# Simulating the trash recycler

Thursday Mar 1st 2001
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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.

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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.

This is not a trivial design because it has an added constraint. That’s what makes it interesting – it’s more like the messy problems you’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.

```//: RecycleA.java
// Recycling with RTTI
package c16.recyclea;
import java.util.*;
import java.io.*;

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

class Aluminum extends Trash {
static double val  = 1.67f;
Aluminum(double wt) { super(wt); }
double value() { return val; }
static void value(double newval) {
val = newval;
}
}

class Paper extends Trash {
static double val = 0.10f;
Paper(double wt) { super(wt); }
double value() { return val; }
static void value(double newval) {
val = newval;
}
}

class Glass extends Trash {
static double val = 0.23f;
Glass(double wt) { super(wt); }
double value() { return val; }
static void value(double newval) {
val = newval;
}
}

public class RecycleA {
public static void main(String[] args) {
Vector bin = new Vector();
// Fill up the Trash bin:
for(int i = 0; i &lt; 30; i++)
switch((int)(Math.random() * 3)) {
case 0 :
Aluminum(Math.random() * 100));
break;
case 1 :
Paper(Math.random() * 100));
break;
case 2 :
Glass(Math.random() * 100));
}
Vector
glassBin = new Vector(),
paperBin = new Vector(),
alBin = new Vector();
Enumeration sorter = bin.elements();
// Sort the Trash:
while(sorter.hasMoreElements()) {
Object t = sorter.nextElement();
// RTTI to show class membership:
if(t instanceof Aluminum)
if(t instanceof Paper)
if(t instanceof Glass)
}
Trash.sumValue(alBin);
Trash.sumValue(paperBin);
Trash.sumValue(glassBin);
Trash.sumValue(bin);
}
} ///:~ ```

package c16.recyclea;

This means that in the source code listings available for the book, this file will be placed in the subdirectory recyclea that branches off from the subdirectory c16 (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 package the class names will not clash.

It looks silly to upcast the types of Trash 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’s structure and flexibility can benefit greatly from downcasting.

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