Re: iterators

"Mike Schilling" <>
Sun, 2 Aug 2009 15:25:29 -0700
Lew wrote:

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.
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.

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 =;

while the C# equivalent is

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

The same amount of work, just divided up differently.

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