📄 13_7.htm

📁 翁剀JAVA语言那门课程的教案很多人都看多他的视频教程可惜没有ppt的教案
💻 HTM
字号:
<HTML>
<HEAD>
   <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
   <META NAME="Author" CONTENT="Weng Kai">
   <META NAME="GENERATOR" CONTENT="Mozilla/4.01 [en] (Win95; I) [Netscape]">
   <TITLE>Inheriting a component</TITLE>
</HEAD>
<BODY>

<H2>
13.7 Inheriting a component</H2>

<HR WIDTH="100%">
<BR>Another place where you'll often see variations on the old way of doing
things is when creating a new type of component. Here's an example showing
that here, too, the new way works:

<P>Case Study: <A HREF="case/GoodTechnique.java">GoodTechnique.java</A>

<P>This example also demonstrates the various events that occur and displays
the information about them. The class Display is a way to centralize that
information display. There's an array of Strings to hold information about
each type of event, and the method show( ) takes a handle to whatever Graphics
object you have and writes directly on that surface. The scheme is intended
to be somewhat reusable.

<P>EnabledPanel represents the new type of component. It's a colored panel
with a button at the bottom, and it captures all the events that happen
over it by using inner listener classes for every single event except those
in which EnabledPanel overrides processEvent( ) in the old style (notice
it must also call super.processEvent( )). The only reason for using this
method is that it captures every event that happens, so you can view everything
that goes on. processEvent( ) does nothing more than show the string representation
of each event, otherwise it would have to use a cascade of if statements
to figure out what event it was. On the other hand, the inner listener
classes already know precisely what event occurred. (Assuming you register
them to components in which you don't need any control logic, which should
be your goal.) Thus, they don't have to check anything out; they just do
their stuff.

<P>Each listener modifies the Display string associated with its particular
event and calls repaint( ) so the strings get displayed. You can also see
a trick that will usually eliminate flicker:
<UL>
<PRE>public void update(Graphics g) {</PRE>

<PRE>&nbsp;&nbsp;&nbsp; paint(g);</PRE>

<PRE>}</PRE>
</UL>
You don't always need to override update( ), but if you write something
that flickers, try it. The default version of update clears the background
and then calls paint( ) to redraw any graphics. This clearing is usually
what causes flicker but is not necessary since paint( ) redraws the entire
surface.

<P>You can see that there are a lot of listeners -- however, type checking
occurs for the listeners, and you can't listen for something that the component
doesn't support (unlike BadTechnique.java, which you will see momentarily).

<P>Experimenting with this program is quite educational since you learn
a lot about the way that events occur in Java. For one thing, it shows
a flaw in the design of most windowing systems: it's pretty hard to click
and release the mouse without moving it, and the windowing system will
often think you're dragging when you're actually just trying to click on
something. A solution to this is to use mousePressed( ) and mouseReleased(
) instead of mouseClicked( ), and then determine whether to call your own
"mouseReallyClicked( )"&nbsp; method based on time and about 4 pixels of
mouse hysteresis.
<BR>&nbsp;
<H3>
Ugly component inheritance</H3>
The alternative, which you will see put forward in many published works,
is to call enableEvents( ) and pass it the masks corresponding to the events
you want to handle. This causes those events to be sent to the old-style
methods (although they're new to Java 1.1) with names like processFocusEvent(
). You must also remember to call the base-class version. Here's what it
looks like:

<P>Case Study: <A HREF="case/BadTechnique.java">BadTechnique.java</A>

<P>Sure, it works. But it's ugly and hard to write, read, debug, maintain,
and reuse. So why bother when you can use inner listener classes?

<P>
<HR WIDTH="100%">
<DIV ALIGN=right><A HREF="../Lesson14/14.htm">Next Page</A></DIV>

</BODY>
</HTML>
⌨️ 快捷键说明

复制代码 Ctrl + C
搜索代码 Ctrl + F
全屏模式 F11
切换主题 Ctrl + Shift + D
显示快捷键 ?
增大字号 Ctrl + =
减小字号 Ctrl + -