How To Write an Equality Method in Java

Re:How To Write an Equality Method in Java by amalcon on Tuesday June 09, 2009 @02:52PM (#649647)

Technically it's not a problem to define equals based on mutable data, but it is a problem to define hashCode based on mutable data. One could have a class with, say, an immutable X and a mutable Y. You probably want to check equality based on both X and Y, but hash based on X.

Or, y'know, just don't use mutable data for keys. I thought that was what people were supposed to do anyway.

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Re:How To Write an Equality Method in Java by stcredzero on Tuesday June 09, 2009 @04:16PM (#649889)

Maybe languages should have special interfaces for things that can be dictionary keys? That way, when someone implements that interface, you are certifying that the object is suitable for being a key.

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Re:How To Write an Equality Method in Java by pmjordan on Tuesday June 09, 2009 @07:13PM (#650190)

C++'s std::[unordered_]{map|multimap|set|multiset} containers all make const copies of keys. Of course, this being C++ you can wiggle out of this requirement (e.g. using the mutable keyword) but you have to be pretty insistant about it.

On the other hand, C++ containers have all sorts of other problems (iterator invalidation to name one)...

C# has the value type concept, but I don't think it's enforced on container keys.

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