chap09.htm

java书籍《thinking in java》

      <font color=#0000ff>new</font> Weeble(), <font color=#0000ff>new</font> Weeble(), <font color=#0000ff>new</font> Weeble()
    };
    <font color=#009900>// Dynamic aggregate initialization:</font>
    a = <font color=#0000ff>new</font> Weeble[] {
      <font color=#0000ff>new</font> Weeble(), <font color=#0000ff>new</font> Weeble()
    };
    System.out.println(<font color=#004488>"a.length="</font> + a.length);
    System.out.println(<font color=#004488>"b.length = "</font> + b.length);
    <font color=#009900>// The references inside the array are </font>
    <font color=#009900>// automatically initialized to null:</font>
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; b.length; i++)
      System.out.println(<font color=#004488>"b["</font> + i + <font color=#004488>"]="</font> + b[i]);
    System.out.println(<font color=#004488>"c.length = "</font> + c.length);
    System.out.println(<font color=#004488>"d.length = "</font> + d.length);
    a = d;
    System.out.println(<font color=#004488>"a.length = "</font> + a.length);

    <font color=#009900>// Arrays of primitives:</font>
    <font color=#0000ff>int</font>[] e; <font color=#009900>// Null reference</font>
    <font color=#0000ff>int</font>[] f = <font color=#0000ff>new</font> <font color=#0000ff>int</font>[5];
    <font color=#0000ff>int</font>[] g = <font color=#0000ff>new</font> <font color=#0000ff>int</font>[4];
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; g.length; i++)
      g[i] = i*i;
    <font color=#0000ff>int</font>[] h = { 11, 47, 93 };
    <font color=#009900>// Compile error: variable e not initialized:</font>
    <font color=#009900>//!System.out.println("e.length=" + e.length);</font>
    System.out.println(<font color=#004488>"f.length = "</font> + f.length);
    <font color=#009900>// The primitives inside the array are</font>
    <font color=#009900>// automatically initialized to zero:</font>
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; f.length; i++)
      System.out.println(<font color=#004488>"f["</font> + i + <font color=#004488>"]="</font> + f[i]);
    System.out.println(<font color=#004488>"g.length = "</font> + g.length);
    System.out.println(<font color=#004488>"h.length = "</font> + h.length);
    e = h;
    System.out.println(<font color=#004488>"e.length = "</font> + e.length);
    e = <font color=#0000ff>new</font> <font color=#0000ff>int</font>[] { 1, 2 };
    System.out.println(<font color=#004488>"e.length = "</font> + e.length);
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Here&#8217;s the output from the
program:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>b.length = 5
b[0]=<font color=#0000ff>null</font>
b[1]=<font color=#0000ff>null</font>
b[2]=<font color=#0000ff>null</font>
b[3]=<font color=#0000ff>null</font>
b[4]=<font color=#0000ff>null</font>
c.length = 4
d.length = 3
a.length = 3
a.length = 2
f.length = 5
f[0]=0
f[1]=0
f[2]=0
f[3]=0
f[4]=0
g.length = 4
h.length = 3
e.length = 3
e.length = 2</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The array <B>a</B> is initially just a
<A NAME="Index865"></A><B>null </B>reference, and the compiler prevents you from
doing anything with this reference until you&#8217;ve properly initialized it.
The array <B>b</B> is initialized to point to an array of <B>Weeble</B>
references, but no actual <B>Weeble</B> objects are ever placed in that array.
However, you can still ask what the size of the array is, since <B>b</B> is
pointing to a legitimate object. This brings up a slight drawback: you
can&#8217;t find out how many elements are actually <I>in</I> the array, since
<B>length</B> tells you only how many elements <I>can</I> be placed in the
array; that is, the size of the array object, not the number of elements it
actually holds. However, when an array object is created its references are
automatically initialized to <B>null</B>, so you can see whether a particular
array slot has an object in it by checking to see whether it&#8217;s
<B>null</B>. Similarly, an array of primitives is automatically initialized to
zero for numeric types, <B>(char)0 </B>for <B>char</B>, and<B> false</B> for
<B>boolean</B>. 
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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Array <B>c</B> shows the creation of the
array object followed by the assignment of <B>Weeble</B> objects to all the
slots in the array. Array <B>d</B> shows the &#8220;aggregate
initialization&#8221; syntax that causes the array object to be created
(implicitly with <B>new</B> on the heap, just like for array <B>c</B>)
<I>and</I> initialized with <B>Weeble</B> objects, all in one statement.

