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Date: Wed, 3 Apr 2013 18:41:39 -0700 (PDT)
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Subject: Re: Usefulness of "final" (Was: Re: Inserting In a List)
From: Lew <lewbloch@gmail.com>
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Xref: csiph.com comp.lang.java.programmer:23270

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.
>=20
>>> If is neither sufficient nor necessary to make them
>>> immutable.
>=20
>> final is necessary.  [...]
>=20
> 	public class Immutable {
> 	    private int value;
> 	    public Immutable(int value) {
> 	        this.value =3D value;
> 	    }
>=20
> 	    public int getValue() {
> 	        return value;
> 	    }
> 	}
>=20
> 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 =3D av;
   }

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

   public void setValue(int av)
   {
     foo =3Dav;
   }
 }

Now you can have code like this:

  MutableImmutable mui =3D new MutableImmutable(0);
  Immutable immu =3D mui;
  System.out.println("Immutable value =3D " + immu.getValue());

  mui.setValue(-1);
  System.out.println("Immutable value =3D " + immu.getValue());

If the "mui" and "immu" code are in different parts of the application, e.g=
., one is=20
inside a method called separately from the other, you can have mutable beha=
vior=20
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=20
mutate the behavior of its parent, and break immutability, which is not pos=
sible if=20
the parent class defines 'value' to be 'final'.

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

--=20
Lew