Why are Java wrapper classes immutable?

Question

I know the usual reasons that apply to general immutable classes, viz

1. can not change as a side effect
2. easy to reason about their state
3. inherently thread safe
4. no need to provide clone/copy constructor/factory copy method
5. instance caching
6. no need for defensive copies.

However, wrapper classes represent primitive types, and primitive types are mutable. So why aren't wrapper classes mutable?

Answer 1

However, wrapper classes represent primitive types, and primitive types (except String) are mutable.

Firstly, String isn't a primitive type.

Secondly, it makes no sense to talk about the primitive types being mutable. If you change the value of a variable like this:

int x = 5;
x = 6;

That's not changing the number 5 - it's changing the value of x.

While the wrapper types could have been made mutable, it would have been annoying to do so, in my view. I frequently use readonly collections of these types, and wouldn't want them to be changeable. Very occasionally I want a mutable equivalent, but in that case it's easy enough to come up with one, or use the Atomic* classes.

I find myself wishing that Date and Calendar were immutable far more often than I find myself wanting Integer to be mutable... (Of course I normally reach for Joda Time instead, but one of the benefits of Joda Time is immutability.)