Why are Java wrapper classes immutable?

java immutability mutable primitive-types
shrini1000 picture shrini1000 · Sep 11, 2012 · Viewed 22.5k times · Source

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

Jon Skeet picture Jon Skeet · Sep 11, 2012

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.)

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