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    <p>Your <code>Pair</code> class needs to implement <code>equals()</code> and <code>hashCode()</code> and you're all set. <code>List.contains()</code> is implemented in terms of the type's <code>equals()</code> method. See the <a href="http://download.oracle.com/javase/6/docs/api/java/util/List.html#contains%28java.lang.Object%29" rel="nofollow noreferrer">API for <code>List.contains()</code></a>. (Edited a bit to address comments from @maaartinus, whose answer you should read b/c the observations are solid, and it's a bit ridiculous for me to fold them in here. As maaartinus points out, a best-practice here would be to avoid error-prone manual definitions for equals and hashcode, and instead build on Guava's helper functions for <a href="http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/base/Objects.html#equal%28java.lang.Object,%20java.lang.Object%29" rel="nofollow noreferrer">nullable equals</a> and <a href="http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/base/Objects.html#hashCode%28java.lang.Object...%29" rel="nofollow noreferrer">hashCode for n objects</a>).</p> <pre><code>final class Pair&lt;T&gt; { final T left; final T right; public Pair(T left, T right) { if (left == null || right == null) { throw new IllegalArgumentException("left and right must be non-null!"); } this.left = left; this.right = right; } public boolean equals(Object o) { // see @maaartinus answer if (! (o instanceof Pair)) { return false; } Pair p = (Pair)o; return left.equals(p.left) &amp;&amp; right.equals(p.right); } public int hashCode() { return 7 * left.hashCode() + 13 * right.hashCode(); } } </code></pre> <p>With suitable <code>equals()</code>, you can now do:</p> <pre><code> lp.add(new Pair("1", "2")); assert lp.contains(new Pair("1","2")); </code></pre> <p>Responding to the comments below, perhaps it would be good to include a good reference for "Why do I need to implement <code>hashCode()</code>?"</p> <ul> <li><p>JavaPractices.com &mdash; <a href="http://www.javapractices.com/topic/TopicAction.do?Id=17" rel="nofollow noreferrer">Implementing <code>equals()</code></a> &mdash; "if you override equals, you must override hashCode"</p></li> <li><p><a href="http://download.oracle.com/javase/6/docs/api/java/lang/Object.html#equals%28java.lang.Object%29" rel="nofollow noreferrer"><code>Object.equals()</code> contract as defined in the API documentation</a></p></li> <li><p><a href="https://stackoverflow.com/questions/27581/overriding-equals-and-hashcode-in-java/27609#27609">StackOverflow answer</a></p></li> </ul>
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    1. COGood answer. I think I pushed you over 3000. Cool.
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    2. COAnd a tip of the hat to you, Hovercraft...
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    3. COThx @spong for the fix.
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