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    <p>There are two versions of <code>Collections.sort()</code> . </p> <p><a href="http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#sort%28java.util.List%29" rel="nofollow">Collections.sort(List)</a> and <a href="http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#sort%28java.util.List,java.util.Comparator%29" rel="nofollow">Collections.sort(List,Comparator)</a> . One takes a <code>List</code>and another takes a <code>List</code> and an instance of <code>Comparator</code>. The single argument <code>sort()</code> will expect your class to have implemented <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Comparable.html" rel="nofollow">Comparable</a> and overridden the <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Comparable.html#compareTo%28T%29" rel="nofollow">compareTo()</a> method. The two argument <code>sort()</code> method expects an instance of <a href="http://docs.oracle.com/javase/6/docs/api/java/util/Comparator.html" rel="nofollow">Comparator</a>, wherein you have implemented the <code>Comparator</code> and overridden its <a href="http://docs.oracle.com/javase/6/docs/api/java/util/Comparator.html#compare%28T,T%29" rel="nofollow">compare()</a> method.</p> <p>An object should implement <code>Comparable</code> if that is the clear natural way to sort the class, and anyone would need to sort the class would generally want to do it that way.</p> <p>If, however, the sorting was an unusual use case of the class or there can be multiple sort orders then a <code>Comparator</code> is a better option.</p> <p><em>Also I have the Collections.sort(items) call right after I declare my ArrayList, is this okay?</em></p> <p>You will call <code>sort()</code> when you need to sort the <code>List</code>. Adding items after <code>sort()</code> won't automatically sort the <code>List</code> to accommodate the change in ordering of list items due to addition of new elements.</p> <p>Sample implementation:</p> <pre><code>public int compareTo (Item other) { return new Integer(this.getID()).compareTo(new Integer(other.getID())); } </code></pre>
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    1. POSorting ArrayList full of objects by their id using Collections.sort(arrayListName)
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    1. COI have to use the one that just calls Collections.sort(items)
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    2. COOk , but based on what field ?
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    3. COWhat do you mean, I don't understand sorry
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