⭐ 欢迎来到虫虫下载站！ | 📦 资源下载 📁 资源专辑 ℹ️ 关于我们
⭐ 虫虫下载站
📤 上传资源

📄 tij0181.html

📁 学习java的经典书籍
💻 HTML
字号:
🔍 
<html><body>

<table width="100%"><tr>
<td>
<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
</td>
<td align="right">
<a href="tij_c.html">Contents</a> | <a href="tij0180.html">Prev</a> | <a href="tij0182.html">Next</a>
</td>
</tr></table>
<hr>

<H2 ALIGN=LEFT>
RTTI
considered harmful?
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Various
designs in this chapter attempt to remove RTTI, which might give you the
impression that it&#8217;s &#8220;considered harmful&#8221; (the condemnation
used for poor, ill-fated 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which was thus never put into Java). This isn&#8217;t true; it is the <A NAME="Index3004"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>misuse</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of RTTI that is the problem. The reason our designs removed RTTI is because the
misapplication of that feature prevented <A NAME="Index3005"></A>extensibility,
while the stated goal was to be able to add a new type to the system with as
little impact on surrounding code as possible. Since RTTI is often misused by
having it look for every single type in your system, it causes code to be
non-extensible: when you add a new type, you have to go hunting for all the
code in which RTTI is used, and if you miss any you won&#8217;t get help from
the compiler.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">However,
RTTI doesn&#8217;t automatically create non-extensible code. Let&#8217;s
revisit the trash recycler once more. This time, a new tool will be introduced,
which I call a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TypeMap</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
It contains a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Hashtable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that holds 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s,
but the interface is simple: you can 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>add(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
a new object, and you can 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>get(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
containing all the objects of a particular type. The keys for the contained 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Hashtable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are the types in the associated 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The beauty of this design (suggested by Larry O&#8217;Brien) is that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TypeMap</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
dynamically adds a new pair whenever it encounters a new type, so whenever you
add a new type to the system (even if you add the new type at run-time), it
adapts.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Our
example will again build on the structure of the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
types in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>package</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>c16.Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(and the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash.dat</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
file used there can be used here without change):
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: DynaTrash.java </font>
<font color="#009900">// Using a Hashtable of Vectors and RTTI</font>
<font color="#009900">// to automatically sort trash into</font>
<font color="#009900">// vectors. This solution, despite the</font>
<font color="#009900">// use of RTTI, is extensible.</font>
<font color="#0000ff">package</font> c16.dynatrash;
<font color="#0000ff">import</font> c16.trash.*;
<font color="#0000ff">import</font> java.util.*;

<font color="#009900">// Generic TypeMap works in any situation:</font>
<font color="#0000ff">class</font> TypeMap {
  <font color="#0000ff">private</font> Hashtable t = <font color="#0000ff">new</font> Hashtable();
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> add(Object o) {
    Class type = o.getClass();
    <font color="#0000ff">if</font>(t.containsKey(type))
      ((Vector)t.get(type)).addElement(o);
    <font color="#0000ff">else</font> {
      Vector v = <font color="#0000ff">new</font> Vector();
      v.addElement(o);
      t.put(type,v);
    }
  }
  <font color="#0000ff">public</font> Vector get(Class type) {
    <font color="#0000ff">return</font> (Vector)t.get(type);
  }
  <font color="#0000ff">public</font> Enumeration keys() { <font color="#0000ff">return</font> t.keys(); }
  <font color="#009900">// Returns handle to adapter class to allow</font>
  <font color="#009900">// callbacks from ParseTrash.fillBin():</font>
  <font color="#0000ff">public</font> Fillable filler() { 
    <font color="#009900">// Anonymous inner class:</font>
    <font color="#0000ff">return</font> <font color="#0000ff">new</font> Fillable() {
      <font color="#0000ff">public</font> <font color="#0000ff">void</font> addTrash(Trash t) { add(t); }
    };
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> DynaTrash {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    TypeMap bin = <font color="#0000ff">new</font> TypeMap();
    ParseTrash.fillBin("Trash.dat",bin.filler());
    Enumeration keys = bin.keys();
    <font color="#0000ff">while</font>(keys.hasMoreElements())
      Trash.sumValue(
        bin.get((Class)keys.nextElement()));
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Although
powerful, the definition for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TypeMap</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is simple. It contains a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Hashtable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>add(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method does most of the work. When you 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>add(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
a new object, the handle for the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object for that type is extracted. This is used as a key to determine whether a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
holding objects of that type is already present in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Hashtable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If so, that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is extracted and the object is added to the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If not, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object and a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are added as a key-value pair.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can get an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Enumeration</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of all the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>keys(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and use each 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object to fetch the corresponding 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>get(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
And that&#8217;s all there is to it.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>filler(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method is interesting because it takes advantage of the design of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ParseTrash.fillBin(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which doesn&#8217;t just try to fill a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
but instead anything that implements the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Fillable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface with its 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>addTrash(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method. All 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>filler(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
needs to do is to return a handle to an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>interface</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that implements 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Fillable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and then this handle can be used as an argument to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>fillBin(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
like this:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">ParseTrash.fillBin("Trash.dat",
bin.filler());
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">To
produce this handle, an <A NAME="Index3006"></A><A NAME="Index3007"></A><A NAME="Index3008"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>anonymous
inner class
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(described in Chapter 7) is used. You never need a named class to implement 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Fillable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you just need a handle to an object of that class, thus this is an appropriate
use of anonymous inner classes.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">An
interesting thing about this design is that even though it wasn&#8217;t created
to handle the sorting, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>fillBin(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is performing a sort every time it inserts a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object into 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>bin</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">Much
of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>class
DynaTrash
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
should be familiar from the previous examples. This time, instead of placing
the new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects into a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>bin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of type 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>bin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is of type 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TypeMap</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so when the trash is thrown into 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>bin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
it&#8217;s immediately sorted by 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TypeMap</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
internal sorting mechanism. Stepping through the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TypeMap</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and operating on each individual 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
becomes a simple matter:
</FONT><P></DIV>

<font color="#990000"><PRE>    Enumeration keys = bin.keys();
    <font color="#0000ff">while</font>(keys.hasMoreElements())
      Trash.sumValue(
        bin.get((Class)keys.nextElement())); </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">As
you can see, adding a new type to the system won&#8217;t affect this code at
all, nor the code in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TypeMap</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This is certainly the smallest solution to the problem, and arguably the most
elegant as well. It does rely heavily on RTTI, but notice that each key-value
pair in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Hashtable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is looking for only one type. In addition, there&#8217;s no way you can
&#8220;forget&#8221; to add the proper code to this system when you add a new
type, since there isn&#8217;t any code you need to add.
</FONT><a name="_Toc408018807"></a><P></DIV>

<div align="right">
<a href="tij_c.html">Contents</a> | <a href="tij0180.html">Prev</a> | <a href="tij0182.html">Next</a>
</div>
</body></html>

⌨️ 快捷键说明

复制代码 Ctrl + C
搜索代码 Ctrl + F
全屏模式 F11
切换主题 Ctrl + Shift + D
显示快捷键 ?
增大字号 Ctrl + =
减小字号 Ctrl + -