Final arguments in interface methods - what's the point?

198
votes

In Java, it is perfectly legal to define final arguments in interface methods and do not obey that in the implementing class, e.g.:

public interface Foo {
    public void foo(int bar, final int baz);
}

public class FooImpl implements Foo {

    @Override
    public void foo(final int bar, int baz) {
        ...
    }
}

In the above example, bar and baz has the opposite final definitions in the class VS the interface.

In the same fashion, no final restrictions are enforced when one class method extends another, either abstract or not.

While final has some practical value inside the class method body, is there any point specifying final for interface method parameters?

5
javaclassinterfacemethodsfinal
final doesn't do anything with native types anyway, since they're copied. – Paul Tomblin
Just as a point of discussion: I just tried it and if two interface definitions vary only in the final attribute of an argument, then the resulting .class files are byte-for-byte identically (and of course javap -v produces the same output). The same is true for two classes that only differ in final on an attribute, by the way! – Joachim Sauer
@Paul: it does exactly the same thing as with reference types: it prevents the arguments itself to be modified (if used in the implementation). – Joachim Sauer
It has as much relevance as the public in the method signature. – Robin
@Deepak: I see you asking for working examples on all kinds of questions, even when it doesn't seem to make a lot of sense. I think you should try learning some abstract thinking: try thinking about a problem without having some executable code in front of you. It will help you a lot in the long run. – Joachim Sauer

5 Answers

112
votes

It doesn't seem like there's any point to it. According to the Java Language Specification 4.12.4:

Declaring a variable final can serve as useful documentation that its value will not change and can help avoid programming errors.

However, a final modifier on a method parameter is not mentioned in the rules for matching signatures of overridden methods, and it has no effect on the caller, only within the body of an implementation. Also, as noted by Robin in a comment, the final modifier on a method parameter has no effect on the generated byte code. (This is not true for other uses of final.)

25
votes

Some IDEs will copy the signature of the abstract/interface method when inserting an implementing method in a sub class.

I don't believe it makes any difference to the compiler.

EDIT: While I believe this was true in the past, I don't think current IDEs do this any more.

18
votes

Final annotations of method parameters are always only relevant to the method implementation never to the caller. Therefore, there is no real reason to use them in interface method signatures. Unless you want to follow the same consistent coding standard, which requires final method parameters, in all method signatures. Then it is nice to be able to do so.

6
votes

Update: Original answer below was written without fully understanding the question, and therefore does not directly address the question :) Nevertheless, it must be informative for those looking to understand the general use of final keyword.

As for the question, I would like to quote my own comment from below.

I believe you're not forced to implement the finality of an argument to leave you free to decide whether it should be final or not in your own implementation.

But yes, it sounds rather odd that you can declare it final in the interface, but have it non-final in the implementation. It would have made more sense if either:

a. final keyword was not allowed for interface (abstract) method arguments (but you can use it in implementation), or
b. declaring an argument as final in interface would force it to be declared final in implementation (but not forced for non-finals).

I can think of two reasons why a method signature can have final parameters: Beans and Objects (Actually, they are both the same reason, but slightly different contexts.)

Objects:

public static void main(String[] args) {
    StringBuilder cookingPot = new StringBuilder("Water ");
    addVegetables(cookingPot);
    addChicken(cookingPot);
    System.out.println(cookingPot.toString());
    // ^--- OUTPUT IS: Water Carrot Broccoli Chicken ChickenBroth 
    //      We forgot to add cauliflower. It went into the wrong pot.
}

private static void addVegetables(StringBuilder cookingPot) {
    cookingPot.append("Carrot ");
    cookingPot.append("Broccoli ");
    cookingPot = new StringBuilder(cookingPot.toString());
    //   ^--- Assignment allowed...
    cookingPot.append("Cauliflower ");
}

private static void addChicken(final StringBuilder cookingPot) {
    cookingPot.append("Chicken ");
    //cookingPot = new StringBuilder(cookingPot.toString());
    //     ^---- COMPILATION ERROR! It is final.
    cookingPot.append("ChickenBroth ");
}

The final keyword ensured that we will not accidentally create a new local cooking pot by showing a compilation error when we attempted to do so. This ensured the chicken broth is added to our original cooking pot which the addChicken method got. Compare this to addVegetables where we lost the cauliflower because it added that to a new local cooking pot instead of the original pot it got.

Beans: It is the same concept as objects (as shown above). Beans are essentially Objects in Java. However, beans (JavaBeans) are used in various applications as a convenient way to store and pass around a defined collection of related data. Just as the addVegetables could mess up the cooking process by creating a new cooking pot StringBuilder and throwing it away with the cauliflower, it could also do the same with a cooking pot JavaBean.

2
votes

I believe it may be a superfluous detail, as whether it's final or not is an implementation detail.

(Sort of like declaring methods/members in an interface as public.)