Declaring Integer Variables

As you saw earlier, you can declare a variable of type long with the statement:

long bigOne;

This statement is a declaration for the variable bigOne. This specifies that the variable bigOne will store a value of type long. When this statement is compiled, 8 bytes of memory will be allocated for the variable bigOne. Java does not automatically initialize a variable such as this. If you want your variables to have an initial value rather than a junk value left over from when the memory was last used, you must specify your own value in the declaration. To declare and initialize the variable bigOne to 2999999999, you just write:

long bigOne = 2999999999L;

The variable will be set to the value following the equal sign. It is good practice to always initialize your variables when you declare them. Note that if you try to use a variable in a calculation that has not had a value assigned to it, your program will not compile. There are also circumstances where the compiler cannot determine whether or not a variable has been initialized before it is used if you don’t initialize it when you declare it, even though it may be obvious to you that it has been. This will also be flagged as an error, but if you get into the habit of always initializing variables when you declare them, you’ll avoid all of these problems.

You can declare a variable just about anywhere in your program, but you must declare each variable before you use it in a calculation. The placement of the declaration has an effect on whether a particular variable is accessible at a given point in a program, and we will look deeper into the significance of this in the next chapter. Broadly, you should group related variable declarations together, immediately before the block of code that uses them. You can declare and define multiple variables in a single statement. For example:

long bigOne = 999999999L, largeOne = 100000000L;

Here I have declared two variables of type long. A comma separates each variable from the next. You can declare as many variables as you like in a single statement, although it is usually better to stick to declaring one variable in each statement, as it helps to make your programs easier to read. A possible exception occurs with variables that are closely related - an (x,y) coordinate pair representing a point, for example, which you might reasonably declare as:

int xCoord = 0, yCoord = 0; // Point coordinates

On the same line as the declaration of these two variables, we have a comment following the double slash, explaining what they are about. The compiler ignores everything from the double slash (//) until the end of the line. Explaining in comments what your variables are for is a good habit to get into, as it can be quite surprising how something that was as clear as crystal when you wrote it transmogrifies into something as clear as mud a few weeks later. You can add comments to your programs in other ways that we will see a little later in this blog. You can also spread a single declaration over several lines if you want. This also can help to make your program more readable. For example:

int miles = 0, // One mile is 8 furlongs

furlongs = 0, // One furlong is 220 yards

yards = 0, // One yard is 3 feet

feet = 0;

This defines four variables of type int in a single statement with the names miles, furlongs, yards, and feet. Each variable has 0 as its initial value. Naturally, you must be sure that an initializing value for a variable is within the range of the type concerned; otherwise, the compiler will complain. Your compiler is intelligent enough to recognize that you can’t get a quart into a pint pot, or, alternatively, a long constant into a variable of type int, short, or byte. Because the statement is spread over four lines, I am able to add a comment on each of the first three lines to explain something about the variable that appears on it. To complete the set of variables that store integers you can declare and initialize a variable of type byte and one of type short with the following two statements:

byte luckyNumber = 7;

short smallNumber = 1234;

Here the compiler can deduce that the integer literals are to be of type byte and short, respectively, and convert the literals to the appropriate type. It is your responsibility to make sure the initial value will fit within the range of the variable that you are initializing. If it doesn’t, the compiler will reject the statement and output an error message.

Most of the time you will find that variables of type int will cover your needs for dealing with integers, with type long being necessary now and again when you have some really big integer values to deal with. Variables of type byte and short do save a little memory, but unless you have a lot of values of these types to store, that is, values with a very limited range, they won’t save enough to be worth worrying about. They also introduce complications when you use them in calculations, as you’ll see shortly, so generally you should not use them unless it is absolutely necessary. Of course, when you are reading data from some external source, a disk file for instance, you’ll need to make the type of variable for each data value correspond to what you expect to read.