[CTJUG Forum] Help with comparing generics

[Fritz Meissner]

Hi all,

Sorry for the verbosity of the post. I have been looking all over the net to find the solution to this one. I'm sure that this must be obvious if you know a lot (or some) about generics. I don't. :/

I have written a class called Proposition, which has a String field called Name. I have two HashSet<Proposition> collections, and I want to perform set operations on them - specifically, I am calling HashSet.retainAll() to perform an intersect-like operation on the two sets. I want the Proposition objects to be compared *by their Name field only*, so I wrote the Proposition class to implement Comparable<Proposition>, and overwrote the compareTo() method in the interface. I know that this works, because if do I this:

Proposition prop1 = new Proposition("test");
Proposition prop2 = new Proposition("test");

and I call prop1==prop2 I get the right result  (I also made the compareTo() method give output so that I know I am calling this method). But now if I do THIS:

         HashSet<Proposition> testSet1 = new HashSet<Proposition>();
         HashSet<Proposition> testSet2 = new HashSet<Proposition>();
        
         Proposition prop1 = new Proposition("test");
         Proposition prop2 = new Proposition("goneElement");
         Proposition prop3 = new Proposition("test");
        
         testSet1.add(prop1);
         testSet1.add(prop2);
        
         testSet2.add(prop3);
        
         testSet1.retainAll(testSet2);
       
         System.out.println(testSet1.size());       
        
I get "0". It is clearly not seeing prop3 and prop1 as equal - but then this is not surprising, since the output I put in Proposition's compareTo() method is not being displayed. Presumably for some obscure reason Java is calling something else to do the comparison. Someone please tell me where, and what I do to make it behave the way I want it to ;-).


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