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学习java的经典书籍

is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>protected</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so no one but 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Parcel3</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
classes in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Parcel3</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
package (since 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>protected</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
also gives package access; that is, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>protected</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is also &#8220;friendly&#8221;), and the inheritors of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Parcel3
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">can
access 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PDestination</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This means that the client programmer has restricted knowledge and access to
these members. In fact, you can&#8217;t even downcast to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
inner class (or a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>protected</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
inner class unless you&#8217;re an inheritor), because you can&#8217;t access
the name, as you can see in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>class
Test
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Thus, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is useless from the client programmer&#8217;s perspective since the client
programmer cannot access any additional methods that aren&#8217;t part of the
public 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class. This also provides an opportunity for the Java compiler to generate more
efficient code.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Normal
(non-inner) classes cannot be made 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>private</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
or 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>protected
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8211;
only 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>public</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
or &#8220;friendly.&#8221;
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Contents</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
doesn&#8217;t need to be an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class. You could use an ordinary class here as well, but the most typical
starting point for such a design is an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><a name="_Toc408018545"></a><P></DIV>
<A NAME="Heading223"></A><H3 ALIGN=LEFT>
Inner
classes in methods and scopes
<P><A NAME="Index621"></A><A NAME="Index622"></A><A NAME="Index623"></A><A NAME="Index624"></A></H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">What
you&#8217;ve seen so far encompasses the typical use for inner classes. In
general, the code that you&#8217;ll write and read involving inner classes will
be &#8220;plain&#8221; 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><P></DIV>
<OL>
<LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	As
shown previously, you&#8217;re implementing an interface of some kind so that
you can create and return a handle.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	You&#8217;re
solving a complicated problem and you want to create a class to aid in your
solution, but you don&#8217;t want it publicly available.
</FONT></OL><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
the following examples, the previous code will be modified to use:
</FONT><P></DIV>
<OL>
<LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	A
class defined within a method 
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	A
class defined within a scope inside a method
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	An
<A NAME="Index625"></A><A NAME="Index626"></A>anonymous
class implementing an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	An
anonymous class extending a class that has a non-default constructor
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	An
anonymous class that performs field initialization
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	An
anonymous class that performs <A NAME="Index627"></A>construction
using instance initialization (anonymous inner classes cannot have constructors)
</FONT></OL><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
will all take place within the package 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>innerscopes</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
First, the common interfaces from the previous code will be defined in their
own files so they can be used in all the examples:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Destination.java</font>
<font color="#0000ff">package</font> c07.innerscopes;

<font color="#0000ff">interface</font> Destination {
  String readLabel();
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
point has been made that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Contents</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
could be an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>abstract</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class, so here it will be in a more natural form, as an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Contents.java</font>
<font color="#0000ff">package</font> c07.innerscopes;

<font color="#0000ff">interface</font> Contents {
  <font color="#0000ff">int</font> value();
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Although
it&#8217;s an ordinary class with an implementation, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wrapping</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is also being used as a common &#8220;interface&#8221; to its derived classes:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Wrapping.java</font>
<font color="#0000ff">package</font> c07.innerscopes;

<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">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You&#8217;ll
notice above that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Wrapping</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
has a constructor that requires an argument, to make things a bit more
interesting.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
first example shows the creation of an entire class within the scope of a
method (instead of the scope of another class):
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Parcel4.java</font>
<font color="#009900">// Nesting a class within a method</font>
<font color="#0000ff">package</font> c07.innerscopes;

<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("Tanzania");
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PDestination</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is part of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>dest(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
rather than being part of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Parcel4.</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(Also notice that you could use the class identifier 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PDestination
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">for
an inner class inside each class in the same subdirectory without a name
clash.) Therefore, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PDestination
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">cannot
be accessed outside of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>dest(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Notice
the upcasting that occurs in the return statement &#8211; nothing comes out of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>dest(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
except a handle to the base class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Destination</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Of course, the fact that the name of the class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PDestination</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is placed inside 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>dest(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
doesn&#8217;t mean that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>PDestination</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is not a valid object once 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>dest(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
returns.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
next example shows how you can nest an <A NAME="Index628"></A><A NAME="Index629"></A><A NAME="Index630"></A>inner
class within any arbitrary scope:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Parcel5.java</font>
<font color="#009900">// Nesting a class within a scope</font>
<font color="#0000ff">package</font> c07.innerscopes;

<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("slip");
      String s = ts.getSlip();