Thursday, September 8, 2011

immutables -- total costs of ownership

I always maintain that one of the practical cures for deadlock is immutable object, but there are several costs.

* every time you deep-clone, you must ensure all the objects cloned are immutable.
* every method and constructor taking in an object arg must deep-clone that object.
* every method returning an object must deep-clone the object. Note Java strings are inherently immutable
* deep-cloning usually require massive chain-reaction. If MyImm class has a pointer field of MyImm, then the deep-cloning will clone a linked list. Immutability means object state immutable. If your definition of "State" includes that MyImm field, then the entire linked list is part of the state of the head node! Cloning could lead to stack overflow.

* It's easy to overlook a detail, and unknowingly create an almost-immutable, with (possibly disastrous) consequences in production
* immutable classes are harder to extend, limiting reusability, flexibility and usefulness.

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