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学习java的经典书籍

      getAppletContext().showStatus("Button 1");
    <font color="#0000ff">else</font> <font color="#0000ff">if</font>(evt.target.equals(b2))
      getAppletContext().showStatus("Button 2");
    <font color="#009900">// Let the base class handle it:</font>
    <font color="#0000ff">else</font> 
      <font color="#0000ff">return</font> <font color="#0000ff">super</font>.action(evt, arg);
    <font color="#0000ff">return</font> <font color="#0000ff">true</font>; <font color="#009900">// We've handled it here</font>
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">To
see what the <A NAME="Index1591"></A><A NAME="Index1592"></A>target
is, ask 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 what its 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>target</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
member is and then use the <A NAME="Index1593"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>equals(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method to see if it matches the target object handle you&#8217;re interested
in. When you&#8217;ve written handlers for all the objects you&#8217;re
interested in you must call <A NAME="Index1594"></A><A NAME="Index1595"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>super.action(evt,
arg)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>else</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
statement at the end, as shown above. Remember from Chapter 7 (polymorphism)
that your overridden method is called instead of the base class version.
However, the base-class version contains code to handle all of the cases that
you&#8217;re not interested in, and it won&#8217;t get called unless you call
it explicitly. The return value indicates whether you&#8217;ve handled it or
not, so if you do match an event you should return 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>true</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
otherwise return whatever the base-class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>event(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
returns.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">For
this example, the simplest action is to print what button is pressed. Some
systems allow you to pop up a little window with a message in it, but applets
discourage this. However, you can put a message at the bottom of the <A NAME="Index1596"></A><A NAME="Index1597"></A><A NAME="Index1598"></A><A NAME="Index1599"></A>Web
browser window on its 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>status
line
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
by calling the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Applet</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method <A NAME="Index1600"></A><A NAME="Index1601"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getAppletContext(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to get access to the browser and then <A NAME="Index1602"></A><A NAME="Index1603"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>showStatus(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to put a string on the status line.
</FONT><A NAME="fnB56" HREF="#fn56">[56]</A><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
You can print out a complete description of an event the same way, with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getAppletContext().showStatus(evt
+ "" ). 
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">(The
empty 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
forces the compiler to convert 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>evt</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String.</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
Both of these reports are really useful only for testing and debugging since
the browser might overwrite your message.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Strange
as it might seem, you can also <A NAME="Index1604"></A><A NAME="Index1605"></A>match
an event to the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>text</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that&#8217;s on a button through the second argument in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>event(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Using this technique, the example above becomes:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Button3.java</font>
<font color="#009900">// Matching events on button text</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> Button3 <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>(arg.equals("Button 1"))
      getAppletContext().showStatus("Button 1");
    <font color="#0000ff">else</font> <font color="#0000ff">if</font>(arg.equals("Button 2"))
      getAppletContext().showStatus("Button 2");
    <font color="#009900">// Let the base class handle it:</font>
    <font color="#0000ff">else</font> 
      <font color="#0000ff">return</font> <font color="#0000ff">super</font>.action(evt, arg);
    <font color="#0000ff">return</font> <font color="#0000ff">true</font>; <font color="#009900">// We've handled it here</font>
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It&#8217;s
difficult to know exactly what the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>equals(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is doing here. The biggest problem with this approach is that most new
Java programmers who start with this technique spend at least one frustrating
session discovering that they&#8217;ve gotten the capitalization or spelling
wrong when comparing to the text on a button. (I had this experience.) Also, if
you change the text of the button, the code will no longer work (but you
won&#8217;t get any compile-time or run-time error messages). You should avoid
this approach if possible.
</FONT><a name="_Toc375545449"></a><a name="_Toc408018685"></a><P></DIV>
<HR><DIV ALIGN=LEFT><A NAME="fn56" HREF="#fnB56">[56]</A><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black"><B>ShowStatus(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
is also a method of Applet, so you can call it directly, without calling 
</FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black"><B>getAppletContext(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">.</FONT><P></DIV>


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