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学习java的经典书籍

    <font color="#0000ff">super</font>.cleanup();
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> CADSystem <font color="#0000ff">extends</font> Shape {
  <font color="#0000ff">private</font> Circle c;
  <font color="#0000ff">private</font> Triangle t;
  <font color="#0000ff">private</font> Line[] lines = <font color="#0000ff">new</font> Line[10];
  CADSystem(<font color="#0000ff">int</font> i) {
    <font color="#0000ff">super</font>(i + 1);
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> j = 0; j &lt; 10; j++)
      lines[j] = <font color="#0000ff">new</font> Line(j, j*j);
    c = <font color="#0000ff">new</font> Circle(1);
    t = <font color="#0000ff">new</font> Triangle(1);
    System.out.println("Combined constructor");
  }
  <font color="#0000ff">void</font> cleanup() {
    System.out.println("CADSystem.cleanup()");
    t.cleanup();
    c.cleanup();
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; lines.length; i++)
      lines[i].cleanup();
    <font color="#0000ff">super</font>.cleanup();
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    CADSystem x = <font color="#0000ff">new</font> CADSystem(47);
    <font color="#0000ff">try</font> {
      <font color="#009900">// Code and exception handling...</font>
    } <font color="#0000ff">finally</font> {
      x.cleanup();
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Everything
in this system is some kind of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Shape</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(which is itself a kind of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
since it&#8217;s implicitly inherited from the root class). Each class redefines 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Shape</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>cleanup(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method in addition to calling the base-class version of that method using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>super</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The specific 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Shape</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
classes 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Circle</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Triangle</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Line</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
all have constructors that &#8220;draw,&#8221; although any method called
during the lifetime of the object could be responsible for doing something that
needs cleanup. Each class has its own 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>cleanup(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method to restore non-memory things back to the way they were before the object
existed.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you can see two keywords that are new, and won&#8217;t officially be introduced
until Chapter 9: <A NAME="Index468"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>try</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index469"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>finally</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>try</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keyword indicates that the block that follows (delimited by curly braces) is a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>guarded
region
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which means that it is given special treatment. One of these special treatments
is that the code in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>finally</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
clause following this guarded region is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>always</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
executed, no matter how the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>try</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
block exits. (With exception handling, it&#8217;s possible to leave a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>try</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
block in a number of non-ordinary ways.) Here, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>finally</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
clause is saying &#8220;always call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>cleanup(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>x</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
no matter what happens.&#8221; These keywords will be explained thoroughly in
Chapter 9.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that in your cleanup method you must also pay attention to the calling order
for the base-class and member-object cleanup methods in case one subobject
depends on another. In general, you should follow the same form that is imposed
by a C++ compiler on its destructors: First perform all of the work specific to
your class (which might require that base-class elements still be viable) then
call the base-class cleanup method, as demonstrated here.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">There
can be many cases in which the cleanup issue is not a problem; you just let the
garbage collector do the work. But when you must do it explicitly, diligence
and attention is required.
</FONT><a name="_Toc312374020"></a><P></DIV>
<A NAME="Heading188"></A><H4 ALIGN=LEFT>
Order
of garbage collection
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">There&#8217;s
not much you can rely on when it comes to <A NAME="Index470"></A>garbage
collection. The garbage collector might never be called. If it is, it can
reclaim objects in any order it wants. In addition, implementations of the
garbage collector in Java 1.0<A NAME="Index471"></A>
often don&#8217;t call the <A NAME="Index472"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>finalize(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods. It&#8217;s best to not rely on garbage collection for anything but
memory reclamation. If you want cleanup to take place, make your own cleanup
methods and don&#8217;t rely on <A NAME="Index473"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>finalize(&#160;).</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(As mentioned earlier, Java 1.1<A NAME="Index474"></A>
can be forced to call all the finalizers.)
</FONT><a name="_Toc312374021"></a><a name="_Toc375545311"></a><a name="_Toc408018514"></a><P></DIV>
<A NAME="Heading189"></A><H3 ALIGN=LEFT>
Name
hiding
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Only
C++ programmers might be surprised by name hiding, since it works differently
in that language. <A NAME="Index475"></A><A NAME="Index476"></A><A NAME="Index477"></A><A NAME="Index478"></A>If
a Java base class has a method name that&#8217;s overloaded several times,
redefining that method name in the derived class will 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>not
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">hide
any of the base-class versions. Thus overloading works regardless of whether
the method was defined at this level or in a base class:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Hide.java</font>
<font color="#009900">// Overloading a base-class method name</font>
<font color="#009900">// in a derived class does not hide the</font>
<font color="#009900">// base-class versions</font>

<font color="#0000ff">class</font> Homer {
  <font color="#0000ff">char</font> doh(<font color="#0000ff">char</font> c) {
    System.out.println("doh(<font color="#0000ff">char</font>)");
    <font color="#0000ff">return</font> 'd';
  }
  <font color="#0000ff">float</font> doh(<font color="#0000ff">float</font> f) {
    System.out.println("doh(<font color="#0000ff">float</font>)");
    <font color="#0000ff">return</font> 1.0f;
  }
}

<font color="#0000ff">class</font> Milhouse {}

<font color="#0000ff">class</font> Bart <font color="#0000ff">extends</font> Homer {
  <font color="#0000ff">void</font> doh(Milhouse m) {}
}

<font color="#0000ff">class</font> Hide {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Bart b = <font color="#0000ff">new</font> Bart();
    b.doh(1); <font color="#009900">// doh(float) used</font>
    b.doh('x');
    b.doh(1.0f);
    b.doh(<font color="#0000ff">new</font> Milhouse());
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">As
you&#8217;ll see in the next chapter, it&#8217;s far more common to override
methods of the same name using exactly the same signature and return type as in
the base class. It can be confusing otherwise (which is why C++ disallows it,
to prevent you from making what is probably a mistake).
</FONT><a name="_Toc305593255"></a><a name="_Toc305628727"></a><a name="_Toc312374023"></a><a name="_Toc375545312"></a><a name="_Toc408018515"></a><P></DIV>

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