Right, that’s exactly what they are warning about.

Not typical. e.g. Java takes the hash key of an object to be its address in memory

No, that’s just the base implementation in Object (and arguably it was a bad idea). All useful “value type” classes will override it with a real hash of the content, including String.

There are some cases in Java where you do want to use IDs instead of values as your map keys, but they’re rare.

Well, this is necessary for a lot of sensible things you'd want to do with non-numeric value types as hash keys...

> including String

...except String is something of an intermediate case. There are loads of use cases where what you're really using is a set of constant strings, not variables that contain arbitrary character data. In that case, you should intern the strings, resulting in non-"value type" keywords where the only thing you care about for equality is whether two keywords do or don't have the same machine address.

I don't actually know how Java handles this, but I had the vague idea that two equal String literals will in fact share their machine address. And String is specifically set up to accommodate this; Strings are immutable, so in theory it could easily be the case that any two equal Strings must share their machine address, even if you got them from user input.

Hey, and now you have two problems: String hashing and finding all strings which are equal to each other in memory

And this isn't exactly some exotic approach; how often do you think people write Hashes in Ruby where the keys they use are all symbols? It's so common that there's dedicated syntax for it.

The problem is, interning is an expensive operation. It means adding to an ever growing database of strings, but first checking if the string isn't already there. You don't want to do that every time you change case or flip a letter in a string, or use it to access a hash table. I'm not saying it can't be done, but I honestly have no idea how to implement sane, generic, automatic interning of strings. I feel more comfortable having a symbol type, and control over turning strings into symbols.

(And on a less fundamental level, the particular Java String class is less string-like and more symbol-like than most string types, and this appears to have been done intentionally.)