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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>
Abstracting
usage
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">With
creation out of the way, it&#8217;s time to tackle the remainder of the design:
where the classes are used. Since it&#8217;s the act of sorting into bins
that&#8217;s particularly ugly and exposed, why not take that process and hide
it inside a class? This is the principle of &#8220;If you must do something
ugly, at least localize the ugliness inside a class.&#8221; It looks like this:
</FONT><P></DIV><DIV ALIGN=LEFT><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>TrashSorter</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object initialization must now be changed whenever a new type of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is added to the model. You could imagine that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TrashSorter</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class might look something like this:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#0000ff">class</font> TrashSorter <font color="#0000ff">extends</font> Vector {
  <font color="#0000ff">void</font> sort(Trash t) { <font color="#009900">/* ... */</font> }
}</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">That
is, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TrashSorter</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of handles to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handles, and with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>addElement(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
you can install another one, like so:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">TrashSorter
ts = new TrashSorter();
</FONT></TT><P><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">ts.addElement(new
Vector());
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Now,
however, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sort(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
becomes a problem. How does the statically-coded method deal with the fact that
a new type has been added? To solve this, the type information must be removed
from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sort(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
so that all it needs to do is call a generic method that takes care of the
details of type. This, of course, is another way to describe a
dynamically-bound method. So 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sort(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
will simply move through the sequence and call a dynamically-bound method for
each 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Since the job of this method is to grab the pieces of trash it is interested
in, it&#8217;s called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>grab(Trash)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The structure now looks like:
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>TrashSorter</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
needs to call each 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>grab(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method and get a different result depending on what type of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the current 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is holding. That is, each 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
must be aware of the type it holds. The classic approach to this problem is to
create a base &#8220;
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
bin&#8221; class and inherit a new derived class for each different type you
want to hold. If Java had a parameterized type mechanism that would probably be
the most straightforward approach. But rather than hand-coding all the classes
that such a mechanism should be building for us, further observation can
produce a better approach.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
basic OOP design principle is &#8220;Use data members for variation in state,
use <A NAME="Index2989"></A>polymorphism
for variation in behavior.&#8221; Your first thought might be that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>grab(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method certainly behaves differently for a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that holds 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Paper</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
than for one that holds 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Glass</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
But what it does is strictly dependent on the type, and nothing else. This
could be interpreted as a different state, and since Java has a class to
represent type (
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
this can be used to determine the type of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
a particular 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tbin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
will hold.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
constructor for this 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tbin
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">requires
that you hand it the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
of your choice. This tells the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
what type it is supposed to hold. Then the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>grab(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method uses 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class
BinType
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and RTTI to see if the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object you&#8217;ve handed it matches the type it&#8217;s supposed to grab.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here
is the whole program. The commented numbers (e.g. (*1*) ) mark sections that
will be described following the code.
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: RecycleB.java</font>
<font color="#009900">// Adding more objects to the recycling problem</font>
<font color="#0000ff">package</font> c16.recycleb;
<font color="#0000ff">import</font> c16.trash.*;
<font color="#0000ff">import</font> java.util.*;

<font color="#009900">// A vector that admits only the right type:</font>
<font color="#0000ff">class</font> Tbin <font color="#0000ff">extends</font> Vector {
  Class binType;
  Tbin(Class binType) { 
    <font color="#0000ff">this</font>.binType = binType; 
  }
  <font color="#0000ff">boolean</font> grab(Trash t) {
    <font color="#009900">// Comparing class types:</font>
    <font color="#0000ff">if</font>(t.getClass().equals(binType)) {
      addElement(t);
      <font color="#0000ff">return</font> <font color="#0000ff">true</font>; <font color="#009900">// Object grabbed</font>
    }
    <font color="#0000ff">return</font> <font color="#0000ff">false</font>; <font color="#009900">// Object not grabbed</font>
  }
}

<font color="#0000ff">class</font> TbinList <font color="#0000ff">extends</font> Vector { <font color="#009900">//(*1*)</font>
  <font color="#0000ff">boolean</font> sort(Trash t) {
    Enumeration e = elements();
    <font color="#0000ff">while</font>(e.hasMoreElements()) {
      Tbin bin = (Tbin)e.nextElement();
      <font color="#0000ff">if</font>(bin.grab(t)) <font color="#0000ff">return</font> <font color="#0000ff">true</font>;
    }
    <font color="#0000ff">return</font> <font color="#0000ff">false</font>; <font color="#009900">// bin not found for t</font>
  }
  <font color="#0000ff">void</font> sortBin(Tbin bin) { <font color="#009900">// (*2*)</font>
    Enumeration e = bin.elements();
    <font color="#0000ff">while</font>(e.hasMoreElements())
      <font color="#0000ff">if</font>(!sort((Trash)e.nextElement()))
        System.out.println("Bin not found");
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> RecycleB {
  <font color="#0000ff">static</font> Tbin bin = <font color="#0000ff">new</font> Tbin(Trash.<font color="#0000ff">class</font>);
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#009900">// Fill up the Trash bin:</font>
    ParseTrash.fillBin("Trash.dat", bin);

    TbinList trashBins = <font color="#0000ff">new</font> TbinList();
    trashBins.addElement(
      <font color="#0000ff">new</font> Tbin(Aluminum.<font color="#0000ff">class</font>));
    trashBins.addElement(
      <font color="#0000ff">new</font> Tbin(Paper.<font color="#0000ff">class</font>));
    trashBins.addElement(
      <font color="#0000ff">new</font> Tbin(Glass.<font color="#0000ff">class</font>));
    <font color="#009900">// add one line here: (*3*)</font>
    trashBins.addElement(
      <font color="#0000ff">new</font> Tbin(Cardboard.<font color="#0000ff">class</font>));

    trashBins.sortBin(bin); <font color="#009900">// (*4*)</font>

    Enumeration e = trashBins.elements();
    <font color="#0000ff">while</font>(e.hasMoreElements()) {
      Tbin b = (Tbin)e.nextElement();
      Trash.sumValue(b);
    }
    Trash.sumValue(bin);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV>
<OL>
<LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>	TbinList</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
holds a set of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tbin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
handles, so that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sort(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
can iterate through the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tbin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
when it&#8217;s looking for a match for the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object you&#8217;ve handed it.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>	sortBin(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
allows you to pass an entire 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tbin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in, and it moves through the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tbin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
picks out each piece of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Trash</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and sorts it into the appropriate specific 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Tbin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Notice the genericity of this code: it doesn&#8217;t change at all if new types
are added. If the bulk of your code doesn&#8217;t need changing when a new type
is added (or some other change occurs) then you have an easily-extensible system.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Now
you can see how easy it is to add a new type. Few lines must be changed to
support the addition. If it&#8217;s really important, you can squeeze out even
more by further manipulating the design.
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	One
method call causes the contents of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>bin</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to be sorted into the respective specifically-typed bins.
</FONT><a name="_Toc408018803"></a><a name="_Toc375545417"></a></OL>
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