tij313.htm

这也是我们java老师给我们的thinking in java的一些资料

<p>In many situations this syntax provides a more convenient way to write code. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1259" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>The expression:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>a = d;</PRE></FONT></BLOCKQUOTE><p><br></p>
<p>shows how you can take a reference that&#146;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. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1260" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>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. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1261" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h4>
<a name="_Toc375545349"></a><a name="Heading10284"></a>Containers of
primitives<br></h4>
<p><a name="Index974"></a><a name="Index975"></a>Container classes can hold only references to <b>Object</b>s. An array, however, can be created to hold primitives directly, as well as references to <b>Object</b>s. It <i>is</i> possible to use the &#147;wrapper&#148; 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&#146;s much more efficient to create and access an array of primitives than a container of wrapped primitives. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1262" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>Of course, if you&#146;re using a primitive type and you need the flexibility of a container that automatically expands when more space is needed, the array won&#146;t work, and you&#146;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&#146;t provide this for you.<sup><a name="fnB52" href="#fn52">[52]</a></sup> <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1263" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h3>
<a name="_Toc24775736"></a><a name="Heading10288"></a>Returning an array</h3>
<p>Suppose you&#146;re writing a method and you don&#146;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&#146;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 size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1264" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p><a name="Index976"></a><a name="Index977"></a>Java takes a similar approach, but you just &#147;return an array.&#148; Unlike C++, with Java you never worry about responsibility for that array&#151;it will be around as long as you need it, and the garbage collector will clean it up when you&#146;re done. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1265" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>As an example, consider returning an array of <b>String</b>:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c11:IceCream.java</font>
<font color=#009900>// Returning arrays from methods.</font>
<font color=#0000ff>import</font> com.bruceeckel.simpletest.*;
<font color=#0000ff>import</font> java.util.*;

