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POOptimization: replace for loop with ListIterator
 

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    <p>It's my first working on a quite big project, and I've been asked to obtain the best performances.</p> <p>So I've thouhgt to replace my for loops with a <code>ListIterator</code>, because I've got around 180 loops which call <code>list.get(i)</code> on lists with about 5000 elements.</p> <p>So I've got two questions.</p> <p>1) Are those 2 snippets equal? I mean, do them produce the same output? If no, how can I correct the <code>ListIterator</code> thing?</p> <pre><code>ListIterator&lt;Corsa&gt; ridesIterator = rides.listIterator(); while (ridesIterator.hasNext()) { ridesIterator.next(); Corsa previous = ridesIterator.previous(); //rides.get(i-1) Corsa current = ridesIterator.next(); //rides.get(i) if (current.getOP() &lt; d.getFFP() &amp;&amp; previous.getOA() &gt; d.getIP() &amp;&amp; current.wait(previous) &gt; DP) { doSomething(); break; } } </code></pre> <p>__</p> <pre><code>for (int i = 1; i &lt; rides.size(); i++) { if (rides.get(i).getOP() &lt; d.getFP() &amp;&amp; rides.get(i - 1).getOA() &gt; d.getIP() &amp;&amp; rides.get(i).getOP() - rides.get(i - 1).getOA() &gt; DP) { doSomething(); break; } } </code></pre> <p>2) How will it be the first snippet if I've got something like this? (changed i and its exit condition)</p> <pre><code>for (int i = 0; i &lt; rides.size() - 1; i++) { if (rides.get(i).getOP() &lt; d.getFP() &amp;&amp; rides.get(i + 1).getOA() &gt; d.getIP() &amp;&amp; rides.get(i).getOP() - rides.get(i + 1).getOA() &gt; DP) { doSomething(); break; } } </code></pre> <p>I'm asking because it's the first time that I'm using a <code>ListIterator</code> and I can't try it now!</p> <p>EDIT: I'm not using an ArrayList, it's a custom List based on a LinkedList</p> <p>EDIT 2 : I'm adding some more infos. I can't use a caching system because my data is changing on evry iteration and managing the cache would be hard as I'd have to deal with inconsistent data. I can't even merge some of this loops into one big loop, as I've got them on different methods because they need to do a lot of different things.</p> <p>So, sticking on this particular case, what do you think is the best pratice? Is ListIterator the best way to deal with my case? And how can I use the ListIterator if my for loop works between 0 and size-1 ?</p>
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    1. COSidenote: `rides.get(i-1)` will throw exception for first element, approach with iterators will do the same. Plus, code with iterator will throw exception for last element in the list on line with `ridesIterator.next()`.
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    2. COMy mistake, the second for should have `rides.get(i+1)` instead of `rides.get(i-1)`. Fixed
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    3. COIf `rides` is an instance of `ArrayList`, then `rides.get(i)` will finish in O(1). In this case, your transformation will be unlikely to make the program significantly faster.
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