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From: Jan Burse <janburse@fastmail.fm>
Newsgroups: comp.lang.java.programmer
Subject: Re: Immutable Datastructures with good Sharing
Date: Sat, 05 Nov 2011 19:30:44 +0100
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Stefan Ram schrieb:
>    In the functional lanugage it is exactly the other way
>    round: It has a result specified, but no effect specified.

I admit that the question might need some out of the box
thinking, especially when somebody makes a strict divide
between Java (destructive, etc..) and functional programming
(non-destructive, etc..).

But I am looking for:

     Some stack class, where: (Easy)
         pop() creates a new immutable stack
         push() creates a new immutable stack
         With good sharing.

     Some queue class, where: (Hard?)
         enqueue() creates a new immutable queue
         dequeue() creates a new immutable queue
         With good sharing.

All of that in !Java!.

The stack is quite easy, a good indicative that
something is immutable are the keywords final
before field variables:

      public class Stack {
           final Object element;
           final Stack next;

           Stack(e, n) {
              element = e;
              next = n;
           }

           public Stack push(Object e) {
              return new Stack(e,this);
           }

           public Object top() {
              return element;
           }

           public Stack pop() {
              return next;
           }
       }

But how about the Queue?

Best Regards

P.S.: In LISP one just uses the CONS for
push, the CAR for top, and the CDR for pop.