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</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><A NAME="Index866"></A><A NAME="Index867"></A><FONT FACE="Georgia">The
next array initialization could be thought of as a &#8220;dynamic aggregate
initialization.&#8221; The aggregate initialization used by <B>d</B> must be
used at the point of <B>d</B>&#8217;s definition, but with the second syntax you
can create and initialize an array object anywhere. For example, suppose
<B>hide(&#160;)</B> is a method that takes an array of <B>Weeble</B> objects.
You could call it by saying:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>hide(d);</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">but you can also dynamically create the
array you want to pass as the argument:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>hide(<font color=#0000ff>new</font> Weeble[] { <font color=#0000ff>new</font> Weeble(), <font color=#0000ff>new</font> Weeble() });</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">In some situations this new syntax
provides a more convenient way to write code.

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</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The expression:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>a = d;</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">shows how you can take a reference
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 reference. Now both <B>a</B>
and <B>d</B> are pointing to the same array object on the heap.

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The second part of <B>ArraySize.java</B>
shows that primitive arrays work just like object arrays <I>except</I> that
primitive arrays hold the primitive values directly.

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</FONT><A NAME="_Toc375545349"></A><BR></P></DIV>
<A NAME="Heading279"></A><FONT FACE = "Verdana"><H4 ALIGN="LEFT">
Containers of primitives<BR><A NAME="Index868"></A><A NAME="Index869"></A></H4></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Container classes can hold only
references to objects. An array, however, can be created to hold primitives
directly, as well as references to objects. It <I>is</I> possible to use the
&#8220;wrapper&#8221; classes such as <B>Integer</B>, <B>Double,</B> etc. to
place primitive values inside a container, but the wrapper classes for
primitives can be awkward to use. In addition, it&#8217;s much more efficient to
create and access an array of primitives than a container of wrapped primitives.

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</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Of course, if you&#8217;re using a
primitive type and you need the flexibility of a container that automatically
expands when more space is needed, the array won&#8217;t work and you&#8217;re
forced to use a container of wrapped primitives. You might think that there
should be a specialized type of <B>ArrayList</B> 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="fnB45" HREF="#fn45">[45]</A><FONT FACE="Georgia">

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</FONT><A NAME="_Toc481064667"></A><BR></P></DIV>
<A NAME="Heading280"></A><FONT FACE = "Verdana"><H3 ALIGN="LEFT">
Returning an array</H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Suppose you&#8217;re writing a method and
you don&#8217;t just want to return just 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. 
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</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><A NAME="Index870"></A><A NAME="Index871"></A><FONT FACE="Georgia">Java
takes a similar approach, but you just &#8220;return an array.&#8221; Actually,
of course, you&#8217;re returning a reference to an array, but with Java you
never worry about responsibility for that array&#8212;it will be around as long
as you need it, and the garbage collector will clean it up when you&#8217;re
done. 
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</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">As an example, consider returning an
array of <B>String</B>:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c09: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 = {
    <font color=#004488>"Chocolate"</font>, <font color=#004488>"Strawberry"</font>,
    <font color=#004488>"Vanilla Fudge Swirl"</font>, <font color=#004488>"Mint Chip"</font>,
    <font color=#004488>"Mocha Almond Fudge"</font>, <font color=#004488>"Rum Raisin"</font>,
    <font color=#004488>"Praline Cream"</font>, <font color=#004488>"Mud Pie"</font> 
  };
  <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>boolean</font>[] picked = 
      <font color=#0000ff>new</font> <font color=#0000ff>boolean</font>[flav.length];
    <font color=#0000ff>for</font> (<font color=#0000ff>int</font> i = 0; i &lt; n; i++) {
      <font color=#0000ff>int</font> t;
      <font color=#0000ff>do</font> 
        t = (<font color=#0000ff>int</font>)(Math.random() * flav.length);
      <font color=#0000ff>while</font> (picked[t]);
      results[i] = flav[t];
      picked[t] = <font color=#0000ff>true</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(
        <font color=#004488>"flavorSet("</font> + i + <font color=#004488>") = "</font>);
      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(<font color=#004488>"\t"</font> + fl[j]);
    }
  }