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

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 nod开发者_Go百科e.

The CCMN class includes different data structures maintaining the states required to implement the drop policies. For instance, Pmap resolves a content identifier to a content object, and freq a content identifier to the number of times a content object has been used.

public HashMap <String, Content> Pmap;
public HashMap<String, Integer> freq;

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:

if(dp.equals(DroppingPolicy.LFU)){

lfu less_frequent=new lfu(); av.raw_drop=new PriorityQueue(1, less_frequent); } The comparator are declared as follows:

public class lfu implements Comparator<String> {

public int compare(String s0, String s1) { if(freq.get(s0) > freq.get(s1)) return 1; else if(freq.get(s0) < freq.get(s1)) return -1; else return 0; } }

The problem is that I get a NullPointerException exception when i call :

raw_drop.add(ct.getID());

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 freq variable (and the other variables initialized in the clone method) equals null, while the variables defined in the constructor are well defined.

Moreover, I check that freq is well initialized before the add call.

I guess that there is a problem between the definition of the comparators as embedded classes and the clone method.

Any help is welcome.

Regards, Mohamed

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Ok, but this will not change anything to the problem, because the problem is that freq is not initialized in the compare method...

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I have fixed the problem. In fact, the nested comparators referenced the template instance instead of the cloned instance. I add a counter so that the first time, the method the instruction raw_drop.add(ct.getID()) belongs to, is called the priorityQueue used by the instance is initialized in order to have the right initialization.