Couple of things that if you knew you’d
probably be able to solve your problem…
- When
using “Hash” type collection you must override hashCode and
equals and _not_ compareTo.
Compare would work if you were using tree based collections. In other
words, override equals and hashCode and make it work against the name
field of the object.
- I’m
not sure what you mean by “prop1 == prop2”. Whenever you
compare objects like this it compares the references (the value of the
references of the objects). This will not
compare the objects by the values in the objects. The fact that your “prop1
== prop2” is merely a coincidence, you’re just lucky.
From: Fritz Meissner
[mailto:[EMAIL PROTECTED]
Sent: 10 May 2006 05:47 PM
To: [email protected]
Subject: [CTJUG Forum] Help with
comparing generics
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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