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Abstracting usage

With creation out of the way, it’s time to tackle the remainder of the design: where the classes are used. Since it’s the act of sorting into bins that’s particularly ugly and exposed, why not take that process and hide it inside a class? This is the principle of “If you must do something ugly, at least localize the ugliness inside a class.” It looks like this:

The TrashSorter object initialization must now be changed whenever a new type of Trash is added to the model. You could imagine that the TrashSorter class might look something like this:

class TrashSorter extends Vector {
  void sort(Trash t) { /* ... */ }
}

That is, TrashSorter is a Vector of handles to Vectors of Trash handles, and with addElement( ) you can install another one, like so:

TrashSorter ts = new TrashSorter();

ts.addElement(new Vector());

Now, however, sort( ) 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 sort( ) 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 sort( ) will simply move through the sequence and call a dynamically-bound method for each Vector. Since the job of this method is to grab the pieces of trash it is interested in, it’s called grab(Trash). The structure now looks like:

TrashSorter needs to call each grab( ) method and get a different result depending on what type of Trash the current Vector is holding. That is, each Vector must be aware of the type it holds. The classic approach to this problem is to create a base “ Trash bin” 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.

A basic OOP design principle is “Use data members for variation in state, use polymorphism for variation in behavior.” Your first thought might be that the grab( ) method certainly behaves differently for a Vector that holds Paper than for one that holds Glass. 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 ( Class) this can be used to determine the type of Trash a particular Tbin will hold.

The constructor for this Tbin requires that you hand it the Class of your choice. This tells the Vector what type it is supposed to hold. Then the grab( ) method uses Class BinType and RTTI to see if the Trash object you’ve handed it matches the type it’s supposed to grab.

Here is the whole program. The commented numbers (e.g. (*1*) ) mark sections that will be described following the code.

//: RecycleB.java
// Adding more objects to the recycling problem
package c16.recycleb;
import c16.trash.*;
import java.util.*;

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

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

public class RecycleB {
  static Tbin bin = new Tbin(Trash.class);
  public static void main(String[] args) {
    // Fill up the Trash bin:
    ParseTrash.fillBin("Trash.dat", bin);

    TbinList trashBins = new TbinList();
    trashBins.addElement(
      new Tbin(Aluminum.class));
    trashBins.addElement(
      new Tbin(Paper.class));
    trashBins.addElement(
      new Tbin(Glass.class));
    // add one line here: (*3*)
    trashBins.addElement(
      new Tbin(Cardboard.class));

    trashBins.sortBin(bin); // (*4*)

    Enumeration e = trashBins.elements();
    while(e.hasMoreElements()) {
      Tbin b = (Tbin)e.nextElement();
      Trash.sumValue(b);
    }
    Trash.sumValue(bin);
  }
} ///:~

TbinList holds a set of Tbin handles, so that sort( ) can iterate through the Tbins when it’s looking for a match for the Trash object you’ve handed it.
sortBin( ) allows you to pass an entire Tbin in, and it moves through the Tbin, picks out each piece of Trash, and sorts it into the appropriate specific Tbin. Notice the genericity of this code: it doesn’t change at all if new types are added. If the bulk of your code doesn’t need changing when a new type is added (or some other change occurs) then you have an easily-extensible system.
Now you can see how easy it is to add a new type. Few lines must be changed to support the addition. If it’s really important, you can squeeze out even more by further manipulating the design.
One method call causes the contents of bin to be sorted into the respective specifically-typed bins.

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