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

Any object instance which has any mutable aspects must have a unique identity; otherwise, another object instances which at one moment happened to be identical in every way except for its identity might at some other moment be different in its mutable aspect. In many cases, though, it's useful for types not to have an identity--to be able to pass a "4" without having to worry about which "4" one is passing. While there are times when it may be helpful to have a mutable wrapper of a primitive or immutable type, there are many more times when it's useful to have a type where all instances that hold the same data at some moment in time may be regarded as interchangeable.