Simulating the trash recycler

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

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.

// 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)
} ///:~ 


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