tij310.htm

这也是我们java老师给我们的thinking in java的一些资料

<font color=#0000ff>class</font> Parcel3 {
  <font color=#0000ff>private</font> <font color=#0000ff>class</font> PContents <font color=#0000ff>implements</font> Contents {
    <font color=#0000ff>private</font> <font color=#0000ff>int</font> i = 11;
    <font color=#0000ff>public</font> <font color=#0000ff>int</font> value() { <font color=#0000ff>return</font> i; }
  }
  <font color=#0000ff>protected</font> <font color=#0000ff>class</font> PDestination <font color=#0000ff>implements</font> Destination {
    <font color=#0000ff>private</font> String label;
    <font color=#0000ff>private</font> PDestination(String whereTo) {
      label = whereTo;
    }
    <font color=#0000ff>public</font> String readLabel() { <font color=#0000ff>return</font> label; }
  }
  <font color=#0000ff>public</font> Destination dest(String s) {
    <font color=#0000ff>return</font> <font color=#0000ff>new</font> PDestination(s);
  }
  <font color=#0000ff>public</font> Contents cont() {
    <font color=#0000ff>return</font> <font color=#0000ff>new</font> PContents();
  }
}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> TestParcel {
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    Parcel3 p = <font color=#0000ff>new</font> Parcel3();
    Contents c = p.cont();
    Destination d = p.dest(<font color=#004488>"Tanzania"</font>);
    <font color=#009900>// Illegal -- can't access private class:</font>
    <font color=#009900>//! Parcel3.PContents pc = p.new PContents();</font>
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>In the example, <b>main(&#160;) </b>must be in a separate class in order to demonstrate the privateness of the inner class <b>PContents</b>. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1146" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>In <b>Parcel3</b>, something new has been added: The inner class <b>PContents</b> is <b>private</b>,<b> </b>so no one but <b>Parcel3</b> can access it. <b>PDestination</b> is <b>protected</b>, so no one but <b>Parcel3</b>, classes in the same package (since <b>protected</b> also gives package access), and the inheritors of <b>Parcel3 </b>can access <b>PDestination</b>. This means that the client programmer has restricted knowledge and access to these members. In fact, you can&#146;t even downcast to a <b>private</b> inner class (or a <b>protected</b> inner class unless you&#146;re an inheritor), because you can&#146;t access the name, as you can see in <b>class TestParcel</b>. Thus, the <b>private</b> inner class provides a way for the class designer to completely prevent any type-coding dependencies and to completely hide details about implementation. In addition, extension of an <b>interface</b> is useless from the client programmer&#146;s perspective since the client programmer cannot access any additional methods that aren&#146;t part of the <b>public</b> <b>interface</b>. This also provides an opportunity for the Java compiler to generate more efficient code. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1147" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>Normal (non-inner) classes cannot be made <b>private</b> or <b>protected</b>; they may only be given <b>public</b> or package access. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1148" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h3>
<a name="_Toc24775682"></a><a name="Heading7358"></a>Inner classes <br>in
methods and scopes<br></h3>
<p><a name="Index723"></a><a name="Index724"></a><a name="Index725"></a><a name="Index726"></a>What you&#146;ve seen so far encompasses the typical use for inner classes. In general, the code that you&#146;ll write and read involving inner classes will be &#147;plain&#148; inner classes that are simple and easy to understand. However, the design for inner classes is quite complete, and there are a number of other, more obscure, ways that you can use them if you choose; inner classes can be created within a method or even an arbitrary scope. There are two reasons for doing this: <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1149" title="Send BackTalk Comment">Feedback</a></font><br></p>
<ol>
<li>As shown previously, you&#146;re implementing an interface of some kind so
that you can create and return a reference. <font size="-2"><a
href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1150" title="Send BackTalk
Comment">Feedback</a></font></li>
<li>You&#146;re solving a complicated problem and you want to create a class to
aid in your solution, but you don&#146;t want it publicly available. <font
size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1151"
title="Send BackTalk
Comment">Feedback</a></font></li></ol><p>In the following examples, the previous code will be modified to use:  <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1152" title="Send BackTalk Comment">Feedback</a></font><br></p>
<ol>
<li><a name="Index727"></a><a name="Index728"></a><a name="Index729"></a>A class
defined within a method </li>
<li>A class defined within a scope inside a method</li>
<li>An anonymous class implementing an interface</li>
<li>An anonymous class extending a class that has a nondefault constructor</li>
<li>An anonymous class that performs field initialization</li>
<li>An anonymous class that performs construction using instance initialization
(anonymous inner classes cannot have
constructors)</li></ol><p>Although it&#146;s an ordinary class with an implementation, <b>Wrapping</b> is also being used as a common &#147;interface&#148; to its derived classes:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c08:Wrapping.java</font>
<font color=#0000ff>public</font> <font color=#0000ff>class</font> Wrapping {
  <font color=#0000ff>private</font> <font color=#0000ff>int</font> i;
  <font color=#0000ff>public</font> Wrapping(<font color=#0000ff>int</font> x) { i = x; }
  <font color=#0000ff>public</font> <font color=#0000ff>int</font> value() { <font color=#0000ff>return</font> i; }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>You&#146;ll notice that <b>Wrapping</b> has a constructor that requires an argument, to make things a bit more interesting. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1153" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>The first example shows the creation of an entire class within the scope of a method (instead of the scope of another class). This is called a <i>local inner class</i>:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c08:Parcel4.java</font>
<font color=#009900>// Nesting a class within a method.</font>