<font color=#0000ff>public</font> <font color=#0000ff>class</font> IceCream {
  <font color=#0000ff>private</font> <font color=#0000ff>static</font> Test monitor = <font color=#0000ff>new</font> Test();
  <font color=#0000ff>private</font> <font color=#0000ff>static</font> Random rand = <font color=#0000ff>new</font> Random();
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>final</font> String[] flavors = {
    <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>public</font> <font color=#0000ff>static</font> String[] flavorSet(<font color=#0000ff>int</font> n) {
    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>[flavors.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 = rand.nextInt(flavors.length);
      <font color=#0000ff>while</font>(picked[t]);
      results[i] = flavors[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(flavors.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]);
      monitor.expect(<font color=#0000ff>new</font> Object[] {
        <font color=#004488>"%% flavorSet\\(\\d+\\) = "</font>,
        <font color=#0000ff>new</font> TestExpression(<font color=#004488>"%% \\t(Chocolate|Strawberry|"</font>
          + <font color=#004488>"Vanilla Fudge Swirl|Mint Chip|Mocha Almond "</font>
          + <font color=#004488>"Fudge|Rum Raisin|Praline Cream|Mud Pie)"</font>, 8)
      });
    }
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>The method <b>flavorSet(&#160;)</b> creates an array of <b>String</b> called <b>results</b>. The size of this array is <b>n</b>, determined by the argument that you pass into the method. Then it proceeds to choose flavors randomly from the array <b>flavors</b> and place them into <b>results</b>, which it finally returns. Returning an array is just like returning any other object&#151;it&#146;s a reference. It&#146;s not important that the array was created within <b>flavorSet(&#160;)</b>, or that the array was created anyplace else, for that matter. The garbage collector takes care of cleaning up the array when you&#146;re done with it, and the array will persist for as long as you need it. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1266" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>As an aside, notice that when <b>flavorSet(&#160;)</b> chooses flavors randomly, it ensures that a particular choice hasn&#146;t already been selected. This is performed in a <b>do</b> loop that keeps making random choices until it finds one not already in the <b>picked</b> array. (Of course, a <b>String</b> comparison also could have been performed to see if the random choice was already in the <b>results</b> array.) If it&#146;s successful, it adds the entry and finds the next one (<b>i </b>gets incremented). <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1267" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p><b>main(&#160;)</b> prints out 20 full sets of flavors, so you can see that <b>flavorSet(&#160;)</b> chooses the flavors in a random order each time. It&#146;s easiest to see this if you redirect the output into a file. And while you&#146;re looking at the file, remember that you just <i>want</i> the ice cream, you don&#146;t <i>need</i> it. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1268" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h3>
<a name="_Toc24775737"></a><a name="Heading10338"></a>The <b>Arrays</b>
class</h3>
<p>In <b>java.util</b>, you&#146;ll find the <a name="Index978"></a><b>Arrays</b> class, which holds a set of <b>static</b> methods that perform utility functions for arrays. There are four basic methods: <b>equals(&#160;)</b>, to compare two arrays for equality; <b>fill(&#160;)</b>, to fill an array with a value; <b>sort(&#160;)</b>, to sort the array; and <b>binarySearch(&#160;)</b>, to find an element in a sorted array. All of these methods are overloaded for all the primitive types and <b>Object</b>s. In addition, there&#146;s a single <b>asList(&#160;)</b> method that takes any array and turns it into a <b>List</b> container, which you&#146;ll learn about later in this chapter. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1269" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>Although useful, the <b>Arrays</b> class stops short of being fully functional. For example, it would be nice to be able to easily print the elements of an array without having to code a <b>for</b> loop by hand every time. And as you&#146;ll see, the <b>fill(&#160;)</b> method only takes a single value and places it in the array, so if you wanted, for example, to fill an array with randomly generated numbers, <b>fill(&#160;)</b> is no help. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1270" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>Thus it makes sense to supplement the <b>Arrays</b> class with some additional utilities, which will be placed in the <b>package</b> <b>com.bruceeckel.util</b> for convenience. These will print an array of any type and fill an array with values or objects that are created by an object called a <i>generator</i> that you can define. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap09_1271" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p><a name="Index979"></a>Because code needs to be created for each primitive type as well as <b>Object</b>, there&#146;s a lot of nearly duplicated code.<sup><a name="fnB53" href="#fn53">[53]</a></sup> For example, a &#147;generator&#148; interface is required for each type because the return type of <b>next(&#160;)</b> must be different in each case: <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]A0125" title="Send BackTalk Comment">Feedback</a></font><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:Generator.java</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>public</font> <font color=#0000ff>interface</font> Generator { Object next(); } <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:BooleanGenerator.java</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>public</font> <font color=#0000ff>interface</font> BooleanGenerator { <font color=#0000ff>boolean</font> next(); } <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:ByteGenerator.java</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>public</font> <font color=#0000ff>interface</font> ByteGenerator { <font color=#0000ff>byte</font> next(); } <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:CharGenerator.java</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>public</font> <font color=#0000ff>interface</font> CharGenerator { <font color=#0000ff>char</font> next(); } <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:ShortGenerator.java</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>public</font> <font color=#0000ff>interface</font> ShortGenerator { <font color=#0000ff>short</font> next(); } <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:IntGenerator.java</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>public</font> <font color=#0000ff>interface</font> IntGenerator { <font color=#0000ff>int</font> next(); } <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:LongGenerator.java</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>public</font> <font color=#0000ff>interface</font> LongGenerator { <font color=#0000ff>long</font> next(); } <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:FloatGenerator.java</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>public</font> <font color=#0000ff>interface</font> FloatGenerator { <font color=#0000ff>float</font> next(); } <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:DoubleGenerator.java</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>public</font> <font color=#0000ff>interface</font> DoubleGenerator { <font color=#0000ff>double</font> next(); } <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p><a name="Index980"></a><b>Arrays2</b> contains a variety of <b>toString(&#160;)</b> methods, overloaded for each type. These methods allow you to easily print an array. The <b>toString(&#160;)</b> code introduces the use of <b>StringBuffer</b> instead of <b>String</b> objects. This is a nod to efficiency; when you&#146;re assembling a string in a method that might be called a lot, it&#146;s wiser to use the more efficient <b>StringBuffer</b> rather than the more convenient <b>String</b> operations. Here, the <b>StringBuffer</b> is created with an initial value, and <b>Strings</b> are appended. Finally, the <b>result</b> is converted to a <b>String</b> as the return value: <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]A0126" title="Send BackTalk Comment">Feedback</a></font><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: com:bruceeckel:util:Arrays2.java</font>
<font color=#009900>// A supplement to java.util.Arrays, to provide additional</font>
<font color=#009900>// useful functionality when working with arrays. Allows</font>
<font color=#009900>// any array to be converted to a String, and to be filled</font>
<font color=#009900>// via a user-defined "generator" object.</font>
<font color=#0000ff>package</font> com.bruceeckel.util;
<font color=#0000ff>import</font> java.util.*;

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Arrays2 {
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> String toString(<font color=#0000ff>boolean</font>[] a) {
    StringBuffer result = <font color=#0000ff>new</font> StringBuffer(<font color=#004488>"["</font>);
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; a.length; i++) {
      result.append(a[i]);
      <font color=#0000ff>if</font>(i &lt; a.length - 1)
        result.append(<font color=#004488>", "</font>);
    }
    result.append(<font color=#004488>"]"</font>);
    <font color=#0000ff>return</font> result.toString();
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> String toString(<font color=#0000ff>byte</font>[] a) {
    StringBuffer result = <font color=#0000ff>new</font> StringBuffer(<font color=#004488>"["</font>);
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; a.length; i++) {
      result.append(a[i]);
      <font color=#0000ff>if</font>(i &lt; a.length - 1)