'const' vs 'final' - A Discussion

(For the tl;dr, click here.)

My latest forays into the realm of software development have taken me into Java country. I'm going to be taking my first classes toward my Master's in Computer Science in the coming months, and the bulk of the courses taught at my university are in Java. It will be my first time working with Java, so I thought it'd be a good idea to test the waters and get up to speed.

When I started writing my first entry level Java programs, one of the first features I instinctively tried to carry over from my C++ background was the const concept. After some research, I discovered that there is no const keyword in Java. Or, more accurately, const is a reserved word, that Java prohibits you from using in your code. It has no function in Java, but you may not use it as a variable or function name, either.

Instead, Java has a keyword called final, which seems on the surface to have a similar function to C++'s const. I wondered what the difference was between the two, and why Java's creators decided to block the const keyword from their language, I went on a quest to find out!

I came across a variety of myths along my search that I quickly busted via simple testing. For example, some folks believe that Java's final keyword can't be applied to primitive data types such as int, double, or char; but it can. Others believe that const C++ references are special, in that while they are immutable, the data they point to is mutable; however, this is also the case with a Java final.

As a side note, I found some information indicating that you do have the option in C++ of differentiating between a constant pointer to data, a pointer to constant data, and a pointer that is declared as constant to protect the otherwise non-constant data that can be dereferenced through it. If that sounds a bit confusing, it's much easier to understand when you see the code:

Cases 1 and 3 are simple enough, as far as syntax, though case 3 may be a little more confusing because of how we are protecting our data. Case 2, on the other hand, is a bit more confusing in terms of syntax; the const keyword appears after the pointer operator, even though the pointer itself is where we are applying the const functionality.

It would make more sense to me if the const keyword immediately preceded the item that it modified, in this case, the pointer operator. But sometimes you just have to accept that things are the way they are, and forge ahead!

So what is the difference between const and final? It turns out the primary difference is in initialization. In C++, a const must be defined in the same statement in which it is declared. In other words, you can't declare a C++ const and define it later in a separate statement.

On the other hand, a final in Java can be initialized in the same statement that it is declared; but you can also choose to declare it and then define it later in the program. This allows for each instance of a final to be defined by one of a variety of values (as you can see in the below example), but still ensures that once it is defined, it is constant forevermore.

As you can see in the above example, depending on whether 'someBool' is true, x might end up being defined as 10 or 20, but once it is set to one or the other, it can never be changed from there on out because it is final. Another important thing to note: when you are using a Java final without defining and declaring it in the same statement, you must be careful to define it before it is used, or you will have an error.

Hopefully this will help clear up some of the idiosyncrasies of and differences between C++'s const and Java's final!

Thanks for reading!
-- Steven Kitzes

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