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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>
Simulating
the trash recycler
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
nature of this problem is that the trash is thrown unclassified into a single
bin, so the specific type information is lost. But later, the specific type
information must be recovered to properly sort the trash. In the initial
solution, RTTI (described in Chapter 11) is used.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
is not a trivial design because it has an added constraint. That&#8217;s what
makes it interesting &#8211; it&#8217;s more like the messy problems
you&#8217;re likely to encounter in your work. The extra constraint is that the
trash arrives at the trash recycling plant all mixed together. The program must
model the sorting of that trash. This is where RTTI comes in: you have a bunch
of anonymous pieces of trash, and the program figures out exactly what type
they are.
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: RecycleA.java </font>
<font color="#009900">// Recycling with RTTI</font>
<font color="#0000ff">package</font> c16.recyclea;
<font color="#0000ff">import</font> java.util.*;
<font color="#0000ff">import</font> java.io.*;

<font color="#0000ff">abstract</font> <font color="#0000ff">class</font> Trash {
  <font color="#0000ff">private</font> <font color="#0000ff">double</font> weight;
  Trash(<font color="#0000ff">double</font> wt) { weight = wt; }
  <font color="#0000ff">abstract</font> <font color="#0000ff">double</font> value();
  <font color="#0000ff">double</font> weight() { <font color="#0000ff">return</font> weight; }
  <font color="#009900">// Sums the value of Trash in a bin:</font>
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> sumValue(Vector bin) {
    Enumeration e = bin.elements();
    <font color="#0000ff">double</font> val = 0.0f;
    <font color="#0000ff">while</font>(e.hasMoreElements()) {
      <font color="#009900">// One kind of RTTI:</font>
      <font color="#009900">// A dynamically-checked cast</font>
      Trash t = (Trash)e.nextElement();
      <font color="#009900">// Polymorphism in action:</font>
      val += t.weight() * t.value();
      System.out.println(
        "weight of " +
        <font color="#009900">// Using RTTI to get type</font>
        <font color="#009900">// information about the class:</font>
        t.getClass().getName() +
        " = " + t.weight());
    }
    System.out.println("Total value = " + val);
  }
}

<font color="#0000ff">class</font> Aluminum <font color="#0000ff">extends</font> Trash {
  <font color="#0000ff">static</font> <font color="#0000ff">double</font> val  = 1.67f;
  Aluminum(<font color="#0000ff">double</font> wt) { <font color="#0000ff">super</font>(wt); }
  <font color="#0000ff">double</font> value() { <font color="#0000ff">return</font> val; }
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> value(<font color="#0000ff">double</font> newval) {
    val = newval;
  }
}

<font color="#0000ff">class</font> Paper <font color="#0000ff">extends</font> Trash {
  <font color="#0000ff">static</font> <font color="#0000ff">double</font> val = 0.10f;
  Paper(<font color="#0000ff">double</font> wt) { <font color="#0000ff">super</font>(wt); }
  <font color="#0000ff">double</font> value() { <font color="#0000ff">return</font> val; }
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> value(<font color="#0000ff">double</font> newval) {
    val = newval;
  }
}

