HashMap with a custom equality check

[Meidan]

Hi,

Is there a java class that gives the functionality of a Hashtable or a
HashMap but doesn't use the keys' equals() method, but rather a given
equality comparar? Something like IEqualityComparer in c#:
http://msdn2.microsoft.com/en-us/library/ms132151.aspx

I wouldn't want to use a TreeMap with a Comparator as I don't need any
sorting.

Thanks.

 

[Patricia Shanahan]

Hi,

Is there a java class that gives the functionality of a Hashtable or a
HashMap but doesn't use the keys' equals() method, but rather a given
equality comparar? Something like IEqualityComparer in c#:
http://msdn2.microsoft.com/en-us/library/ms132151.aspx

I wouldn't want to use a TreeMap with a Comparator as I don't need any
sorting.

Thanks.

To make HashMap work you would also need to supply a hashCode method
that is consistent with the equality test you are using in the map.

You could use a wrapper class that keeps a reference to the object, and
overrides both equals and hashCode.

Patricia

 

[Meidan]

To make HashMap work you would also need to supply a hashCode method
that is consistent with the equality test you are using in the map.

You could use a wrapper class that keeps a reference to the object, and
overrides both equals and hashCode.

Patricia

I've thought about it, but code is much clearer when you don't have to
go through a wrapper.

 

[Lew]

I've thought about it, but code is much clearer when you don't have to
go through a wrapper.

Rrr?

 

[Lasse Reichstein Nielsen]

I've thought about it, but code is much clearer when you don't have to
go through a wrapper.

Then make an adapter class that does it for you:

Map myMap = new EqualityMapAdapter(new HashMap(), myEqualityRelation);

Should be fairly easy to write. Remember that your equality relation
should also provide compatible hash codes, e.g.

public interface Equality {
boolean equals(Object obj1, Object obj2);
int hashCodeOf(Object obj);
}

/L

 

[Meidan]

Then make an adapter class that does it for you:

Map myMap = new EqualityMapAdapter(new HashMap(), myEqualityRelation);

Should be fairly easy to write. Remember that your equality relation
should also provide compatible hash codes, e.g.

public interface Equality {
boolean equals(Object obj1, Object obj2);
int hashCodeOf(Object obj);
}

/L

Thanks. I just figured someone had already written something like that.

 

[Lasse Reichstein Nielsen]

Thanks. I just figured someone had already written something like that.

I'm guessing Commons Collections, but do tell so we can learn

/L

 

[Roedy Green]

Is there a java class that gives the functionality of a Hashtable or a
HashMap but doesn't use the keys' equals() method, but rather a given
equality comparar? Something like IEqualityComparer in c#:
http://msdn2.microsoft.com/en-us/library/ms132151.aspx

You would subclass that object and provide your custom equals. It
would have a copy constructor i.e. one that ate one of the old class
objects. Then you store these new objects in your HashMap.

Be careful to make sure your hashCode function maintains the contract
with equals.


Your new objects might just POINT to an old object.