Question

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Why can't strings be mutable in Java and .NET?
Why .NET String is immutable?

Several languages have chosen for this, such as C#, Java, and Python. If it is intended to save memory or gain efficiency for operations like compare, what effect does it have on concatenation and other modifying operations?

Answer 1

Immutable types are a good thing generally:

• They work better for concurrency (you don’t need to lock something that can’t change!)
• They reduce errors: mutable objects are vulnerable to being changed when you don’t expect it which can introduce all kinds of strange bugs (“action at a distance”)
• They can be safely shared (i.e. multiple references to the same object) which can reduce memory consumption and improve cache utilisation.
• Sharing also makes copying a very cheap O(1) operation when it would be O(n) if you have to take a defensive copy of a mutable object. This is a big deal because copying is an incredibly common operation (e.g. whenever you want to pass parameters around….)

As a result, it’s a pretty reasonable language design choice to make strings immutable.

Some languages (particularly functional languages like Haskell and Clojure) go even further and make pretty much everything immutable. This enlightening video is very much worth a look if you are interested in the benefits of immutability.

There are a couple of minor downsides for immutable types:

• Operations that create a changed string like concatenation are more expensive because you need to construct new objects. Typically the cost is O(n+m) for concatenating two immutable Strings, though it can go as low as O(log (m+n)) if you use a tree-based string data structure like a Rope. Plus you can always use special tools like Java’s StringBuilder if you really need to concatenate Strings efficiently.
• A small change on a large string can result in the need to construct a completely new copy of the large String, which obviously increases memory consumption. Note however that this isn’t usually a big issue in garbage-collected languages since the old copy will get garbage collected pretty quickly if you don’t keep a reference to it.

Overall though, the advantages of immutability vastly outweigh the minor disadvantages. Even if you are only interested in performance, the concurrency advantages and cheapness of copying will in general make immutable strings much more performant than mutable ones with locking and defensive copying.