Re: iterators
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.
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