String is Immutable in Java because String objects are cached in String pool. Since cached String literal is shared between multiple client there is always a risk, where one client's action would affect all other client. For example, if one client changes value of String "Test" to "TEST", all other client will also see that value as explained in first example. Since caching of String objects was important from performance reason this risk was avoided by making String class Immutable. At the same time, String was made final so that no one can compromise invariant of String class e.g. Immutability, Caching, hascode calculation etc by extending and overriding behaviors. Another reason of why String class is immutable could de due to HashMap. Since Strings are very popular as HashMap key, it's important for them to be immutable so that they can retrieve the value object which was stored in HashMap. Since HashMap works in principle of hashing, which requires same has value to function properly. Mutable String would produce two different hashcode at the time of insertion and retrieval if contents of String was modified after insertion, potentially losing the value object in map. If you are an Indian cricket fan, you may be able to correlate with my next sentence. String is VVS Laxman of Java, i.e. very very special class. I have not seen a single Java program which is written without using String. That's why solid understanding of String is very important for a Java developer. Important and popularity of String as data type, transfer object and mediator has also make it popular on Java interviews. Why String is immutable in Java is one of the most frequently asked String Interview questions in Java, which starts with discussion of, What is String, How String in Java is different than String in C and C++, and then shifted towards what is immutable object in Java , what are the benefits of immutable object, why do you use them and which scenarios should you use them. This question some time also asked as "Why String is final in Java". e
Why String is Final in Java
As I said, there could be many possible answer of this question, and only designer of String class can answer it with confidence, I think following two reasons make a lot of sense on why String class is made Immutable or final in Java :
1) Imagine String pool facility without making string immutable , its not possible at all because in case of string pool one string object/literal e.g. "Test" has referenced by many reference variables , so if any one of them change the value others will be automatically gets affected i.e. lets say
String A = "Test"
String B = "Test"
Now String B called "Test".toUpperCase() which change the same object into "TEST" , so A will also be "TEST" which is not desirable.
2)String has been widely used as parameter for many Java classes e.g. for opening network connection, you can pass hostname and port number as string , you can pass database URL as string for opening database connection, you can open any file in Java by passing name of file as argument to File I/O classes.
In case, if String is not immutable, this would lead serious security threat , I mean some one can access to any file for which he has authorization, and then can change the file name either deliberately or accidentally and gain access of those file. Because of immutability, you don't need to worry about those kind of threats. This reason also gel with, Why String is final in Java, by making java.lang.String final, Java designer ensured that no one overrides any behavior of String class.
3)Since String is immutable it can safely shared between many threads ,which is very important for multithreaded programming and to avoid any synchronization issues in Java, Immutability also makes String instance thread-safe in Java, means you don't need to synchronize String operation externally. Another important point to note about String is memory leak caused by SubString, which is not a thread related issues but something to be aware of.
4) Another reason of Why String is immutable in Java is to allow String to cache its hashcode , being immutable String in Java caches its hashcode, and do not calculate every time we call hashcode method of String, which makes it very fast as hashmap key to be used in hashmap in Java. This one is also suggested by Jaroslav Sedlacek in comments below. In short because String is immutable, no one can change its contents once created which guarantees hashCode of String to be same on multiple invocation.
5) Another good reason of Why String is immutable in Java suggested by Dan Bergh Johnsson on comments is: The absolutely most important reason that String is immutable is that it is used by the class loading mechanism, and thus have profound and fundamental security aspects. Had String been mutable, a request to load "java.io.Writer" could have been changed to load "mil.vogoon.DiskErasingWriter"
Security and String pool being primary reason of making String immutable, I believe there could be some more very convincing reasons as well, Please post those reasons as comments and I will include those on this post. By the way, above reason holds good to answer, another Java interview questions "Why String is final in Java". Also to be immutable you have to be final, so that your subclass doesn't break immutability. what do you guys think ?
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