Nava-Why Strings are Immutable?

In Java, String object is defined as immutable...meaning it cannot be changed, everytime you change the object [concatenate,replace etc], VM actually creates a new string object and update the references if required

[code]
String x="Nava";
x=x+" World";
S.O.P(x); // Nava World
/* Here actually three String objects are created--"Nava", "World" and "Nava World" and x is refrenced to third object, whereas other two String objects do not have any references which can be considered as "Lost" */
[/code]

I used to wonder, why such a concept "immutability" used in Java for String object.....and finally got elucidated.

Strings are the most commonly used objects in any programming languages, especially in Java. In terms of memory usage, they are on the top most place.

JVM designers well considered this aspect and constructed a special area of memory called the "String constant pool". When the compiler encounters a String literal, it checks the pool to see if an identical String already exists. If a match is found, the reference to the new literal is directed to the existing String, and no new String literal object is created.The existing String simply has an additional reference.

Ah! I hear how you appreciate.. why making String objects immutable is such a good idea.If several reference variables refer to the same String,without even knowing it, it would be bad if any of them could change the String's value.

Cool..but what if one overides the String Object? ..well String is a FINAL class :-)