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POusing comparators as embedded classes, the mother class (initialized in clone ) fields are not visible in the compare method
 

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  1. POusing comparators as embedded classes, the mother class (initialized in clone ) fields are not visible in the compare method
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    <p>I have developed a server-to-server protocol called CCMN and different drop policies for the messages cached by each server. The PEERSIM simulator creates a template node with the CCMN protocol and then clones this template node. </p> <p>The CCMN class includes different data structures maintaining the states required to implement the drop policies. For instance, <em>Pmap</em> resolves a content identifier to a content object, and <em>freq</em> a content identifier to the number of times a content object has been used.</p> <pre><code>public HashMap &lt;String, Content&gt; Pmap; public HashMap&lt;String, Integer&gt; freq; </code></pre> <p>The initialization of these data structures is done in the clone method of CCMN. In order to support the drop policies, I use a priorityQueue initialized with a comparator corresponding to the policy to use. In the clone method:</p> <pre><code>if(dp.equals(DroppingPolicy.LFU)){ </code></pre> <p>lfu less_frequent=new lfu(); av.raw_drop=new PriorityQueue(1, less_frequent); } The comparator are declared as follows:</p> <pre><code>public class lfu implements Comparator&lt;String&gt; { </code></pre> <p>public int compare(String s0, String s1) { if(freq.get(s0) > freq.get(s1)) return 1; else if(freq.get(s0) &lt; freq.get(s1)) return -1; else return 0; } }</p> <p><strong>The problem is that</strong> I get a NullPointerException exception when i call :</p> <pre><code>raw_drop.add(ct.getID()); </code></pre> <p>By debugging, I found that the add works for the first element, but the exception occurs when a second element is added. I traced the exception to the compare method, by checking the variables I find that the <em>freq</em> variable (and the other variables initialized in the clone method) equals <em>null</em>, while the variables defined in the constructor are well defined.</p> <p>Moreover, I check that <em>freq</em> is well initialized before the <em>add</em> call.</p> <p>I guess that there is a problem between the definition of the comparators as embedded classes and the <em>clone</em> method.</p> <p>Any help is welcome.</p> <p>Regards, Mohamed</p>
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      1. USJames P.
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    1. COSorry man for a fairly related comment, but why do you use such weird comparator? Isn't it easier to write return freq.get(s0).compareTo(freq.get(s1)); ?
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      1. USdchekmarev
 

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