Re: iterators

Lew <>
Sun, 02 Aug 2009 16:05:12 -0400
Mike Schilling wrote:

Stefan Ram wrote: (Stefan Ram) writes:

class Example
implements de.dclj.ram.Value<T>, de.dclj.ram.Advanceable
{ public T value(){ ... }
 public boolean advance(){ ... }}

 I made a design error:

 If there is not even one first value available,
 the client can not detect this. So now, I prefer:

class ExampleIterator
{ public boolean IsAvailable(){ ... } [sic]
 public T value() { ... }
 public void advance() { ... }}

I'd call this a "cursor", and propbably call the boolean method
isValid()". Note that C# presents things more the way you like. Its
IEnumerator interface has:

1. The property Current, like your method value().
2. The boolean method MoveNext(), which is like your advance() plus
your isAvailable() put together.

You could build one from a ListIterator.

public interface Advancer <E> extends java.util.ListIterator <E>
   public E value();

You would simply use 'hasNext()' instead of 'isAvailable()'.

'next()' is already like 'MoveNext()', but uses an exception instead of a
boolean return. I suspect this is to obviate a test-and-branch in both the
source and the runtime for the usual case of having a next element.

A simple implementation of 'value()' could be put together from a combination
of 'hasNext()', 'hasPrevious()', 'next()' and 'previous()' or use a local
'current' and a little bit of state.


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