Why are Strings Immutable ?

[Arvind Sampath]

Strings in Java are Immutable. But then ,why is it so ?

I have gone through quite a few resources in the Net which deal with Immutability of Strings. But still am not able to get into a conclusion as to why Strings were made immutable at first. I have a fairly good understanding of the String Literal Pool concept. I guess am missing something.

Pls help me on this. Thanks in Advance.

Regards,
Arvind

[Mani Ram]

Performance, Safety, Simplicity.

Think about it for sometime. See whether you can appreciate the idea of having the String class as immutable based on the above mentioned points.

[Arvind Sampath]

Thanks Mani!

Can u pls elaborate a little? Am still not clear.


Regards,
Arvind

[Kai Witte]

hello,

I recommend "Effective Java" (Bloch), "Chapter 13: Favor immutability" about this topic.

Beside the advantages Mani listed immutable data objects are thread-safe, can be shared freely(*), and they are great "building blocks" for other objects.

The apparent disadvantages (a separate object is required for each distinct value, thus they are not good for multistep operations) can easily be negated by providing a mutable companion type, like StringBuilder and StringBuffer for String.

However, this is implemented poorly in the core api. The best general approach for providing an immutable and a mutable type is to have one interface for the immutable type, and one interface that extends this interface for the mutable type.

I have gone through quite a few resources in the Net which deal with Immutability of Strings.

Big mistake. Just get "Effective Java" (Bloch).

Kai

(*) Consider an

interface Customer {
    public String getName();
}

In the implementation getName can simply return a private String name without making a copy of it. The client of the api can not modify the internals of Customer by modifying the String, because it is immutable.

[Arvind Sampath]

Thanks Kai!

In the implementation getName can simply return a private String name without making a copy of it. The client of the api can not modify the internals of Customer by modifying the String, because it is immutable.

I have been using these getter methods all along, but never thought on these lines.I can now appreciate the reasons for making Strings immutable.


Regards,
Arvind