Re: More Flexible "final"

From:
Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups:
comp.lang.java.programmer
Date:
Sun, 28 Oct 2007 15:21:12 -0400
Message-ID:
<2uydnaMoHMm3fLnanZ2dnUVZ_rOqnZ2d@comcast.com>
Christian wrote:

Hello

I have been wondering why the final keyword for instance variables is
not more flexible.
Often in my code I find situation when it would have been
helpful/produced nicer code if I were able to change a final variable
while being in the constructor .

Is there any reason behind it that we can't change a final variable
multiple times while we are in the constructor?


     The only scenario I can think of where a `semifinal' might
help is when there are multiple constructors, and one of them
chains to another before setting the variable:

    private Something instanceVariable;

    public SomeClass() {
        doLotsOfStuff();
        instanceVariable = new Something(42);
    }

    public SomeClass(int value) {
        this();
        instanceVariable = new Something(value);
    }

The obvious answer in this case is to rearrange responsibilities:

    private Something instanceVariable;

    public SomeClass() {
        this(42);
    }

    public SomeClass(int value) {
        doLotsOfStuff();
        instanceVariable = new Something(value);
    }

     Or perhaps you're worried about running a loop that might
set the variable several times, as in:

    private Something instanceVariable();

    public SomeClass(Collection<Something> things) {
        for (Something thing : things)
            if (thing.somePredicate())
                instanceVariable = thing;
    }

This is easily fixed by introducing a local variable:

    private Something instanceVariable;

    public SomeClass(Collection<Something> things) {
        Something chosen = null;
        for (Something thing : things)
            if (thing.somePredicate())
                chosen = thing;
        instanceVariable = chosen;
    }

     If you have some other situation that seems to cry out for
a `semifinal' variable, please post it.

It would be interesting to know why it is like it is in java.
What benefit does java get by not allowing this?


     Simplicity: If the rules are easy to state ("one and only
one assignment") things are easier for both the compiler and
for the human reader. Error detection: Did the programmer truly
intend to make multiple assignments, or did some execution path
escape his notice? Maybe other reasons, too.

     Let's turn the question around: What benefit would accrue
from *allowing* `semifinal' variables?

--
Eric Sosman
esosman@ieee-dot-org.invalid

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