Why String is immutable or final in Java

Question

As i was told this is important String Interview question in Java, which starts with discussion of " What is String ", how String is different in java than in C or C++ and then you are asked about immutable objects and you're asked the main question: " Why String is immutable or final in Java ".

Can you share your Ideas ?

Thanks in advance.

http://javarevisited.blogspot.co.uk/2010/10/why-string-is-immutable-in-java.html — NPE, Apr 12 '13 at 09:31
Related: [What's the advantage of a String being Immutable?](http://stackoverflow.com/questions/3407403) — McDowell, Apr 12 '13 at 10:06

Mohan Raj B · Accepted Answer · 2013-04-12T09:59:32.173

27

It is mainly for security reasons. String is used as parameter in network connection, database url etc. It can be easily attacked if it is mutable

Immutability of String solves some synchronization issues, it makes the String thread safe

To support StringPool facility

To cache the hashcode of String

To support class loading mechanism in which String is used as arguments. String being mutable results in wrong class being loaded

edited Apr 12 '13 at 09:59

answered Apr 12 '13 at 09:43

Mohan Raj B

1,015
7
14

I know you have covered all points. But explanation to each point would be fantastic – Nandish A Jun 09 '13 at 17:43
@mohan Raj : you have mentioned the issues , can you please put a little more explanation to each point , may be a little practical example or something – Hussain Akhtar Wahid 'Ghouri' Nov 05 '13 at 06:39

score 8 · Answer 2 · answered Apr 12 '13 at 09:35

8

The two main reasons why strings are immutable in many modern languages, including Java, are security and performance (or, more precisely, chance for optimizations).

The fact that strings are final is to ensure their immutability (by forbidding anyone from extending them and making them mutable again).

answered Apr 12 '13 at 09:35

Theodoros Chatzigiannakis

28,773
8
68
104

2

I concur. Also related to performance and optimisation, immutable data structures are much easier to reason about especially with multi threaded algorithms. But the core reason in Java originally for java.lang.String being immutable was Security. – Chris K Apr 12 '13 at 09:37

Nomesh Gajare · Answer 3 · 2013-04-12T09:53:24.030

The most important reason is security.

A lot of security risks would appear if a malicious thread could gain a reference to a mutable String, which is about to be passed into a method that has to validate the String before it performs an important operation. It would be possible for the thread to change the string after it was validated, and then the operation would be carried out using a dangerous String.

Another reason of Why String is immutable in Java is to allow String to cache its hashcode

As mentioned above - the most important reason - security & thread safety.

Consider a scenario, in a banking application for money transfer - the beneficiary account number is defined in a string as "0789567345". If by mistake/intentionally this acc. number is changed, money will go to a wrong account.

Another scenario - if someone change the class name anywhere between processing as ..

getClass().getName().subString(0, 5);

The Class loader will simply say 'Class Not Found

Why String is immutable or final in Java

3 Answers3

Linked