Simulating the trash recycler

Bruce Eckel’s Thinking in Java Contents | Prev | Next

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.

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.

// Recycling with RTTI
package c16.recyclea;
import java.util.*;
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();
        "weight of " +
        // Using RTTI to get type
        // information about the class:
        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 < 30; i++)
      switch((int)(Math.random() * 3)) {
        case 0 :
            Aluminum(Math.random() * 100));
        case 1 :
            Paper(Math.random() * 100));
        case 2 :
            Glass(Math.random() * 100));
      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)
} ///:~ 

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

looks silly to upcast the types of
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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