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学习java的经典书籍

and
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
false
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean</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">Array
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>c</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
shows the creation of the array object followed by the assignment of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Weeble</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects to all the slots in the array. Array 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>d</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
shows the &#8220;aggregate initialization&#8221; syntax that causes the array
object to be created (implicitly with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
on the heap, just like for array 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>c</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>and</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
initialized with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Weeble</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects, all in one statement.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
expression
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">a
= d;
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">shows
how you can take a handle that&#8217;s attached to one array object and assign
it to another array object, just as you can do with any other type of object
handle. Now both 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>a</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>d</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are pointing to the same array object on the heap.
</FONT><P></DIV><DIV ALIGN=LEFT><A NAME="Index734"></A><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Java
1.1 adds a new array initialization syntax, which could be thought of as a
&#8220;dynamic aggregate initialization.&#8221; The Java 1.0<A NAME="Index735"></A>
aggregate initialization used by 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>d</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
must be used at the point of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>d</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
definition, but with the Java 1.1 syntax you can create and initialize an array
object anywhere. For example, suppose 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>hide(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is a method that takes an array of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Weeble</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects. You could call it by saying:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">hide(d);</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">but
in Java 1.1 you can also dynamically create the array you want to pass as the
argument:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">hide(new
Weeble[] { new Weeble(), new Weeble() });
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
new syntax provides a more convenient way to write code in some situations.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
second part of the above example shows that primitive arrays work just like
object arrays 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>except</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that primitive arrays hold the primitive values directly.
</FONT><a name="_Toc375545349"></a><P></DIV>
<A NAME="Heading243"></A><H4 ALIGN=LEFT>
Collections
of primitives
<P><A NAME="Index736"></A><A NAME="Index737"></A></H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Collection
classes can hold only handles to objects. An array, however, can be created to
hold primitives directly, as well as handles to objects. It 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>is</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
possible to use the &#8220;wrapper&#8221; classes such as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Integer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Double,</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
etc. to place primitive values inside a collection, but as you&#8217;ll see
later in this chapter in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>WordCount.java</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
example, the wrapper classes for primitives are only somewhat useful anyway.
Whether you put primitives in arrays or wrap them in a class that&#8217;s
placed in a collection is a question of efficiency. It&#8217;s much more
efficient to create and access an array of primitives than a collection of
wrapped primitives.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Of
course, if you&#8217;re using a primitive type and you need the flexibility of
a collection that automatically expands when more space is needed, the array
won&#8217;t work and you&#8217;re forced to use a collection of wrapped
primitives. You might think that there should be a specialized type of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for each of the primitive data types, but Java doesn&#8217;t provide this for
you. Some sort of templatizing mechanism might someday provide a better way for
Java to handle this problem.
</FONT><A NAME="fnB32" HREF="#fn32">[32]</A><a name="_Toc408018565"></a><P></DIV>
<A NAME="Heading244"></A><H3 ALIGN=LEFT>
Returning
an array
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Suppose
you&#8217;re writing a method and you don&#8217;t just want to return one
thing, but a whole bunch of things. Languages like C and C++ make this
difficult because you can&#8217;t just return an array, only a pointer to an
array. This introduces problems because it becomes messy to control the
lifetime of the array, which easily leads to memory leaks.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Java
takes a similar approach, but you just &#8220;return an array.&#8221; Actually,
of course, you&#8217;re returning a handle to an array, but with Java you never
worry about responsibility for that array &#8211; it will be around as long as
you need it, and the garbage collector will clean it up when you&#8217;re done.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">As
an example, consider returning an array of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: IceCream.java</font>
<font color="#009900">// Returning arrays from methods</font>

<font color="#0000ff">public</font> <font color="#0000ff">class</font> IceCream {
  <font color="#0000ff">static</font> String[] flav = {
    "Chocolate", "Strawberry",
    "Vanilla Fudge Swirl", "Mint Chip",
    "Mocha Almond Fudge", "Rum Raisin",
    "Praline Cream", "Mud Pie" 
  };
  <font color="#0000ff">static</font> String[] flavorSet(<font color="#0000ff">int</font> n) {
    <font color="#009900">// Force it to be positive &amp; within bounds:</font>
    n = Math.abs(n) % (flav.length + 1);
    String[] results = <font color="#0000ff">new</font> String[n];
    <font color="#0000ff">int</font>[] picks = <font color="#0000ff">new</font> <font color="#0000ff">int</font>[n];
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; picks.length; i++)
      picks[i] = -1;
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; picks.length; i++) {
      retry:
      <font color="#0000ff">while</font>(<font color="#0000ff">true</font>) {
        <font color="#0000ff">int</font> t =
          (<font color="#0000ff">int</font>)(Math.random() * flav.length);
        <font color="#0000ff">for</font>(<font color="#0000ff">int</font> j = 0; j &lt; i; j++)
          <font color="#0000ff">if</font>(picks[j] == t) <font color="#0000ff">continue</font> retry;
        picks[i] = t;
        results[i] = flav[t];
        <font color="#0000ff">break</font>;
      }
    }
    <font color="#0000ff">return</font> results;
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; 20; i++) {
      System.out.println(
        "flavorSet(" + i + ") = ");
      String[] fl = flavorSet(flav.length);
      <font color="#0000ff">for</font>(<font color="#0000ff">int</font> j = 0; j &lt; fl.length; j++)
        System.out.println("\t" + fl[j]);
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>flavorSet(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
creates an array of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>results</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The size of this array is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>n</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
determined by the argument you pass into the method. Then it proceeds to choose
flavors randomly from the array 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>flav</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and place them into 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>results</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which it finally returns. Returning an array is just like returning any other
object &#8211; it&#8217;s a handle. It&#8217;s not important that the array was
created within 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>flavorSet(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
or that the array was created anyplace else, for that matter. The garbage
collector takes care of cleaning up the array when you&#8217;re done with it,
and the array will persist for as long as you need it.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">As
an aside, notice that when 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>flavorSet(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
chooses flavors randomly, it ensures that a random choice hasn&#8217;t been
picked before. This is performed in a seemingly infinite 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>while</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
loop that keeps making random choices until it finds one that&#8217;s not
already in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>picks</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
array. (Of course, a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
comparison could also have been performed to see if the random choice was
already in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>results</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
array, but 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
comparisons are inefficient.) If it&#8217;s successful it adds the entry and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>break</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
out to go find the next one (
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">gets
incremented). But if 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>t</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is a number that&#8217;s already in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>picks</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
then a labeled 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>continue</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is used to jump back two levels, which forces a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>t</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to be selected. It&#8217;s particularly convincing to watch this happen with a
debugger.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
prints out 20 full sets of flavors, so you can see that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>flavorSet(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
chooses the flavors in a random order each time. It&#8217;s easiest to see this
if you redirect the output into a file. And while you&#8217;re looking at the
file, remember, you&#8217;re not really hungry. (You just 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>want</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the ice cream, you don&#8217;t 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>need</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
it.)
</FONT><a name="_Toc375545350"></a><a name="_Toc408018566"></a><P></DIV>
<HR><DIV ALIGN=LEFT><A NAME="fn32" HREF="#fnB32">[32]</A><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
This is one of the places where C++ is distinctly superior to Java, since C++
supports 
</FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black"><I>parameterized
types
</I></FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
with the 
</FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black"><B>template</B></FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
keyword.
</FONT><P></DIV>


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