You need to override the equals method on Proposition : ( Collections
use the equals method to check for equality )
You have to take care when you override the equals method that you
implement the contract of the equals method properly... ( Check the
JavaDoc on java.lang.Object )...
If you don't... you get some VERY funky behaviour on collections... :)
.... this SHOULD implement the contract of equals() correctly... just
give it another pair of eyballs...
public boolean equals( Object o ) {
if ( ! o instanceof Proposition ) {
return false;
}
if ( o == null ) {
return false;
}
return ObjectUtils.equals(getName(),other.getName());
}
ObjectUtils is part of commons-lang and takes care of all the
additional null checking...
The lazy developers friend.. :):):)
Cheers,
Renier
On 5/10/06, Fritz Meissner <[EMAIL PROTECTED]> wrote:
> 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 ;-).
>
>
> >
>
--
Regards,
Renier Rhode
mobile: 083 556 0804
e-mail: [EMAIL PROTECTED]
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