Well it is immutable because of the memory concern. For 
every new string if there is new object then it will take 
lot of memory and if there is such requirement then there 
is StringBuffer class provided.

Since, return type of the string class cannot be altered 
with substrings or any ammendments to it .

Immutable means which cant not be changed. whenever you call
a method though an instance of String class it will give you
the result as new String object. It wont change the existing
String object.

String class objects are made immutable so that if the
String is logically same,then more than one object can refer
the same single String in memory. in this way it also saves
memory. 

Example: String a="ABC";
         String b="ABC";

in above example JVM will create only one object with the
value "ABC" and both the reference variables a and b will
refer to the same. So it is saving memory.


If you need to change a String object very frequently, it is
not adviced to use a String object as it creates a new
object whenever you change its value. Its a best practice to
use StringBuffer. it is mutable.

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