Why would you declare a parameter as final ?

Question

"It is good practice to specify parameters that are class objects as final if you do not intend to alter them in a method."
- Ivor Horton, Begining Java 2 .
Why ?
Declaring an object parameter as final or not still allows you to change the properties (instance variables) of the object. Rather, declaring the parameter as final only ensures that the reference to the object cannot be changed.
Have i got this right ?
Any help will be appreciated.
Ezaz

Answer 1

I often define my collections as final
class Whatever {
  final ArrayList arList = new ArrayList();
}Then you never have to test arList for null because any attempt to reset arList is flagged as a compiler error - e.g.
arList = null;    // Compiler error
arList = new ArrayList();    // Compiler errorThe compiler and/or JVM can also do any optimzations knowing this as well.