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?
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.
It’s mainly intended to prevent programming errors. For example, Strings are frequently used as keys in hashtables. If they could change, the hashtable would become corrupted. And that’s just one example where having a piece of data change while you’re using it causes problems. Security is another: if you checking whether a user is allowed to access a file at a given path before executing the operation they requested, the string containing the path better not be mutable…
It becomes even more important when you’re doing multithreading. Immutable data can be safely passed around between threads while mutable data causes endless headaches.
Basically, immutable data makes the code that works on it easier to reason about. Which is why purely functional languages try to keep everything immutable.
In Java not only String but all primitive Wrapper classes (Integer, Double, Character etc) are immutable. I am not sure of the exact reason but I think these are the basic data types on which all the programming schemes work. If they change, things could go wild. To be more specific, I’ll use an example: Say you have opened a socket connection to a remote host. The host name would be a String and port would be Integer. What if these values are modified after the connection is established.
As far as performance is concerned, Java allocates memory to these classes from a separate memory section called Literal Pool, and not from stack or Heap. The Literal Pool is indexed and if you use a string “String” twice, they point to the same object from Literal pool.
Having strings as immutable also allows the new string references easy, as the same/similar strings will be readily available from the pool of the Strings previously created. Thereby reducing the cost of new object creation.