Why java created Strings as IMMUTABLE CLASS? [duplicate]

Because if the String class was not final then you could declare a subclass of String which breaks any number of the contracts of the String class, especially immutability, and wreak havoc on unsuspecting programmers.

Imagine:

class MyString extends String {
  // Override methods which mutate this instance...
}

Now whenever a MyString is passed around as a String it could cause all sorts of problems, especially regarding hashing, which depends on the immutability of String.

Because it makes them much easier to use:

• thread safe without synchronization
• usable as a key in Maps
• no need to create defensive copies each time you pass or return them
• poolable
• ...