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. inhe

Answer 1

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

No they're not (and String isn't a primitive type). But since primitive types aren't objects anyway, they can't really be called mutable / immutable in the first place.

Regardless, the fact that wrapper classes are immutable is a design decision (a good one IMO.) They could have just has easily been made mutable, or mutable alternatives provided too (indeed several libraries provide this, and other languages do by default.)