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    <p>I'd move to lists and solve it this way:<br/></p> <ol> <li>Sort both lists by id ascending using custom <a href="http://java.sun.com/j2se/1.5.0/docs/api/index.html?java/util/Comparator.html" rel="nofollow noreferrer">Comparator</a> if objects in lists aren't <a href="http://java.sun.com/j2se/1.5.0/docs/api/index.html?java/lang/Comparable.html" rel="nofollow noreferrer">Comparable</a></li> <li>Iterate over elements in both lists like in merge phase in <a href="http://en.wikipedia.org/wiki/Merge_sort" rel="nofollow noreferrer">merge sort algorithm</a>, but instead of merging lists, you check your logic.</li> </ol> <p>The code would be more or less like this:</p> <pre><code>/* Main method */ private void execute(Collection&lt;Foo&gt; oldSet, Collection&lt;Foo&gt; newSet) { List&lt;Foo&gt; oldList = asSortedList(oldSet); List&lt;Foo&gt; newList = asSortedList(newSet); int oldIndex = 0; int newIndex = 0; // Iterate over both collections but not always in the same pace while( oldIndex &lt; oldList.size() &amp;&amp; newIndex &lt; newIndex.size()) { Foo oldObject = oldList.get(oldIndex); Foo newObject = newList.get(newIndex); // Your logic here if(oldObject.getId() &lt; newObject.getId()) { doRemove(oldObject); oldIndex++; } else if( oldObject.getId() &gt; newObject.getId() ) { doAdd(newObject); newIndex++; } else if( oldObject.getId() == newObject.getId() &amp;&amp; isModified(oldObject, newObject) ) { doUpdate(oldObject, newObject); oldIndex++; newIndex++; } else { ... } }// while // Check if there are any objects left in *oldList* or *newList* for(; oldIndex &lt; oldList.size(); oldIndex++ ) { doRemove( oldList.get(oldIndex) ); }// for( oldIndex ) for(; newIndex &lt; newList.size(); newIndex++ ) { doAdd( newList.get(newIndex) ); }// for( newIndex ) }// execute( oldSet, newSet ) /** Create sorted list from collection If you actually perform any actions on input collections than you should always return new instance of list to keep algorithm simple. */ private List&lt;Foo&gt; asSortedList(Collection&lt;Foo&gt; data) { List&lt;Foo&gt; resultList; if(data instanceof List) { resultList = (List&lt;Foo&gt;)data; } else { resultList = new ArrayList&lt;Foo&gt;(data); } Collections.sort(resultList) return resultList; } </code></pre>
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    1. USBartosz Bierkowski
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