Re: the benefits of immutability

[Pascal Costanza <address@hidden>]

Vadim Nasardinov wrote:

>    class Subclass extends Superclass {
>        private int m_num;
> 
>        public Subclass(String str, int num) {
>            super(str);
>            m_num = num;
>        }
> 
>        public boolean equals(Object obj) {
>            // try and replace this with an implementation that takes
>            //  m_num into account
>            return super.equals(obj);
>        }
>    }

What's the problem with this?

public boolean equals(Object obj) {
   if (!(obj instanceof Subclass)) return false;

   Subclass that = (Subclass)obj;

   return super.equals(that) && (this.m_num == that.m_num);
}


I think the main design problem in Java is that immutability is just a 
convention, but inappropriate language constructs are used to enforce 
that convention. Either you completely forget about enforcing 
immutability and rely on intelligent programmers to do it right, or 
invent a language construct that explicitly says what you want. In Java, 
one could have added a class modifier "immutable" and, for example, 
require that all fields in an immutable class are final.

(I suspect there are some nasty corner cases, but it should be doable...)


Pascal

-- 
Pascal Costanza               University of Bonn
mailto:costanza@web.de        Institute of Computer Science III
http://www.pascalcostanza.de  Ro"merstr. 164, D-53117 Bonn (Germany)