Immutability — Notes

Heuristics

• Default to immutable. Make a class mutable only when there's a reason.
• Value objects (Money, Date, IP, URL) almost always immutable.
• Entity objects (User, Account) often need controlled mutation.
• Containers passed across thread / module boundaries → immutable.

Costs

• Allocations + GC pressure.
• Awkward when you need to update one field of a 30-field object — language sugar helps (copy(x = ...) in Kotlin, records' withers).

Language support

• Java records, record keyword.
• Kotlin data class val.
• Python frozen=True dataclass; tuple, frozenset.
• C# records, readonly struct, ImmutableList.
• Rust default-immutable; mut is opt-in.
• Clojure / Haskell — immutable from the ground up.

Refs

• Effective Java — Item 17 (minimize mutability).
• "Out of the Tar Pit" — Moseley & Marks, 2006.
• Rich Hickey: "The Value of Values".