why String is final ?

[yashendra chandrakar]

Hi
friends,
What's the reason behind making String immutable.

[Thomas Paul]

Performance. Immutable strings can be placed in a string pool. Final classes and methods don't participate in polymorphism and can be inlined at compile time.

[Sonny Gill]

I read somewhere that it also has to do something with security. So, someone cant write a String subclass and get at some secret information stored in Strings, something like that...

[John Dale]

Consider

class SpyString extends String {
 List stolenData = new ArrayList();
 public boolean equals(String s) {
  stolenData.add(s);
  return super.equals(s);
 }
}

It would be good if nobody could make such a SpyString and pass it into code that might be expected to compare the it to some confidential information, like a password or PIN.

[Sayed Ibrahim Hashimi]

The String class being final is different then the fact that strings are immutable.
Being final means that the String class cannot be subclassed. And the fact the strings are immutable means that once a String has been created it can't be changed. You can only what the reference points to.
so

String beg = new String("begining ");
String end = new String("end");
end = beg + end;
//you just change what end points to, you don't change
//the String that it is pointing to.
//it creates a new String.

[John Dale]

Right. I was trying to speak to the earlier point of subclassing. Since I don't think the question has been answered yet...
One reason for making string immutable:
Suppose a string is passed to an object which needs to save the the information in that string for future use. If the string were not immutable, it would need to make a copy of the information in string, at extra cost in memory and time, or save a reference to the string and run the risk that somebody else will change the value of the string in the future, which is generally unacceptable. Thus we need an immutable String class.