Fair point, you're absolutely correct. Admittedly I haven't given this a TON of thought, because it seems like something that can't be added anymore. Maybe the interfaces should be siblings, but since Java doesn't have a disjoint type, you might run into problems if something says it implements both MutableList and ImmutableList. Not sure if Java's type system is strong enough to deal with this problem.
I think the Map interface should have been composed of two interfaces, ReadableMap and WriteableMap. Then APIs could return ReadableMap, which would lack the mutator methods altogether.
Then a Map would be a ReadableMap, but a ReadableMap wouldn't be a Map. If a method expects a Map, you would't be able to put a ReadableMap (or a WriteableMap) into it.
My solution so far is to avoid to return a map or to expect a map as argument.
