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Thinking in Java
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Vector & Enumeration

The only self-expanding sequence in Java 1.0/1.1 was the Vector, so it saw a lot of use. Its flaws are too numerous to describe here (see the first edition of this book, available as a free download from www.BruceEckel.com). Basically, you can think of it as an ArrayList with long, awkward method names. In the Java 2 container library, Vector was adapted so that it could fit as a Collection and a List, so in the following example, the Collections2.fill( ) method is successfully used. This turns out to be a bit perverse, as it may confuse some people into thinking that Vector has gotten better, when it is actually included only to support pre-Java 2 code.

The Java 1.0/1.1 version of the iterator chose to invent a new name, “enumeration,” instead of using a term that everyone was already familiar with. The Enumeration interface is smaller than Iterator, with only two methods, and it uses longer method names: boolean hasMoreElements( ) produces true if this enumeration contains more elements, and Object nextElement( ) returns the next element of this enumeration if there are any more (otherwise it throws an exception).

Enumeration is only an interface, not an implementation, and even new libraries sometimes still use the old Enumeration, which is unfortunate but generally harmless. Even though you should always use Iterator when you can in your own code, you must be prepared for libraries that want to hand you an Enumeration.

In addition, you can produce an Enumeration for any Collection by using the Collections.enumeration( ) method, as seen in this example:

//: c11:Enumerations.java
// Java 1.0/1.1 Vector and Enumeration.
import java.util.*;
import com.bruceeckel.util.*;

public class Enumerations {
  public static void main(String[] args) {
    Vector v = new Vector();
    Collections2.fill(v, Collections2.countries, 100);
    Enumeration e = v.elements();
    while(e.hasMoreElements())
      System.out.println(e.nextElement());
    // Produce an Enumeration from a Collection:
    e = Collections.enumeration(new ArrayList());
  }
} ///:~


The Java 1.0/1.1 Vector has only an addElement( ) method, but fill( ) uses the add( ) method that was pasted on while Vector was being turned into a List. To produce an Enumeration, you call elements( ), then you can use it to perform a forward iteration.

The last line creates an ArrayList and uses enumeration( ) to adapt an Enumeration from the ArrayList Iterator. Thus, if you have old code that wants an Enumeration, you can still use the new containers.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire