Why String is immutable in Java?

Why String is immutable in Java?

String is immutable for several reasons, here is a summary:

·         Security: Parameters are typically represented as String in network connections, database connection urls, usernames/passwords etc. If it were mutable, these parameters could be easily changed.

·         String pool : If String weren’t immutable , then we never had String pool great feature in String class. Let say  :

String a=“Paras”;

String b=“Paras”

We all know that a and b points to same Object in heap, if a changed to ”Chawla”, b will still point to “Paras” which is not possible if String weren’t immutable.

String a="Paras";

String b=a;

System.out.println(a==b);

a="Chawla";

System.out.println(a +b);

Output:

true

ChawlaParas

·         Synchronization and concurrency: making String immutable automatically makes them thread safe thereby solving the synchronization issues. Since String is immutable it can safely share 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.

·         Caching: when compiler optimizes your String objects, it sees that if two objects have same value (a="test", and b="test") and thus you need only one string object (for both a and b, these two will point to the same object). String is immutable, no one can change its contents once created which guarantees hashCode of String to be same on multiple invocations.

·         Class loading: String is used as arguments for class loading. If mutable, it could result in wrong class being loaded (because mutable objects change their state). Had String been mutable, a request to load "java.io.Writer" could have been changed to load "mil.vogoon.DiskErasingWriter"

If I have class with all static members is it immutable?

If your class has only static members, then objects of this class are immutable, because you cannot change the state of that object ( you probably cannot create it either :) )