Why is StringBuilder a modifiable interface variable ?

[Graeme Byers]

Why don't StringBuffer and StringBuilder when declared in interfaces behave as expected ?
Thank you.

    
interface Interfaceable {
          StringBuffer  BUF = new StringBuffer ("A") ;
          StringBuilder BLD = new StringBuilder("C") ;
          String        S   = "E" ;
          Integer       INT = new Integer(1) ;
}
public class GBTest implements Interfaceable {
         public static void main (String[] args) {
                 BUF.append("B") ; System.out.println (BUF) ;      // AB
                 BLD.append("D") ; System.out.println (BLD) ;      // CD 
             //  S = S + "F" ;       // Cannot assign value to a final variable (Expected behaviour)
             //  INT = new Integer(2) ; // Ditto     
           
       }
}  

[Ernest Friedman-Hill]

Java has no concept of making a particular instance of a class "unmodifiable." If you use an instance of a modifiable (usually we say "mutable") class as a member of an interface, then it's going to be possible to modify that object.

Note that in your example, you didn't show something like

BUF = new StringBuffer("replacement");

If you tried this, you'd find that because BUF is final, this is illegal. The object that BUF points to can be modified, but BUF, being final, can not.

Make sense?

[Vikas Kapoor]

And We all know that STRING IS IMMUTABLE.