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POGeneric method to Sort a Map on Values
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  1. POGeneric method to Sort a Map on Values
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    copied!<p>I have a small program that is supposed to sort a map based on its values. Here is what I have so far:</p> <pre><code> public static &lt;K, V extends Comparable&lt; ? extends V&gt;&gt; Map&lt;K, V&gt; sortByValues(final Map &lt;K, V&gt; mapToSort) { List&lt;Map.Entry&lt;K, V&gt;&gt; entries = new ArrayList&lt;Map.Entry&lt;K, V&gt;&gt;(mapToSort.size()); entries.addAll(mapToSort.entrySet()); Collections.sort(entries, new Comparator&lt;Map.Entry&lt;K, V&gt;&gt;() { public int compare( final Map.Entry&lt;K, V&gt; entry1, final Map.Entry&lt;K, V&gt; entry2) { return entry1.getValue().compareTo(entry2.getValue()); } }); Map&lt;K, V&gt; sortedMap = new LinkedHashMap&lt;K, V&gt;(); for (Map.Entry&lt;K, V&gt; entry : entries) { sortedMap.put(entry.getKey(), entry.getValue()); } return sortedMap; } </code></pre> <p>I want my generic value V to be comparable to anything that is <strong>either V or is a at least a subclass of V</strong>. </p> <p>I get the following error for the code piece :</p> <pre><code>public static &lt;K, V extends Comparable&lt; ? extends V&gt;&gt; </code></pre> <blockquote> <p>Bound mismatch: The method compareTo(? extends V) of type V is not applicable for the arguments (V). The wildcard parameter ? extends V has no lower bound, and may actually be more restrictive than argument V</p> </blockquote> <p><strong>How can it be more restrictive?</strong></p> <p>If I change the declaration to:</p> <pre><code>public static &lt;K, V extends Comparable&lt; ? super V&gt;&gt; </code></pre> <p>then there is no error. But this is not what I want.</p> <p>One workaround I have is that, I can change the declaration to:</p> <pre><code>public static &lt;K, V extends Comparable&lt;V&gt;&gt; </code></pre> <p>but doing this I lose the flexibility in that I cannot pass a Map whose value implements Comparable with a subclass of itself.</p> <p>Apologies for such a long question. Thanks in advance.</p>
 

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