusEnabled method prevents a
   // component from being clickable, but it can still
   // get the focus through the keyboard - this ensures
   // user accessibility.
   display.setRequestFocusEnabled(false);
   display.addFocusListener(this);
   JScrollPane displayScrollPane = new JScrollPane(display);
   gridbag.setConstraints(displayScrollPane, c);
   add(displayScrollPane);
   setPreferredSize(new Dimension(450, 450));
   ((JPanel) pane).setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));
 }
 public void focusGained(FocusEvent e) {
   displayMessage("Focus gained", e);
 }
 public void focusLost(FocusEvent e) {
   displayMessage("Focus lost", e);
 }
 void displayMessage(String prefix, FocusEvent e) {
   display.append(prefix
       + (e.isTemporary() ? " (temporary):" : ":")
       + e.getComponent().getClass().getName()
       + "; Opposite component: "
       + (e.getOppositeComponent() != null ? e.getOppositeComponent()
           .getClass().getName() : "null") + newline);
   display.setCaretPosition(display.getDocument().getLength());
 }
 /**
  * Create the GUI and show it. For thread safety, this method should be
  * invoked from the event dispatch thread.
  */
 private static void createAndShowGUI() {
   // Create and set up the window.
   FocusEventDemo frame = new FocusEventDemo("FocusEventDemo");
   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   // Set up the content pane.
   frame.addComponentsToPane(frame.getContentPane());
   // Display the window.
   frame.pack();
   frame.setVisible(true);
 }
 public static void main(String[] args) {
   /* Use an appropriate Look and Feel */
   try {
     // UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
     UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
   } catch (UnsupportedLookAndFeelException ex) {
     ex.printStackTrace();
   } catch (IllegalAccessException ex) {
     ex.printStackTrace();
   } catch (InstantiationException ex) {
     ex.printStackTrace();
   } catch (ClassNotFoundException ex) {
     ex.printStackTrace();
   }
   /* Turn off metal"s use of bold fonts */
   UIManager.put("swing.boldMetal", Boolean.FALSE);
   // Schedule a job for the event dispatch thread:
   // creating and showing this application"s GUI.
   javax.swing.SwingUtilities.invokeLater(new Runnable() {
     public void run() {
       createAndShowGUI();
     }
   });
 }

}</source>

How to Write a Focus Listener

Focus events are fired whenever a component gains or loses the keyboard focus.

   <source lang="java">

import java.awt.event.FocusEvent; import java.awt.event.FocusListener; import javax.swing.JFrame; import javax.swing.JTextField; public class UsingFocusListener {

 public static void main(String[] a) {
   JFrame frame = new JFrame();
   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   JTextField textField = new JTextField("A TextField");
   textField.addFocusListener(new FocusListener() {
     public void focusGained(FocusEvent e) {
       displayMessage("Focus gained", e);
     }
     public void focusLost(FocusEvent e) {
       displayMessage("Focus lost", e);
     }
     void displayMessage(String prefix, FocusEvent e) {
       System.out.println(prefix
           + (e.isTemporary() ? " (temporary):" : ":")
           + e.getComponent().getClass().getName()
           + "; Opposite component: "
           + (e.getOppositeComponent() != null ? e.getOppositeComponent().getClass().getName()
               : "null"));
     }
   });
   frame.add(textField,"North");
   frame.add(new JTextField(),"South");
   frame.setSize(300, 200);
   frame.setVisible(true);
 }

}</source>

The event IDs defined in the FocusEvent class

FOCUS_GAINED
FOCUS_LOST

The FocusListener Interface: respond to a component gaining or losing the focus

Defined MethodsDescriptionfocusGained(FocusEvent e)Called when a component gains the keyboard focusfocusLost(FocusEvent e)Called when a component loses the keyboard focus

Use Focus Events in Swing

   <source lang="java">

import java.awt.Dimension; import java.awt.event.FocusEvent; import java.awt.event.FocusListener; import java.util.Vector; import javax.swing.BorderFactory; import javax.swing.JButton; import javax.swing.JComboBox; import javax.swing.JComponent; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JList; import javax.swing.JPanel; import javax.swing.JScrollPane; import javax.swing.JTextField; public class Main extends JPanel implements FocusListener {

