Final variables as method arguments

Question

I was looking at some code from a colleague and saw the following method:

public void someMethod(final String str) {
    ...
}

which I found odd, as Strings are immutable, anyway. I think it is safe to delete the final keyword, there.

I also found the following

public void reportProblems(final Collection<IProblem> problems) {
    ...
}

There aren't any inner classes around, so.. is there a point in using the final keyword there? It looks to me that having the final there is absolutely the same as not having. Am I right?

Thanks

Jon Skeet · Accepted Answer · 2011-07-30 07:55:12Z

8

The immutability of string is completely irrelevant to whether it's declared final or not. You could have a final StringBuilder parameter, and it would prevent exactly the same thing: reassignment of the variable within the method. You'd still be able to append to the StringBuilder. final in Java isn't the same as const in C++.

Some people like to make everything final when they can - if you know that the value of a variable doesn't change, it can be easier to reason about it. I have a lot of sympathy with that view, and often take the same approach myself - except that I don't actually force the issue with the final modifier, simply because it adds cruft in the code. If final were the default and you had to explicitly use a mutable modifier, I suspect that most developers wouldn't start making every local variable mutable... they'd just use the default most of the time, as they do now.

So, to come back to the original question: I suspect the developer just wants to be explicit about their style. It's not how I do it, but I can understand the motivation. Another possibility is that at one point there was an inner class, but that's gone now and the developer forgot to remove the final modifier.

Of course, if your colleague wrote the code, you don't have to guess - just ask!

edited Jul 30, 2011 at 7:55

answered Jul 30, 2011 at 7:45

Jon Skeet

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2 Comments

devoured elysium Over a year ago

As this are such simple methods, I guess the intent was to show to the method's clients that the arguments wouldn't be mutated. But that wouldn't ever happen anyway, right?

Jon Skeet Over a year ago

@devoured: The method's clients wouldn't care whether the parameter itself changed value - they wouldn't be able to tell. The final modifier has nothing to do with whether the object that the parameter value refers to is mutated or not. You need to keep the two concepts completely separate in your head. It's possible that the original coder didn't understand this, of course...

gnat · Accepted Answer · 2011-07-30 14:44:08Z

2

It looks to me that having the final there is absolutely the same as not having. Am I right?

You are wrong in that it's not absolutely the same.

Code like below would compile without final...

public void someMethod(String str) {
    str = "nonsense"; // will compile fine
}

...and won't compile if there's final

public void someMethod(final String str) {
    str = "nonsense"; // won't compile
}

"What a difference a word makes"

answered Jul 30, 2011 at 14:44

gnat

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Comments

Alex Gitelman · Accepted Answer · 2011-07-30 07:44:33Z

1

It prevents you from assigning to parameter variable inside the method body. So it does make a difference.

answered Jul 30, 2011 at 7:44

Alex Gitelman

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