Why does final keyword work differently with collection

Question

I have tried final on StringBuffer but when i changed the value of StringBuffer object its throws a compile time error but when i did same thing with vector it work perfectly fine.
any one knows the reason? please send me in details.
final Vector v = new Vector();
v.add("String1");
v.add("String2");
final StringBuffer sb  = new StringBuffer();
sb = sb.append("xyz");

Answer 1

Because you haven't understood the keyword final. when you make a reference final you may not change the object to which it points to.
final StringBuffer sb  = new StringBuffer();This is valid because this is the first time you instatiate sb
sb = sb.append("xyz");this is invalid because you are trying to assign a new object to a reference that may only be assigned to once.
Compare
final int testInt = 1;
testInt = 2; //errorthe vector works because you do not assign vector reference v a new Vector object