Variables

Sometimes we want our program to remember information and reuse the information later. We tell our program to remember something by creating a variable and assigning the information to that variable.

The `var` Keyword

We can use the var keyword followed by a name and initial value to create a new variable. This follows the format: var VARIABLE_NAME = INITIAL_VALUE;. For example, var x = 5; will create a variable named x with an initial value of the number 5. var message = "Hello, World!"; will create a variable named message with an initial value of the text Hello, World!. You can create many types of variables, but for now we'll stick with numbers and text.

The Assignment Operator

The equals sign (=) can be confusing to new programmers. You've likely seen it used in math to establish a relationship between two expressions. For example, we might say "x = y + z" to establish that the variable x is the sum of the variables y and z. We might also make simpler statements, such as "x = 5". However, if we follow "x = 5" with the statement "x = 6", then we've created a contradiction because the variable x cannot equal 5 and 6 at the same time.

In programming, the equals sign means something different. If our code says x = 5;, we are not stating a fact about x as we would be in mathematics. What we're really doing is telling the computer to change the value associated with the variable x to the number 5. This is called an assignment, and the equals sign is the assignment operator. An assignment follows the pattern VARIABLE_NAME = EXPRESSION;. In our x = 5; example, x is the variable name and 5 is the expression. An expression can be a lone number or a single variable name, but it could also be a more complex calculation involving multiple numbers and variables. Expressions are also not restricted to numbers. "Hello, World!" is also an expression.

What does this mean? While the statement x = 5; followed by x = 6; would normally be a contradiction in mathematics, in programming these statements mean "set the value of x to 5" and "set the value of x to 6". They are instructions for the computer to change the value of x rather than declarations about the value of x. If our program follows both of these instructions, the result will be that our variable named x stores the number 6.

Outputting Variables

You can a variable with println to output the information stored in the variable. Copy the following code into a jshell script named variableOutput.jsh and run it with jshell:

var x = 5;
print("x is equal to ");
println(x);
x = 3 * x;
print("x is now equal to ");
println(x);
/exit

A couple of notes about the above program:

The variable x can be used in an assignment statement's expression to calculate a new value for itself. The calculation uses the old value of x, and the result of this calculation becomes the new value of x.
We only use the var keyword when we first create the variable. After it exists we only refer to it by its name, even if we assign a new value to it.

Names

Variable names must consist of letters, numbers, and the underscore (_) character, and they cannot start with a number. Among other things, this means they cannot include spaces. Java also has certain [reserved words] (https://docs.oracle.com/javase/tutorial/java/nutsandbolts/_keywords.html) that cannot be used as variable names.

Types

We'll learn more about types later. For now, keep in mind that a variable's type is set with its initial value and cannot be changed. If you make a variable that stores text, you cannot reassign it to hold a number. Try running the following code in jshell and you'll see an error message:

var x = "hello";
x = 5;

Good Variable Names

Variable names in Java should follow the "camelCase" naming convention. The name should start with a lowercase letter, and the first letter of each word after the first should be capitalized. All other letters are lowercase, and we usually don't use underscores.

Variable names should also be descriptive. If you make a variable to store somebody's age, then the name age describes what is in the variable and makes our code easier to understand. Don't name your variables arbitrary letters like a or x unless those names make sense in context. For example, x would be an appropriate name for an x-coordinate on a graph, and a single arbitrary letter is fine if the information in the variable is only a number with no further meaning in the context of the program.

Your program can still work if you do not follow these rules, but your code will be harder for a human to read. A human has to understand the code in order to use and maintain it, so writing readable code is very important.

CSC110 Notebook