Re: Usefulness of "final" (Was: Re: Inserting In a List)

From:
Lew <lewbloch@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 3 Apr 2013 18:41:39 -0700 (PDT)
Message-ID:
<f3d6284f-0ef8-4167-9fba-089c2ed351ef@googlegroups.com>
Eric Sosman wrote:

markspace wrote:

 Arne Vajh=EF=BF=BDj wrote:

But now you raise the question, then final is only slightly
involved in immutability in Java.

 

If is neither sufficient nor necessary to make them
immutable.

 

final is necessary. [...]

 
    public class Immutable {
        private int value;
        public Immutable(int value) {
            this.value = value;
        }
 
        public int getValue() {
            return value;
        }
    }
 
Claim: Immutable is immutable, despite the lack of "final".


'final' can help in some superficially similar scenarios.

 public class MutableImmutable extends Immutable
 {
   private int foo;

   public MutableImmutable(int av)
   {
     super(av);
     foo = av;
   }

   @Override public int getValue()
   {
     return foo;
   }

   public void setValue(int av)
   {
     foo =av;
   }
 }

Now you can have code like this:

  MutableImmutable mui = new MutableImmutable(0);
  Immutable immu = mui;
  System.out.println("Immutable value = " + immu.getValue());

  mui.setValue(-1);
  System.out.println("Immutable value = " + immu.getValue());

If the "mui" and "immu" code are in different parts of the application, e.g=
.., one is
inside a method called separately from the other, you can have mutable beha=
vior
from an 'Immutable' reference even though the parent class is actually immu=
table.

Furthermore, if 'Immutable#value' were not 'private', then the overriding c=
lass can
mutate the behavior of its parent, and break immutability, which is not pos=
sible if
the parent class defines 'value' to be 'final'.

So while 'final' is not necessary for immutability, it sure does help.

--
Lew

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