How to differentiate the first or last item in Java 5 for-each type of loop?

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whmanutd@gmail.com - 29 Sep 2006 19:21 GMT

Hello there!

I got a code snippet like:

// == start of the code ==

List<String> list = new ArrayList<String>();
...// some code to add String to "list"

for (String item : list) // Java 5 for-each loop
{
// do something.
..
// but also want to something special for the first/last item of the
loop.
}

// == end of the code ==

So my question is: how can I catch the first/last item, without roll
the code back to the old iterator-style of loop.

Thanks a lot for any comments!

Chris

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Patricia Shanahan - 30 Sep 2006 03:40 GMT

> Hello there!
>
[quoted text clipped - 17 lines]
> So my question is: how can I catch the first/last item, without roll
> the code back to the old iterator-style of loop.

There are ways of doing each, but I think the result would be messier
than the iterator-style loop. Iterator based loops are not deprecated,
and there is absolutely nothing wrong with using them. The new forms are
just shorthand for the very simplest, most routine cases.

Here are a couple of techniques to consider if you really do want to try
to bend the new loop forms beyond what they do naturally:

1. Keep a boolean isFirstIteration, initially true. When it is true, do
the extra first element processing and change it to false.

2. In the iterated statement, assign a copy of the item reference to a
variable declared outside the loop. At the end, it will contain a
reference to the last element, and can be used for additional last
element processing.

Patricia

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Hendrik Maryns - 04 Oct 2006 16:25 GMT

Patricia Shanahan schreef:

>> Hello there!
>>
[quoted text clipped - 22 lines]
> and there is absolutely nothing wrong with using them. The new forms are
> just shorthand for the very simplest, most routine cases.

How would you catch the last object with an iterator? The first is no
problem, but there is no way to know you’re handling the last one,
except for checking hasNext() inside the loop once more after doing
next(), which isn’t very nice either. So basically, you have to know
the number of elements and count, or use one of those techniques you
described.

H
- --
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
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Patricia Shanahan - 04 Oct 2006 20:03 GMT

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> Hash: SHA1
[quoted text clipped - 32 lines]
> the number of elements and count, or use one of those techniques you
> described.

I was dealing with the case of adding special work.

If you need to not do the normal processing for the last element, it
gets a bit more complicated but can still be done. Run one iteration
behind. Keep a "previousItem" reference, initially null. Inside the loop:

if(previousItem != null){
// deal with previousItem object.
}
previousItem = item;

At the end of the loop, previousItem points to the last element, and it
has not yet been processed.

However, I still think that it would be better and simpler to use an
explicit iterator, and check for hasNext().

Patricia

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How to differentiate the first or last item in Java 5 for-each type of loop?

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