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📁 学习java的经典书籍
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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<H2 ALIGN=LEFT>
Enumerators
(iterators)
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
any collection class, you must have a way to put things in and a way to get
things out. After all, that&#8217;s the primary job of a collection &#8211; to
hold things. In the <A NAME="Index762"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>addElement(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is the way that you insert objects, and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>elementAt(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>one</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
way to get things out. 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is quite flexible &#8211; you can select anything at any time, and select
multiple elements at once using different indexes.<A NAME="Index763"></A><A NAME="Index764"></A></FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you want to start thinking at a higher level, there&#8217;s a drawback: you
need to know the exact type of the collection in order to use it. This might
not seem bad at first, but what if you start out using a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and later on in your program you decide, for efficiency, that you want to
change to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>List</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(which is part of the Java 1.2<A NAME="Index765"></A>
collections library)? Or you&#8217;d like to write a piece of code that
doesn&#8217;t know or care what type of collection it&#8217;s working with.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
concept of an <A NAME="Index766"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>iterator
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">can
be used to achieve this next level of abstraction. This is an object whose job
is to move through a sequence of objects and select each object in that
sequence without the client programmer knowing or caring about the underlying
structure of that sequence. In addition, an iterator is usually what&#8217;s
called a &#8220;light-weight&#8221; object; that is, one that&#8217;s cheap to
create. For that reason, you&#8217;ll often find seemingly strange constraints
for iterators; for example, some iterators can move in only one direction.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
Java 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Enumeration</B></FONT><A NAME="fnB33" HREF="#fn33">[33]</A><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is an example of an iterator with these kinds of constraints. There&#8217;s not
much you can do with one except:
</FONT><P></DIV>
<OL>
<LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Ask
a collection to hand you an <A NAME="Index767"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Enumeration
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">using
a method called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>elements(&#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">This
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Enumeration
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">will
be ready to return the first element in the sequence on your first call to its 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>nextElement(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Get
the next object in the sequence with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>nextElement(&#160;).</B></FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	See
if there 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>are</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
any more objects in the sequence with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>hasMoreElements(&#160;).</B></FONT></OL><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">That&#8217;s
all. It&#8217;s a simple implementation of an iterator, but still powerful. To
see how it works, let&#8217;s revisit the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CatsAndDogs.java</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
program from earlier in the chapter. In the original version, the method 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>elementAt(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
was used to select each element, but in the following modified version an
enumeration is used:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: CatsAndDogs2.java</font>
<font color="#009900">// Simple collection with Enumeration</font>
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">class</font> Cat2 {
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> catNumber;
  Cat2(<font color="#0000ff">int</font> i) {
    catNumber = i;
  }
  <font color="#0000ff">void</font> print() {
    System.out.println("Cat number " +catNumber);
  }
}

<font color="#0000ff">class</font> Dog2 {
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> dogNumber;
  Dog2(<font color="#0000ff">int</font> i) {
    dogNumber = i;
  }
  <font color="#0000ff">void</font> print() {
    System.out.println("Dog number " +dogNumber);
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> CatsAndDogs2 {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Vector cats = <font color="#0000ff">new</font> Vector();
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; 7; i++)
      cats.addElement(<font color="#0000ff">new</font> Cat2(i));
    <font color="#009900">// Not a problem to add a dog to cats:</font>
    cats.addElement(<font color="#0000ff">new</font> Dog2(7));
    Enumeration e = cats.elements();
    <font color="#0000ff">while</font>(e.hasMoreElements())
      ((Cat2)e.nextElement()).print();
    <font color="#009900">// Dog is detected only at run-time</font>
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that the only change is in the last few lines. Instead of:
</FONT><P></DIV>

<font color="#990000"><PRE>    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; cats.size(); i++)
      ((Cat)cats.elementAt(i)).print(); </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">an
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Enumeration</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is used to step through the sequence:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#0000ff">while</font>(e.hasMoreElements())
      ((Cat2)e.nextElement()).print(); </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">With
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Enumeration</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you don&#8217;t need to worry about the number of elements in the collection.
That&#8217;s taken care of for you by <A NAME="Index768"></A><A NAME="Index769"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>hasMoreElements(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index770"></A><A NAME="Index771"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>nextElement(&#160;)</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">As
another example, consider the creation of a general-purpose printing method:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: HamsterMaze.java</font>
<font color="#009900">// Using an Enumeration</font>
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">class</font> Hamster {
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> hamsterNumber;
  Hamster(<font color="#0000ff">int</font> i) {
    hamsterNumber = i;
  }
  <font color="#0000ff">public</font> String toString() {
    <font color="#0000ff">return</font> "This is Hamster #" + hamsterNumber;
  }
}

<font color="#0000ff">class</font> Printer {
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> printAll(Enumeration e) {
    <font color="#0000ff">while</font>(e.hasMoreElements())
      System.out.println(
        e.nextElement().toString());
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> HamsterMaze {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Vector v = <font color="#0000ff">new</font> Vector();
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; 3; i++)
      v.addElement(<font color="#0000ff">new</font> Hamster(i));
    Printer.printAll(v.elements());
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Look
closely at the printing method:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#0000ff">static</font> <font color="#0000ff">void</font> printAll(Enumeration e) {
  <font color="#0000ff">while</font>(e.hasMoreElements())
    System.out.println(
      e.nextElement().toString());
}</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that there&#8217;s no information about the type of sequence. All you have is an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Enumeration</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and that&#8217;s all you need to know about the sequence: that you can get the
next object, and that you can know when you&#8217;re at the end. This idea of
taking a collection of objects and passing through it to perform an operation
on each one is powerful and will be seen throughout this book.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
particular example is even more generic, since it uses the ubiquitous <A NAME="Index772"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method (ubiquitous only because it&#8217;s part of the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class). An alternative way to call print (although probably slightly less
efficient, if you could even notice the difference) is: 
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">System.out.println(""
+ e.nextElement());
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><A NAME="Index773"></A><A NAME="Index774"></A><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">which
uses the &#8220;automatic conversion to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8221;
that&#8217;s wired into Java. When the compiler sees a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
followed by a &#8216;
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>+</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;,
it expects another 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to follow and calls 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
automatically. (In <A NAME="Index775"></A>Java
1.1, the first 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is unnecessary; any object will be converted to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String.</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
You can also perform a cast, which has the effect of calling 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>toString(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">System.out.println((String)e.nextElement());</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
general, however, you&#8217;ll want to do something more than call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods, so you&#8217;ll run up against the type-casting issue again. You must
assume you&#8217;ve gotten an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Enumeration</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to a sequence of the particular type you&#8217;re interested in, and cast the
resulting objects to that type (getting a run-time exception if you&#8217;re
wrong).
</FONT><a name="_Toc375545353"></a><a name="_Toc408018569"></a><P></DIV>
<HR><DIV ALIGN=LEFT><A NAME="fn33" HREF="#fnB33">[33]</A><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
The term 
</FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black"><I>iterator
</I></FONT><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">is
common in C++ and elsewhere in OOP, so it&#8217;s difficult to know why the
Java team used a strange name. The collections library in Java 1.2 fixes this
as well as many other problems.
</FONT><P></DIV>


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