<font color="#0000ff">class</font> Glass <font color="#0000ff">extends</font> Trash {
  <font color="#0000ff">static</font> <font color="#0000ff">double</font> val = 0.23f;
  Glass(<font color="#0000ff">double</font> wt) { <font color="#0000ff">super</font>(wt); }
  <font color="#0000ff">double</font> value() { <font color="#0000ff">return</font> val; }
  <font color="#0000ff">static</font> <font color="#0000ff">void</font> value(<font color="#0000ff">double</font> newval) {
    val = newval;
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> RecycleA {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Vector bin = <font color="#0000ff">new</font> Vector();
    <font color="#009900">// Fill up the Trash bin:</font>
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; 30; i++)
      <font color="#0000ff">switch</font>((<font color="#0000ff">int</font>)(Math.random() * 3)) {
        <font color="#0000ff">case</font> 0 :
          bin.addElement(<font color="#0000ff">new</font>
            Aluminum(Math.random() * 100));
          <font color="#0000ff">break</font>;
        <font color="#0000ff">case</font> 1 :
          bin.addElement(<font color="#0000ff">new</font>
            Paper(Math.random() * 100));
          <font color="#0000ff">break</font>;
        <font color="#0000ff">case</font> 2 :
          bin.addElement(<font color="#0000ff">new</font>
            Glass(Math.random() * 100));
      }
    Vector 
      glassBin = <font color="#0000ff">new</font> Vector(),
      paperBin = <font color="#0000ff">new</font> Vector(),
      alBin = <font color="#0000ff">new</font> Vector();
    Enumeration sorter = bin.elements();
    <font color="#009900">// Sort the Trash:</font>
    <font color="#0000ff">while</font>(sorter.hasMoreElements()) {
      Object t = sorter.nextElement();
      <font color="#009900">// RTTI to show class membership:</font>
      <font color="#0000ff">if</font>(t <font color="#0000ff">instanceof</font> Aluminum)
        alBin.addElement(t);
      <font color="#0000ff">if</font>(t <font color="#0000ff">instanceof</font> Paper)
        paperBin.addElement(t);
      <font color="#0000ff">if</font>(t <font color="#0000ff">instanceof</font> Glass)
        glassBin.addElement(t);
    }
    Trash.sumValue(alBin);
    Trash.sumValue(paperBin);
    Trash.sumValue(glassBin);
    Trash.sumValue(bin);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><a name="_Toc305593314"></a><a name="_Toc305628786"></a><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
first thing you&#8217;ll notice is the <A NAME="Index2942"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>package</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
statement:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">package
c16.recyclea;
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
means that in the source code listings available for the book, this file will
be placed in the subdirectory 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>recyclea</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that branches off from the subdirectory 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>c16</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(for Chapter 16). The unpacking tool in Chapter 17 takes care of placing it
into the correct subdirectory. The reason for doing this is that this chapter
rewrites this particular example a number of times and by putting each version
in its own 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>package</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the class names will not clash.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Several
<A NAME="Index2943"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects are created to hold 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handles. Of course, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">actually
hold 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
so they&#8217;ll hold anything at all. The reason they hold 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">(or
something derived from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
is only because you&#8217;ve been careful to not put in anything except 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If you do put something &#8220;wrong&#8221; into the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
you won&#8217;t get any compile-time warnings or errors &#8211; you&#8217;ll
find out only via an exception at run-time.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handles are added, they lose their specific identities and become simply 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handles (they are <A NAME="Index2944"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>upcast</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">).
However, because of polymorphism <A NAME="Index2945"></A>the
proper behavior still occurs when the dynamically-bound methods<A NAME="Index2946"></A>
are called through the <A NAME="Index2947"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Enumeration</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sorter</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
once the resulting <A NAME="Index2948"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
has been cast back to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sumValue(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
also uses 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
perform operations on every object in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</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">It
looks silly to upcast the types of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
into a collection holding base type handles, and then turn around and downcast.
Why not just put the trash into the appropriate receptacle in the first place?
(Indeed, this is the whole enigma of recycling). In this program it would be
easy to repair, but sometimes a system&#8217;s structure and flexibility can
benefit greatly from downcasting.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
program satisfies the design requirements: it works. This might be fine as long
as it&#8217;s a one-shot solution. However, a useful program tends to evolve
over time, so you must ask, &#8220;What if the situation changes?&#8221; For
example, cardboard is now a valuable recyclable commodity, so how will that be
integrated into the system (especially if the program is large and
complicated). Since the above <A NAME="Index2949"></A>type-check
coding in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>switch</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
statement could be scattered throughout the program, you must go find all that
code every time a new type is added, and if you miss one the compiler
won&#8217;t give you any help by pointing out an error.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
key to the <A NAME="Index2950"></A>misuse
of RTTI here is that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>every
type is tested
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If you&#8217;re looking for only a subset of types because that subset needs
special treatment, that&#8217;s probably fine. But if you&#8217;re hunting for
every type inside a switch statement, then you&#8217;re probably missing an
important point, and definitely making your code less maintainable. In the next
section we&#8217;ll look at how this program evolved over several stages to
become much more flexible. This should prove a valuable example in program
design.
</FONT><a name="_Toc375545414"></a><a name="_Toc408018799"></a><P></DIV>

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