Enumeration interface

[Vijay pillai]

hello
I would like to know about the interface Enumeration.
since Interfaces has methods which are abstract(not defined).
how is it that we find this code running.
Enumeration e=//+a code that returns Enumeration object.
while(e.hasMoreElements())
{
e.nextElement();
}
my doubt is .how could we call the undefined methods of the Enumeration interface without declaring it in any way.
e.hasMoreElements();
e.nextElement();
pls help.
vijay

[Frank Carver]

This is the whole point of interfaces. By using the interface Enumeration, you are indicating that you don't care which actual class the object returned is an instance of, only that it implements Enumeration.
Consider the following code sample:

import java.util.*;
class ArrayEnumeration implements Enumeration
{
private Object[] array;
private int i;

public ArrayEnumeration(Object[] array)
{
this.array = array;
i = 0;
}
public boolean hasMoreElements()
{
return array!=null ? i < array.length : false;
}
public Object nextElement()
throws NoSuchElementException
{
if (!hasNext())
{
throw new NoSuchElementException();
}
return array[i++];
}
}
class ArrayHolder
{
private String[] array = null;

public ArrayHolder(String a)
{
array = new String[1];
array[0] = a;
}

public ArrayHolder(String a, String b)
{
array = new String[2];
array[0] = a;
array[1] = b;
}

public ArrayHolder(String a, String b, String c)
{
array = new String[3];
array[0] = a;
array[1] = b;
array[2] = c;
}

public Enumeration elements()
{
return new ArrayEnumeration(array);
}
}
public class EnumerationExample
{
public static void main(String[] args)
{
Enumeration e;
ArrayHolder two = new ArrayHolder("hello", "there");
ArrayHolder three = new ArrayHolder("hello", "there", "world");

e = two.elements();
while (e.hasMoreElements())
{
System.out.println("element: '" + e.nextElement() + "'");
}

e = three.elements();
while (e.hasMoreElements())
{
System.out.println("element: '" + e.nextElement() + "'");
}
}
}