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    <p>Since <a href="//docs.oracle.com/javase/8/docs/api/java/util/Date.html" rel="noreferrer"><code>Date</code></a> implements <a href="//docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html" rel="noreferrer"><code>Comparable</code></a>, it has a <code>compareTo</code> method just like <code>String</code> does.</p> <p>So your custom <a href="//docs.oracle.com/javase/8/docs/api/java/util/Comparator.html" rel="noreferrer"><code>Comparator</code></a> could look like this:</p> <pre><code>public class CustomComparator implements Comparator&lt;MyObject&gt; { @Override public int compare(MyObject o1, MyObject o2) { return o1.getStartDate().compareTo(o2.getStartDate()); } } </code></pre> <p>The <code>compare()</code> method must return an <code>int</code>, so you couldn't directly return a <code>boolean</code> like you were planning to anyway.</p> <p>Your sorting code would be just about like you wrote:</p> <pre><code>Collections.sort(Database.arrayList, new CustomComparator()); </code></pre> <p>A slightly shorter way to write all this, if you don't need to reuse your comparator, is to write it as an inline anonymous class:</p> <pre><code>Collections.sort(Database.arrayList, new Comparator&lt;MyObject&gt;() { @Override public int compare(MyObject o1, MyObject o2) { return o1.getStartDate().compareTo(o2.getStartDate()); } }); </code></pre> <hr> <h1>Since <a href="/questions/tagged/java-8" class="post-tag" title="show questions tagged &#39;java-8&#39;" rel="tag">java-8</a></h1> <p>You can now write the last example in a shorter form by using a <a href="//docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html" rel="noreferrer">lambda expression</a> for the <code>Comparator</code>:</p> <pre><code>Collections.sort(Database.arrayList, (o1, o2) -&gt; o1.getStartDate().compareTo(o2.getStartDate())); </code></pre> <p>And <code>List</code> has a <a href="//docs.oracle.com/javase/8/docs/api/java/util/List.html#sort-java.util.Comparator-" rel="noreferrer"><code>sort(Comparator)</code></a> method, so you can shorten this even further:</p> <pre><code>Database.arrayList.sort((o1, o2) -&gt; o1.getStartDate().compareTo(o2.getStartDate())); </code></pre> <p>This is such a common idiom that there's <a href="//docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparing-java.util.function.Function-" rel="noreferrer">a built-in method</a> to generate a <code>Comparator</code> for a class with a <code>Comparable</code> key:</p> <pre><code>Database.arrayList.sort(Comparator.comparing(MyObject::getStartDate)); </code></pre> <p>All of these are equivalent forms.</p>
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    1. CO+1 for mentioning that it should return `int` and that you'd better to use `Date#compareTo()` for this. Why this isn't upvoted above the other answer is beyond me. This link may also be useful: [Object Ordering Tutorial at Sun.com](http://java.sun.com/docs/books/tutorial/collections/interfaces/order.html).
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