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    <p>Assuming this is what you meant:</p> <pre><code>// traditional for loop for (int i = 0; i &lt; collection.size(); i++) { T obj = collection.get(i); // snip } // using iterator Iterator&lt;T&gt; iter = collection.iterator(); while (iter.hasNext()) { T obj = iter.next(); // snip } // using iterator internally (confirm it yourself using javap -c) for (T obj : collection) { // snip } </code></pre> <p>Iterator is faster for collections with no random access (e.g. TreeSet, HashMap, LinkedList). For arrays and ArrayLists, performance differences should be negligible. </p> <p>Edit: I believe that micro-benchmarking is root of pretty much evil, just like early optimization. But then again, I think it's good to have a feeling for the implications of such quite trivial things. Hence I've run <a href="http://pastebin.com/f3a2ebd05" rel="noreferrer">a small test</a>: </p> <ul> <li>iterate over a LinkedList and an ArrayList respecively</li> <li>with 100,000 "random" strings</li> <li>summing up their length (just something to avoid that compiler optimizes away the whole loop)</li> <li>using all 3 loop styles (iterator, for each, for with counter)</li> </ul> <p>Results are similar for all but "for with counter" with LinkedList. All the other five took less than 20 milliseconds to iterate over the whole list. Using <code>list.get(i)</code> on a LinkedList 100,000 times took more than 2 minutes (!) to complete (60,000 times slower). Wow! :) Hence it's best to use an iterator (explicitly or implicitly using for each), especially if you don't know what type and size of list your dealing with. </p>
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    1. COYour LinkedList result shows what happens when you go from O(n) to O(n^2) (or more)
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      1. USThorbjørn Ravn Andersen
    2. CO*All the other five took less than 20 milliseconds to iterate over the whole list* looks like the JVM dead-code optimization kicked in... The difference between iteration of LinkedList and ArrayList are significant (in favor of the ArrayList)
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    3. CO@bestsss no, it certainly didn't. I've generated 100,000 random strings (actually UUIDs) and summed their lengths which was printed to stdout after the loop. Sure, UUIDs have the same length which makes the output predictable, but the compiler isn't that smart. Believe it or not, but a modern CPU can do that in 20 ms. To give another perspective: my CPU has 4,000 BogoMips per core. So we're talking about billions of instructions per s or millions per ms. Thus, iterating over 100,000 strings with several millions of instructions is feasible. CPUs are faster than most developers think :)
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