Question

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What's the advantage of a String be Immutable?

Strings are Immutable?

String s = "PROJECT";

Here s cannot be modified. But String s2=s.toLowerCase() will return me a new string. Here still I can get a new copy of the original string! But if Java still wants immutability, then why not restrict the user while modifying the string (throw an exception or something). Then why immutability? Can any one explain why?

Answer 1

Strings are immutable, meaning that this is always true:

String s = "PROJECT";
String s2 = s.toLowerCase();

System.out.println(s.equals("PROJECT")); // Prints true
System.out.println(s.equals(s2));        // Prints false

In comparison, consider what would happen if Strings were mutable.

MutableString ms = "PROJECT";
MutableString ms2 = ms.toLowerCase();

System.out.println(ms.equals("PROJECT")); // Prints false
System.out.println(ms.equals(ms2));       // Prints true

This example may seem trivial, but it means that unless you actually re-assign the reference s then you can guarantee that s will not be changed by any piece of code.