 public Main() {
   JTextField textField = new JTextField("A TextField");
   textField.addFocusListener(this);
   JLabel label = new JLabel("A Label");
   label.addFocusListener(this);
   add(label);
   String comboPrefix = "ComboBox Item #";
   final int numItems = 15;
   Vector vector = new Vector(numItems);
   for (int i = 0; i < numItems; i++) {
     vector.addElement(comboPrefix + i);
   }
   JComboBox comboBox = new JComboBox(vector);
   comboBox.addFocusListener(this);
   add(comboBox);
   JButton button = new JButton("A Button");
   button.addFocusListener(this);
   add(button);
   String listPrefix = "List Item #";
   Vector listVector = new Vector(numItems);
   for (int i = 0; i < numItems; i++) {
     listVector.addElement(listPrefix + i);
   }
   JList list = new JList(listVector);
   list.setSelectedIndex(1); 
   list.addFocusListener(this);
   JScrollPane listScrollPane = new JScrollPane(list);
   listScrollPane.getVerticalScrollBar().setFocusable(false);
   listScrollPane.getHorizontalScrollBar().setFocusable(false);
   add(listScrollPane);
   setPreferredSize(new Dimension(450, 450));
   setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));
 }
 void displayMessage(String prefix, FocusEvent e) {
   System.out.println(prefix
       + (e.isTemporary() ? " (temporary):" : ":")
       + e.getComponent().getClass().getName()
       + "; Opposite component: "
       + (e.getOppositeComponent() != null ? e.getOppositeComponent().getClass().getName()
           : "null") );
 }
 public static void main(String[] args) {
   JFrame frame = new JFrame("FocusEventDemo");
   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   JComponent newContentPane = new Main();
   newContentPane.setOpaque(true); // content panes must be opaque
   frame.setContentPane(newContentPane);
   frame.pack();
   frame.setVisible(true);
 }
 public void focusGained(FocusEvent e) {
   displayMessage("Focus gained", e);

} public void focusLost(FocusEvent e) {

   displayMessage("Focus lost", e);

} }</source>

Validate a value on the lostFocus event

   <source lang="java">

import java.awt.BorderLayout; import java.awt.event.FocusEvent; import java.awt.event.FocusListener; import javax.swing.JComponent; import javax.swing.JFrame; import javax.swing.JTextField; import javax.swing.SwingUtilities; public class Main {

 public static void main(String[] a) {
   final JTextField textField = new JTextField(5);

   textField.addFocusListener(new FocusListener() {
     public void focusGained(FocusEvent e) {
     }
     public void focusLost(FocusEvent e) {
       if (!e.isTemporary()) {
         String content = textField.getText();
         if (!content.equals("a") ) {
           System.out.println("illegal value! " + content);
           SwingUtilities.invokeLater(new FocusGrabber(textField));
         }
       }
     }
   });
   JFrame frame = new JFrame();
   frame.add(textField,BorderLayout.CENTER);
   frame.setSize(300,300);
   frame.setVisible(true);
 
 
 }

} class FocusGrabber implements Runnable {

 private JComponent component;
 public FocusGrabber(JComponent component) {
   this.ruponent = component;
 }
 public void run() {
   component.grabFocus();
 }

}</source>

Validating a JTextField When Permanently Losing the Focus

   <source lang="java">

import java.awt.FlowLayout; import java.awt.event.FocusAdapter; import java.awt.event.FocusEvent; import javax.swing.JFrame; import javax.swing.JTextField; import javax.swing.text.JTextComponent; public class Main {

 public static void main(String[] argv) throws Exception {
   JTextField component = new JTextField(10);
   JTextField component1 = new JTextField(10);
   component.addFocusListener(new MyFocusListener());
   component1.addFocusListener(new MyFocusListener());
   JFrame f = new JFrame();
   f.setLayout(new FlowLayout());
   f.add(component1);
   f.add(component);
   f.pack();
   f.setVisible(true);
 }

} class MyFocusListener extends FocusAdapter {

 boolean showingDialog = false;
 public void focusGained(FocusEvent evt) {
   final JTextComponent c = (JTextComponent) evt.getSource();
   String s = c.getText();
   for (int i = 0; i < s.length(); i++) {
     if (!Character.isDigit(s.charAt(i))) {
       c.setSelectionStart(i);
       c.setSelectionEnd(i);
       break;
     }
   }
 }
 public void focusLost(FocusEvent evt) {
   final JTextComponent c = (JTextComponent) evt.getSource();
   String s = c.getText();
   if (evt.isTemporary()) {
     return;
   }
   for (int i = 0; i < s.length(); i++) {
     if (!Character.isDigit(s.charAt(i))) {
       System.out.println("must only contain digits");
       c.requestFocus();
       break;
     }
   }
 }

}</source>

Java Tutorial/Swing Event/FocusListener

Содержание

FocusListener and FocusEvent

How to Write a Focus Listener

The event IDs defined in the FocusEvent class

The FocusListener Interface: respond to a component gaining or losing the focus

Use Focus Events in Swing

Validate a value on the lostFocus event

Validating a JTextField When Permanently Losing the Focus

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