I've seen it used in an old code base so that a specialized Map implementation could be used with String keys. I'm not even sure of the name of the library it

I've seen it used in an old code base so that a specialized Map implementation could be used with String keys. I'm not even sure of the name of the library it came from or what it was called - maybe "ObjectHashMap" or something like that - but its semantics were that it used reference equality instead of the structural equality used by a regular HashMap. As a result map operations depended on base Object.hashCode and reference equality which are constant and presumably very quick operations. Using a regular HashMap, the map operations would depend on String.hashCode() and String.equals() which are O(n) in the length of the String - although the return value from hashCode is cached. On reflection and this was an old code-base, I'm somewhat sceptical that this was a win given that String keyed-HashMaps are so common that I'd imagine a lot of JVM optimisation effort has gone into this area. I guess it would depend on the pattern of use. If the universe of String keys was known in advance, then I guess it operated like a poor mans perfect hash. On the other hand, this is the kind of optimization in Java which I've found to be unstable with performance varying significantly between JVM versions and platforms.