RTTI considered harmful?

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

//: DynaTrash.java 
// Using a Hashtable of Vectors and RTTI
// to automatically sort trash into
// vectors. This solution, despite the
// use of RTTI, is extensible.
package c16.dynatrash;
import c16.trash.*;
import java.util.*;
 
// Generic TypeMap works in any situation:
class TypeMap {
  private Hashtable t = new Hashtable();
  public void add(Object o) {
    Class type = o.getClass();
    if(t.containsKey(type))
      ((Vector)t.get(type)).addElement(o);
    else {
      Vector v = new Vector();
      v.addElement(o);
      t.put(type,v);
    }
  }
  public Vector get(Class type) {
    return (Vector)t.get(type);
  }
  public Enumeration keys() { return t.keys(); }
  // Returns handle to adapter class to allow
  // callbacks from ParseTrash.fillBin():
  public Fillable filler() { 
    // Anonymous inner class:
    return new Fillable() {
      public void addTrash(Trash t) { add(t); }
    };
  }
}
 
public class DynaTrash {
  public static void main(String[] args) {
    TypeMap bin = new TypeMap();
    ParseTrash.fillBin("Trash.dat",bin.filler());
    Enumeration keys = bin.keys();
    while(keys.hasMoreElements())
      Trash.sumValue(
        bin.get((Class)keys.nextElement()));
  }
} ///:~ 

Although powerful, the definition for TypeMap is simple. It contains a Hashtable, and the add( ) method does most of the work. When you add( ) a new object, the handle for the Class object for that type is extracted. This is used as a key to determine whether a Vector holding objects of that type is already present in the Hashtable. If so, that Vector is extracted and the object is added to the Vector. If not, the Class object and a new Vector are added as a key-value pair.

You can get an Enumeration of all the Class objects from keys( ), and use each Class object to fetch the corresponding Vector with get( ). And that’s all there is to it.

The filler( ) method is interesting because it takes advantage of the design of ParseTrash.fillBin( ), which doesn’t just try to fill a Vector but instead anything that implements the Fillable interface with its addTrash( ) method. All filler( ) needs to do is to return a handle to an interface that implements Fillable, and then this handle can be used as an argument to fillBin( ) like this:

ParseTrash.fillBin("Trash.dat", bin.filler());

    Enumeration keys = bin.keys();
    while(keys.hasMoreElements())
      Trash.sumValue(
        bin.get((Class)keys.nextElement()));

As you can see, adding a new type to the system won’t affect this code at all, nor the code in TypeMap. This is certainly the smallest solution to the problem, and arguably the most elegant as well. It does rely heavily on RTTI, but notice that each key-value pair in the Hashtable is looking for only one type. In addition, there’s no way you can “forget” to add the proper code to this system when you add a new type, since there isn’t any code you need to add.



Comments

  • There are no comments yet. Be the first to comment!

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • For the first time in modern history, workplace demographics now span four generations. Understanding the unique social influences that have shaped the behaviors of each is instrumental in creating productive work teams. And when managed effectively, their strengths, perspectives, and experiences can bring synergy in the workplace. Download this white paper for best practices on how to better understand and effectively manage a multi-generational workforce.

  • By now you've likely heard of Agile development and building products in small incremental pieces, so you can get real feedback along the way. In fact, you may even be considering using Agile on your next project. But where do you start? Agile can take a lot of forms, such as Scrum or Kanban. Each form has advantages and disadvantages, but both will help your team get the right feedback they need to build great products. Read this white paper to find out which one is right for you.

Most Popular Programming Stories

More for Developers

RSS Feeds

Thanks for your registration, follow us on our social networks to keep up-to-date