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学习java的经典书籍

does. In fact, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
also implements 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which specifies only that there be a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
use of the combined program/thread is not quite so obvious. When you start the
program, you create an object that&#8217;s 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
but you don&#8217;t start the thread. This must be done explicitly. You can see
this in the following program, which reproduces the functionality of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Counter2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Counter3.java</font>
<font color="#009900">// Using the Runnable interface to turn the </font>
<font color="#009900">// main class into a thread.</font>
<font color="#0000ff">import</font> java.awt.*;
<font color="#0000ff">import</font> java.awt.event.*;
<font color="#0000ff">import</font> java.applet.*;

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Counter3 
    <font color="#0000ff">extends</font> Applet <font color="#0000ff">implements</font> Runnable {
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> count = 0;
  <font color="#0000ff">private</font> <font color="#0000ff">boolean</font> runFlag = <font color="#0000ff">true</font>;
  <font color="#0000ff">private</font> Thread selfThread = <font color="#0000ff">null</font>;
  <font color="#0000ff">private</font> Button 
    onOff = <font color="#0000ff">new</font> Button("Toggle"),
    start = <font color="#0000ff">new</font> Button("Start");
  <font color="#0000ff">private</font> TextField t = <font color="#0000ff">new</font> TextField(10);
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> init() {
    add(t);
    start.addActionListener(<font color="#0000ff">new</font> StartL());
    add(start);
    onOff.addActionListener(<font color="#0000ff">new</font> OnOffL());
    add(onOff);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> run() {
    <font color="#0000ff">while</font> (<font color="#0000ff">true</font>) {
      <font color="#0000ff">try</font> {
        selfThread.sleep(100);
      } <font color="#0000ff">catch</font> (InterruptedException e){}
      <font color="#0000ff">if</font>(runFlag) 
        t.setText(Integer.toString(count++));
    }
  }
  <font color="#0000ff">class</font> StartL <font color="#0000ff">implements</font> ActionListener {
    <font color="#0000ff">public</font> <font color="#0000ff">void</font> actionPerformed(ActionEvent e) {
      <font color="#0000ff">if</font>(selfThread == <font color="#0000ff">null</font>) {
        selfThread = <font color="#0000ff">new</font> Thread(Counter3.<font color="#0000ff">this</font>);
        selfThread.start();
      }
    }
  }
  <font color="#0000ff">class</font> OnOffL <font color="#0000ff">implements</font> ActionListener {
    <font color="#0000ff">public</font> <font color="#0000ff">void</font> actionPerformed(ActionEvent e) {
      runFlag = !runFlag;
    }
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Counter3 applet = <font color="#0000ff">new</font> Counter3();
    Frame aFrame = <font color="#0000ff">new</font> Frame("Counter3");
    aFrame.addWindowListener(
      <font color="#0000ff">new</font> WindowAdapter() {
        <font color="#0000ff">public</font> <font color="#0000ff">void</font> windowClosing(WindowEvent e) {
          System.exit(0);
        }
      });
    aFrame.add(applet, BorderLayout.CENTER);
    aFrame.setSize(300,200);
    applet.init();
    applet.start();
    aFrame.setVisible(<font color="#0000ff">true</font>);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Now
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is inside the class, but it&#8217;s still dormant after 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>init(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
completes. When you press the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>start</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
button, the thread is created (if it doesn&#8217;t already exist) in the
somewhat obscure expression:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">new
Thread(Counter3.this);
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
something has a <A NAME="Index2471"></A><A NAME="Index2472"></A><A NAME="Index2473"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface, it simply means that it has a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method, but there&#8217;s nothing special about that &#8211; it doesn&#8217;t
produce any innate threading abilities, like those of a class inherited from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
So to produce a thread from a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object, you must create a thread separately and hand it the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object; there&#8217;s a special constructor for this that takes a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
as its argument. You can then call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>start(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for that thread:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">selfThread.start();</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
performs the usual initialization and then calls 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
convenient aspect about the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable
interface
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is that everything belongs to the same class. If you need to access something,
you simply do it without going through a separate object. The penalty for this
convenience is strict, though &#8211; you can have only a single thread running
for that particular object (although you can create more objects of that type,
or create other threads in different classes).
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface is not what imposes this restriction. It&#8217;s the combination of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and your main class that does it, since you can have only one object of your
main class per application.
</FONT><a name="_Toc375545476"></a><a name="_Toc408018749"></a><P></DIV>
<A NAME="Heading487"></A><H3 ALIGN=LEFT>
Making
many threads
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Consider
the creation of many different threads. You can&#8217;t do this with the
previous example, so you must go back to having separate classes inherited from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to encapsulate the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
But this is a more general solution and easier to understand, so while the
previous example shows a coding style you&#8217;ll often see, I can&#8217;t
recommend it for most cases because it&#8217;s just a little bit more confusing
and less flexible.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
following example repeats the form of the examples above with counters and
toggle buttons. But now all the information for a particular counter, including
the button and text field, is inside its own object that is inherited from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
All the fields in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Ticker</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which means that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Ticker</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
implementation can be changed at will, including the quantity and type of data
components to acquire and display information. When a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Ticker</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object is created, the constructor requires a handle to an AWT 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Container,</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
which 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Ticker
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">fills
with its visual components. This way, if you change the visual components, the
code that uses 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Ticker</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
doesn&#8217;t need to be modified.
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Counter4.java</font>
<font color="#009900">// If you separate your thread from the main</font>
<font color="#009900">// class, you can have as many threads as you</font>
<font color="#009900">// want.</font>
<font color="#0000ff">import</font> java.awt.*;
<font color="#0000ff">import</font> java.awt.event.*;
<font color="#0000ff">import</font> java.applet.*;

<font color="#0000ff">class</font> Ticker <font color="#0000ff">extends</font> Thread {
  <font color="#0000ff">private</font> Button b = <font color="#0000ff">new</font> Button("Toggle");
  <font color="#0000ff">private</font> TextField t = <font color="#0000ff">new</font> TextField(10);
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> count = 0;
  <font color="#0000ff">private</font> <font color="#0000ff">boolean</font> runFlag = <font color="#0000ff">true</font>;
  <font color="#0000ff">public</font> Ticker(Container c) {
    b.addActionListener(<font color="#0000ff">new</font> ToggleL());
    Panel p = <font color="#0000ff">new</font> Panel();
    p.add(t);
    p.add(b);
    c.add(p);
  }
  <font color="#0000ff">class</font> ToggleL <font color="#0000ff">implements</font> ActionListener {
    <font color="#0000ff">public</font> <font color="#0000ff">void</font> actionPerformed(ActionEvent e) {
      runFlag = !runFlag;
    }
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> run() {
    <font color="#0000ff">while</font> (<font color="#0000ff">true</font>) {
      <font color="#0000ff">if</font>(runFlag)
        t.setText(Integer.toString(count++));
       <font color="#0000ff">try</font> {
        sleep(100);
      } <font color="#0000ff">catch</font> (InterruptedException e){}
    }
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Counter4 <font color="#0000ff">extends</font> Applet {
  <font color="#0000ff">private</font> Button start = <font color="#0000ff">new</font> Button("Start");
  <font color="#0000ff">private</font> <font color="#0000ff">boolean</font> started = <font color="#0000ff">false</font>;
  <font color="#0000ff">private</font> Ticker[] s;
  <font color="#0000ff">private</font> <font color="#0000ff">boolean</font> isApplet = <font color="#0000ff">true</font>;
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> size;