Re: iterators

From:

"Mike Schilling" <mscottschilling@hotmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 2 Aug 2009 15:25:29 -0700

Message-ID:

<h5546v$2ld$1@news.eternal-september.org>

Lew wrote:

Mike Schilling wrote:

Stefan Ram wrote:

ram@zedat.fu-berlin.de (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
implements
de.dclj.ram.IsAvailable,
de.dclj.ram.Value<T>,
de.dclj.ram.Advanceable,
{ 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.
<http://java.sun.com/javase/6/docs/api/java/util/ListIterator.html>

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.

It's becausee Java assumes you've called hasNext() first, so next()
failing really is exceptional. C# can't assume anything analogous,
since no "test-only" method exists. That is, the standard Java loop
looks like

    for (Iterator iter = ...; iter.hasNext(); )
    {
        value = iter.next();
    }

while the C# equivalent is

    for (Iterator iter = ...; iter.MoveNext();)
    {
        value = iter.Current;
    }

The same amount of work, just divided up differently.