tij0156.html

学习java的经典书籍

  }
  <font color="#0000ff">public</font> <font color="#0000ff">synchronized</font> <font color="#0000ff">void</font> 
  setCircleSize(<font color="#0000ff">int</font> newSize) {
    cSize = newSize;
  }
  <font color="#0000ff">public</font> <font color="#0000ff">synchronized</font> String getBangText() { 
    <font color="#0000ff">return</font> text; 
  }
  <font color="#0000ff">public</font> <font color="#0000ff">synchronized</font> <font color="#0000ff">void</font> 
  setBangText(String newText) {
    text = newText;
  }
  <font color="#0000ff">public</font> <font color="#0000ff">synchronized</font> <font color="#0000ff">int</font> getFontSize() { 
    <font color="#0000ff">return</font> fontSize; 
  }
  <font color="#0000ff">public</font> <font color="#0000ff">synchronized</font> <font color="#0000ff">void</font> 
  setFontSize(<font color="#0000ff">int</font> newSize) {
    fontSize = newSize;
  }
  <font color="#0000ff">public</font> <font color="#0000ff">synchronized</font> Color getTextColor() {
    <font color="#0000ff">return</font> tColor; 
  }
  <font color="#0000ff">public</font> <font color="#0000ff">synchronized</font> <font color="#0000ff">void</font> 
  setTextColor(Color newColor) {
    tColor = newColor;
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> paint(Graphics g) {
    g.setColor(Color.black);
    g.drawOval(xm - cSize/2, ym - cSize/2, 
      cSize, cSize);
  }
  <font color="#009900">// This is a multicast listener, which is</font>
  <font color="#009900">// more typically used than the unicast</font>
  <font color="#009900">// approach taken in BangBean.java:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">synchronized</font> <font color="#0000ff">void</font> addActionListener (
      ActionListener l) {
    actionListeners.addElement(l);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">synchronized</font> <font color="#0000ff">void</font> removeActionListener(
      ActionListener l) {
    actionListeners.removeElement(l);
  }
  <font color="#009900">// Notice this isn't synchronized:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> notifyListeners() {
    ActionEvent a =
      <font color="#0000ff">new</font> ActionEvent(BangBean2.<font color="#0000ff">this</font>,
        ActionEvent.ACTION_PERFORMED, <font color="#0000ff">null</font>);
    Vector lv = <font color="#0000ff">null</font>;
    <font color="#009900">// Make a copy of the vector in case someone</font>
    <font color="#009900">// adds a listener while we're </font>
    <font color="#009900">// calling listeners:</font>
    <font color="#0000ff">synchronized</font>(<font color="#0000ff">this</font>) {
      lv = (Vector)actionListeners.clone();
    }
    <font color="#009900">// Call all the listener methods:</font>
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; lv.size(); i++) {
      ActionListener al = 
        (ActionListener)lv.elementAt(i);
      al.actionPerformed(a);
    }
  }
  <font color="#0000ff">class</font> ML <font color="#0000ff">extends</font> MouseAdapter {
    <font color="#0000ff">public</font> <font color="#0000ff">void</font> mousePressed(MouseEvent e) {
      Graphics g = getGraphics();
      g.setColor(tColor);
      g.setFont(
        <font color="#0000ff">new</font> Font(
          "TimesRoman", Font.BOLD, fontSize));
      <font color="#0000ff">int</font> width = 
        g.getFontMetrics().stringWidth(text);
      g.drawString(text, 
        (getSize().width - width) /2,
        getSize().height/2);
      g.dispose();
      notifyListeners();
    }
  }
  <font color="#0000ff">class</font> MM <font color="#0000ff">extends</font> MouseMotionAdapter {
    <font color="#0000ff">public</font> <font color="#0000ff">void</font> mouseMoved(MouseEvent e) {
      xm = e.getX();
      ym = e.getY();
      repaint();
    }
  }
  <font color="#009900">// Testing the BangBean2:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    BangBean2 bb = <font color="#0000ff">new</font> BangBean2();
    bb.addActionListener(<font color="#0000ff">new</font> ActionListener() {
      <font color="#0000ff">public</font> <font color="#0000ff">void</font> actionPerformed(ActionEvent e){
        System.out.println("ActionEvent" + e);
      }
    });
    bb.addActionListener(<font color="#0000ff">new</font> ActionListener() {
      <font color="#0000ff">public</font> <font color="#0000ff">void</font> actionPerformed(ActionEvent e){
        System.out.println("BangBean2 action");
      }
    });
    bb.addActionListener(<font color="#0000ff">new</font> ActionListener() {
      <font color="#0000ff">public</font> <font color="#0000ff">void</font> actionPerformed(ActionEvent e){
        System.out.println("More action");
      }
    });
    Frame aFrame = <font color="#0000ff">new</font> Frame("BangBean2 Test");
    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(bb, BorderLayout.CENTER);
    aFrame.setSize(300,300);
    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">Adding
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to the methods is an easy change. However, notice in <A NAME="Index2512"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>addActionListener(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index2513"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>removeActionListener(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ActionListener</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
are now added to and removed from a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so you can have as many as you want.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that the method <A NAME="Index2514"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyListeners(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>not</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
It can be called from more than one thread at a time. It&#8217;s also possible
for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>addActionListener(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
or 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>removeActionListener(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to be called in the middle of a call to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyListeners(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which is a problem since it traverses the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector
actionListeners
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
To alleviate the problem, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is cloned inside a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
clause and the clone is traversed. This way the original 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
can be manipulated without impact on 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>notifyListeners(&#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
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is also not <A NAME="Index2515"></A><A NAME="Index2516"></A><A NAME="Index2517"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Deciding whether to synchronize overridden methods is not as clear as when
you&#8217;re just adding your own methods. In this example it turns out that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
seems to work OK whether it&#8217;s 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
or not. But the issues you must consider are:
</FONT><P></DIV>
<OL>
<LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Does
the method modify the state of &#8220;critical&#8221; variables within the
object? To discover whether the variables are &#8220;critical&#8221; you must
determine whether they will be read or set by other threads in the program. (In
this case, the reading or setting is virtually always accomplished via 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods, so you can just examine those.) In the case of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
no modification takes place.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Does
the method depend on the state of these &#8220;critical&#8221; variables? If a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method modifies a variable that your method uses, then you might very well want
to make your method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
as well. Based on this, you might observe that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>cSize</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is changed by 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods and therefore 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
should be 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Here, however, you can ask &#8220;What&#8217;s the worst thing that will happen
if 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>cSize</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is changed during a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">?&#8221;
When you see that it&#8217;s nothing too bad, and a transient effect at that,
it&#8217;s best to leave 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
un-
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to prevent the extra overhead from the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method call.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	A
third clue is to notice whether the base-class version of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which it isn&#8217;t. This isn&#8217;t an airtight argument, just a clue. In
this case, for example, a field that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>is</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
changed via 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods (that is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>cSize</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
has been mixed into the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>paint(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
formula and might have changed the situation. Notice, however, that <A NAME="Index2518"></A><A NAME="Index2519"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
doesn&#8217;t inherit &#8211; that is, if a method is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in the base class then it 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>is
not
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
automatically 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>synchronized</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in the derived class overridden version.
</FONT></OL><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
test code in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TestBangBean2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
has been modified from that in the previous chapter to demonstrate the
multicast ability of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>BangBean2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
by adding extra listeners.
</FONT><a name="_Toc375545481"></a><a name="_Toc408018755"></a><P></DIV>

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