Focus Listener Event in Advanced Java Programming

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Focus events are sent when a component gains or loses keyboard input focus. The following is the definition of the FocusEvent class.

public class FocusEvent extends ComponentEvent { public static final int FOCU_GAINED; public static final int FOCU_LOST; public FocusEvent(Component source, int id, boolean temporary); public FocusEvent(Compoent sourec, int id); public boolean isTemporary(); public String paramString(); }

A component can process its own focus events by calling
enableEvents(AWTEvent.FOCUS_EVENT_MASK) and providing a processFocusEvent() method. Alternatively, you can delegate focus events to listener that implements the FocusListener interface.

public interface FocusListener extends EventListener { public void focusGained(FocusEvent fe); public void focusLost(FocusEvent fe); }

The following applet uses a BorderLayout manager to put a text field at "north" and a textarea at "center". The textfield delegates its focus events to the applet. The text area is a sub-class that handles its own focus events. Depending upon whether focus gained or lost, both the event handlers print a message to the console.

/* =========================================================== File Name : FocusListenerTest.java WebSite : http://javaproglang.blogspot.in/ Facebook : https://www.facebook.com/AdvanceJavaProgramming Twitter ID : https://twitter.com/AdvanceJava Created By : Bintu Chaudhary =========================================================== */ import java.applet.Applet; import java.awt.*; import java.awt.event.*; /* <APPLET Code="FocusListenerTest" Width=300 Height=300> </APPLET> */ public class FocusListenerTest extends Applet implements FocusListener { public void init() { setLayout(new BorderLayout()); // A text field that delegates to the applet TextField txt=new TextField(); txt.addFocusListener(this); add(txt,"North"); // A subclass that handles its own focus events SelfTextArea slfta = new SelfTextArea(); add(slfta,"Center"); } public void focusGained(FocusEvent FocEvt) { System.out.println("Text field gained focus"); } public void focusLost(FocusEvent FocEvt) { System.out.println("Text field lost focus"); } } class SelfTextArea extends TextArea { public SelfTextArea() { enableEvents(AWTEvent.FOCUS_EVENT_MASK); } public void processFocusEvent(FocusEvent FocEvt) { if(FocEvt.getID() == FocusEvent.FOCUS_GAINED) System.out.println("Text area gained focus"); else System.out.println("Text area lost focus"); } }

The above code will be stored in the file called FocusListenerTest.java, which on compilation will crate a class file called FocusListenerTest.class. Now compile this file using below commands in the DOS mode:

C:\>jdk1.4\bin>javac FocusListenerTest.java
C:\>jdk1.4\bin>appletviewer FocusListenerTest.java

"Output of FocusListenerTest.class"

"Output of FocusListenerTest.class in DOS mode"

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Event Listener

Action Listener	Adjustment Listener	Mouse Listener
Mouse Motion Listener	Window Listener	Key Listener
Focus Listener	ITEM Listener	Text Listener