Re: Why doesn't Integer have a setValue(int i), and is there a way of changing its value.
From: natG (natgross.rentalsystems_at_verizon.net)
Date: 12/21/03
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Date: Sun, 21 Dec 2003 18:51:32 GMT
"bagbourne" <noway@noway.com> wrote in message
news:bs1rtc$4jp$1@hercules.btinternet.com...
> natG wrote:
>
> > "Bent C Dalager" <bcd@pvv.ntnu.no> wrote in message
> > news:brsjhp$t6r$1@tyfon.itea.ntnu.no...
> >
> >>In article <vu3hd8iad8546c@corp.supernews.com>,
> >>Alex Hunsley <lard@tardis.ed.ac.molar.uk> wrote:
> >>
> >>>natG wrote:
> >>>
> >>>
> >>>>Can you please explain "safer"? Do I practice SAFE Java<g>?
> >>>>Seriously, safer in what respect?
> >>
> >>Two cases spring immediately to mind.
> >>
> >>If you use instances of the class as keys in a Map, or you have them
> >>in a Collection that you expect to always be sorted, then immutability
> >>guarantees that your sorting won't inadvertantly get screwed up by
> >>someone calling a setter at inopportune times. With immutable keys,
> >>the only thing that can mess with your sorting is when the
> >>Collection's setters are called. The alternative to immutability would
> >>be for the class to fire events whenever you changed something and the
> >>Collections classes to be sensitive to such events and be able to
> >>auto-resort themselves, but this gets real messy real quick.
> >
> >
> > Question: Suppose:
> > Integer a = new Integer(10);
> > Integer b = new Integer(20);
> > Integer c = new Integer(30);
> > TreeMap tm = new TreeMap();
> > tm.put(a);
> > tm.put(b);
> > tm.put(c);
> > And then:
> > b = new Integer(60);
> > Now, I assume that the second Integer added to the Map, will return an int
value
> > of 60.
> > If so, what happens to the sort at that point?
>
> You have a fundamental misunderstanding. I'm surprised nobody picked up
> on this. You sent a reference to an Integer object (stored in varable
> named "b") to TreeMap.put(). It stored that reference.
>
> Then you create a *totally new instance* of an Integer with 60 in it,
> and put a reference to that object into variable b.
>
> That has nothing to do with the reference that you previously put into
> the TreeMap.
>
> If it helps, think of object variables as smart pointers that cannot be
> corrupted to point to anything else other than what they were declared as.
>
> When you assign using "=", you just change the variable to *reference*
> that new object.
I am surprised at myself as well. It seems like that in hurry to pick an example
of changing keys in HashMap, I blinded myself.
-nat
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