ということ。中身が不変なので、マルチスレッド処理中にロックをかける必要がないから。
Immutable objects greatly simplify your program, since they:
- are simple to construct, test, and use
- are automatically thread-safe and have no synchronization issues
- don't need a copy constructor
- don't need an implementation of clone
- allow hashCode to use lazy initialization, and to cache its return value
- don't need to be copied defensively when used as a field
- make good Map keys and Set elements (these objects must not change state while in the collection)
- have their class invariant established once upon construction, and it never needs to be checked again
- always have "failure atomicity" (a term used by Joshua Bloch): if an immutable object throws an exception, it's never left in an undesirable or indeterminate state
