tij0038.html

学习java的经典书籍

Date());
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Consider
the argument: a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Date</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object is being created just to send its value to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>println(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
As soon as this statement is finished, that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Date</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is unnecessary, and the garbage collector can come along and get it anytime. We
don&#8217;t need to worry about cleaning it up.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
second line calls 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>System.getProperties(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If you consult the online documentation using your Web browser, you&#8217;ll
see that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getProperties(&#160;)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">is
a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">method
of class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>System</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Because it&#8217;s 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you don&#8217;t need to create any objects in order to call the method; a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">method
is always available whether an object of its class exists or not. When you call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getProperties(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
it
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">produces
the system properties as an object of class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Properties</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The handle that comes back is stored in a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Properties</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handle called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>p</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
In line three, you can see that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Properties</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object has a method called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>list(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that sends its entire contents to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PrintStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object that you pass as an argument.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
fourth and sixth lines 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">
are typical print statements. Note that to print multiple 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
values, we simply separate them with &#8216;
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;
signs. However, there&#8217;s something strange going on here. The &#8216;
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;
sign doesn&#8217;t mean addition when it&#8217;s used with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects. Normally, you wouldn&#8217;t ascribe any meaning to &#8216;
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;
when you think of strings. However, the Java 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class is blessed with something called &#8220;operator overloading.&#8221; That
is, the &#8216;
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;
sign, only when used with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects, behaves differently from the way it does with everything else. For 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s,
it means &#8220;concatenate these two strings.&#8221;
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">But
that&#8217;s not all. If you look at the statement:
</FONT><P></DIV>

<font color="#990000"><PRE>    System.out.println("Total Memory = "
                       + rt.totalMemory()
                       + " Free Memory = "
                       + rt.freeMemory()); </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>totalMemory(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>freeMemory(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
return 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>numerical
values
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and not 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects. What happens when you &#8220;add&#8221; a numerical value 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">?
The compiler sees the problem and magically calls a method that turns that
numerical value (
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>float</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
etc.) into a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which can then be &#8220;added&#8221; with the plus sign. This 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>automatic
type conversion
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
also falls into the category of operator overloading.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Much
of the Java literature states vehemently that operator overloading (a feature
in C++) is bad, and yet here it is! However, this is wired into the compiler,
only for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects, and you can&#8217;t overload operators for any of the code you write.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
fifth line 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">
creates a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runtime</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object by calling the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>static</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>getRuntime(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for the class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runtime</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
What&#8217;s returned is a handle to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runtime</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object; whether this is a static object or one created with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
doesn&#8217;t need to concern you, since you can use the objects without
worrying about who&#8217;s responsible for cleaning them up. As shown, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runtime</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object can tell you information about memory usage.
</FONT><a name="_Toc375545234"></a><a name="_Toc408018435"></a><P></DIV>
<HR><DIV ALIGN=LEFT><A NAME="fn15" HREF="#fnB15">[15]</A><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
Some programming environments will flash programs up on the screen and close
them before you've had a chance to see the results. You can put in the
following bit of code at the end of 
</FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
to pause the output:
</FONT><P></DIV>

<font color="#990000"><PRE>  <font color="#0000ff">try</font> {
      Thread.currentThread().sleep(5 * 1000);
    } <font color="#0000ff">catch</font>(InterruptedException e) {}</PRE></font><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=2 COLOR="Black">
 }
</FONT></TT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">This
will pause the output for five seconds. This code involves concepts that will
not be introduced until much later in the book, so you won&#8217;t understand
it until then, but it will do the trick.
</FONT><P></DIV>


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