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    <p>You are right that a <code>TrieEntry&lt;T&gt;</code> is a <code>Map.Entry&lt;String, T&gt;</code>. A <code>HashSet&lt;TrieEntry&lt;T&gt;&gt;</code> is also a <code>Set&lt;TrieEntry&lt;T&gt;&gt;</code>, but it is <strong>not</strong> a <code>Set&lt;Map.Entry&lt;String, T&gt;&gt;</code>.</p> <p>If it were you could do this:</p> <pre><code>Set&lt;TrieEntry&lt;T&gt;&gt; trieSet = ...; Set&lt;Map.Entry&lt;String, T&gt;&gt; mapSet = trieSet; mapSet.add(mapEntry); </code></pre> <p>So now trieSet would now contain a <code>Map.Entry&lt;String, T&gt;</code>! This would break generics.</p> <p>So how to solve this particular problem? Easy - use a bounded wildcard:</p> <pre><code>Set&lt;? extends Map.Entry&lt;String, T&gt;&gt; x = new HashSet&lt;TrieEntry&lt;T&gt;&gt;(); </code></pre> <p>You can read <code>? extends Map.Entry&lt;String, T&gt;</code> as "anything which is at least a <code>Map.Entry&lt;String, T&gt;</code>".</p> <hr> <p>Okay, now to your implementation problem. I actually believe, that this is <strong>not solveable</strong> in this way because of the current definition of the method entrySet(). It <em>should</em> return <code>? extends Map.Entry&lt;String, T&gt;</code>, but it <em>does</em> return <code>Map.Entry&lt;String, T&gt;</code>.</p> <p>There is actually a <a href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6231312" rel="nofollow">bug report</a> (or feature request) for exactly this problem. Looking at the submit date and the priority, a fix can't be expected soon.</p> <p>So you have two options:</p> <ol> <li><p>Drop your <code>TrieEntry</code> class and try to use <code>Map.Entry</code> instead, storing information in your keys and/or values.</p></li> <li><p>Drop the <code>Map</code> interface and let your custom <code>entrySet</code> method return <code>? extends Map.Entry&lt;String, T&gt;</code>.</p></li> </ol> <p>Both solutions may not be ideal, and there may be better solutions, but this is all what I currently can tell you.</p>
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    1. COYeah, I was little bit confused with type casting. Thanks for clarification.
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    2. COwhy not solveable? what's wrong with just creating a `Set<Map.Entry<String, T>> x = new HashSet<Map.Entry<String, T>>();` and then just putting `TrieEntry` objects into it?
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    3. CO@newacct By "not solvable" I mean, that it is not possible to let `entrySet()` return `Set<TrieEntry<T>>` - of course you can return a `Set<Map.Entry<String, T>>` instead, putting `TrieEntry`s into it (see Jespers answer!), but this won't be typesafe, and this is kinda all what generics is about. For example one could inherit from `Trie` and make a subclass where `entrySet()` doesn't return `TrieEntry`s anymore - so you would need typechecks and typecasts to guarantee typesafety.
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