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Parcel4 {
  <font color=#0000ff>public</font> Destination dest(String s) {
    <font color=#0000ff>class</font> PDestination <font color=#0000ff>implements</font> Destination {
      <font color=#0000ff>private</font> String label;
      <font color=#0000ff>private</font> PDestination(String whereTo) {
        label = whereTo;
      }
      <font color=#0000ff>public</font> String readLabel() { <font color=#0000ff>return</font> label; }
    }
    <font color=#0000ff>return</font> <font color=#0000ff>new</font> PDestination(s);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    Parcel4 p = <font color=#0000ff>new</font> Parcel4();
    Destination d = p.dest(<font color=#004488>"Tanzania"</font>);
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>The class <b>PDestination</b> is part of <b>dest(&#160;)</b> rather than being part of <b>Parcel4</b>. (Also notice that you could use the class identifier <b>PDestination </b>for an inner class inside each class in the same subdirectory without a name clash.) Therefore, <b>PDestination </b>cannot be accessed outside of <b>dest(&#160;)</b>.<b> </b>Notice the upcasting that occurs in the return statement&#151;nothing comes out of <b>dest(&#160;)</b> except a reference to <b>Destination</b>, the base class. Of course, the fact that the name of the class <b>PDestination</b> is placed inside <b>dest(&#160;)</b> doesn&#146;t mean that <b>PDestination</b> is not a valid object once <b>dest(&#160;)</b> returns. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1154" title="Send BackTalk Comment">Feedback</a></font><br></p>
<p>The next example shows how you can nest an inner class within any arbitrary scope:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c08:Parcel5.java</font>
<font color=#009900>// Nesting a class within a scope.</font>

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Parcel5 {
  <font color=#0000ff>private</font> <font color=#0000ff>void</font> internalTracking(<font color=#0000ff>boolean</font> b) {
    <font color=#0000ff>if</font>(b) {
      <font color=#0000ff>class</font> TrackingSlip {
        <font color=#0000ff>private</font> String id;
        TrackingSlip(String s) {
          id = s;
        }
        String getSlip() { <font color=#0000ff>return</font> id; }
      }
      TrackingSlip ts = <font color=#0000ff>new</font> TrackingSlip(<font color=#004488>"slip"</font>);
      String s = ts.getSlip();
    }
    <font color=#009900>// Can't use it here! Out of scope:</font>
    <font color=#009900>//! TrackingSlip ts = new TrackingSlip("x");</font>
  }
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> track() { internalTracking(<font color=#0000ff>true</font>); }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    Parcel5 p = <font color=#0000ff>new</font> Parcel5();
    p.track();
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>The class <b>TrackingSlip</b> is nested inside the scope of an <b>if</b> statement. This does not mean that the <i>class</i> is conditionally created&#151;it gets compiled along with everything else. However, it&#146;s not available outside the scope in which it is defined.<b> </b>Other than that, it looks just like an ordinary class. <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1155" title="Send BackTalk Comment">Feedback</a></font><br></p>
<h3>
<a name="_Toc24775683"></a><a name="Heading7428"></a>Anonymous inner
classes</h3>
<p>The next example looks a little strange:<br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c08:Parcel6.java</font>
<font color=#009900>// A method that returns an anonymous inner class.</font>

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Parcel6 {
  <font color=#0000ff>public</font> Contents cont() {
    <font color=#0000ff>return</font> <font color=#0000ff>new</font> Contents() {
      <font color=#0000ff>private</font> <font color=#0000ff>int</font> i = 11;
      <font color=#0000ff>public</font> <font color=#0000ff>int</font> value() { <font color=#0000ff>return</font> i; }
    }; <font color=#009900>// Semicolon required in this case</font>
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    Parcel6 p = <font color=#0000ff>new</font> Parcel6();
    Contents c = p.cont();
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE><p><br></p>
<p>The <b>cont(&#160;)</b> method combines the creation of the return value with the definition of the class that represents that return value! In addition, the class is anonymous; it has no name. To make matters a bit worse, it looks like you&#146;re starting out to create a <b>Contents</b> object: <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1156" title="Send BackTalk Comment">Feedback</a></font><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#0000ff>return</font> <font color=#0000ff>new</font> Contents()</PRE></FONT></BLOCKQUOTE><p><br></p>
<p>But then, before you get to the semicolon, you say, &#147;But wait, I think I&#146;ll slip in a class definition&#148;: <font size="-2"><a href="mailto:[e-mail]?Subject=[TIJ3]Chap08_1157" title="Send BackTalk Comment">Feedback</a></font><br></p>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#0000ff>return</font> <font color=#0000ff>new</font> Contents() {
  <font color=#0000ff>private</font> <font col