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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<H2 ALIGN=LEFT>
Capturing
an event
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You’ll
notice that if you compile and run the applet above, nothing happens when you
press the buttons. This is where you must step in and write some code to
determine what will happen. The basis of <A NAME="Index1576"></A><A NAME="Index1577"></A><A NAME="Index1578"></A>event-driven
programming, which comprises a lot of what a GUI is about, is tying events to
code that responds to those events.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">After
working your way this far through the book and grasping some of the
fundamentals of object-oriented programming, you might think that of course
there will be some sort of object-oriented approach to handling events. For
example, you might have to inherit each button and override some “button
pressed” method (this, it turns out, is too tedious and restrictive). You
might also think there’s some master “event” class that
contains a method for each event you want to respond to.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Before
objects, the typical approach to handling events was the “giant switch
statement.” Each event would have a unique integer value and inside the
master event handling method you’d write a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>switch</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
on that value.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
AWT in Java 1.0<A NAME="Index1579"></A>
doesn’t use any object-oriented approach. Neither does it use a giant
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>switch</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
statement that relies on the assignment of numbers to events. Instead, you must
create a cascaded set of
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>if</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
statements. What you’re trying to do with the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>if</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
statements is detect the object that was the <A NAME="Index1580"></A><A NAME="Index1581"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>target</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of the event. That is, if you click on a button, then that particular button is
the target. Normally, that’s all you care about – if a button is
the target of an event, then it was most certainly a mouse click and you can
continue based on that assumption. However, events can contain other
information as well. For example, if you want to find out the pixel location
where a mouse click occurred so you can draw a line to that location, the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Event</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object will contain the location. (You should also be aware that Java 1.0
components can be limited in the kinds of events they generate, while Java 1.1
and Swing/JFC components produce a full set of events.)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
Java 1.0<A NAME="Index1582"></A>
AWT method where your cascaded
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>if</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
statement resides is called <A NAME="Index1583"></A><A NAME="Index1584"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>action( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Although the whole Java 1.0<A NAME="Index1585"></A>
Event model has been deprecated in Java 1.1<A NAME="Index1586"></A>,
it is still widely used for simple applets and in systems that do not yet
support Java 1.1, so I recommend you become comfortable with it, including the
use of the following
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>action()</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method approach.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>action( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
has two arguments: the first is of type <A NAME="Index1587"></A><A NAME="Index1588"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Event</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and contains all the information about the event that triggered this call to
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>action( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
For example, it could be a mouse click, a normal keyboard press or release, a
special key press or release, the fact that the component got or lost the
focus, mouse movements, or drags, etc. The second argument is usually the
target of the event, which you’ll often ignore. The second argument is
also encapsulated in the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Event</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object so it is redundant as an argument.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
situations in which
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>action( )
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">gets
called are extremely limited: When you place controls on a form, some types of
controls (buttons, check boxes, drop-down lists, menus) have a “standard
action” that occurs, which causes the call to
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>action( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
with the appropriate
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Event</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object. For example, with a button the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>action( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is called when the button is pressed and at no other time. Usually this
is just fine, since that’s what you ordinarily look for with a button.
However, it’s possible to deal with many other types of events via the <A NAME="Index1589"></A><A NAME="Index1590"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>handleEvent( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method as we will see later in this chapter.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
previous example can be extended to handle button clicks as follows:
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: Button2.java</font>
<font color="#009900">// Capturing button presses</font>
<font color="#0000ff">import</font> java.awt.*;
<font color="#0000ff">import</font> java.applet.*;
<font color="#0000ff">public</font> <font color="#0000ff">class</font> Button2 <font color="#0000ff">extends</font> Applet {
Button
b1 = <font color="#0000ff">new</font> Button("Button 1"),
b2 = <font color="#0000ff">new</font> Button("Button 2");
<font color="#0000ff">public</font> <font color="#0000ff">void</font> init() {
add(b1);
add(b2);
}
<font color="#0000ff">public</font> <font color="#0000ff">boolean</font> action(Event evt, Object arg) {
<font color="#0000ff">if</font>(evt.target.equals(b1))
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