Variables in Java

An identifier is a sequence of one or more characters used to name classes, variables, and subroutines.

Programming

Why Programs Need Names

A Java program works with many things at once: classes, variables, and subroutines. To refer to any of them, the program needs a name. In Java, these names are called identifiers. Once you have a name for something, you can read it, change it, and pass it around, so the name is the handle your code uses to reach the thing it stands for.

A common assumption is that any readable text can serve as a name. It cannot. Java applies rules to the characters and structure of every identifier, and this article walks through those rules, then shows how a variable is declared and why that declaration matters. Follow along by tracing what happens to each name as the program is read, one step at a time.

What Makes a Legal Identifier

Definition

An identifier is a sequence of one or more characters used to name classes, variables, and subroutines.

Java adds two rules on top of that definition. An identifier must begin with a letter or an underscore. After the first character, it must consist entirely of letters, digits, and underscores. Identifiers cannot contain spaces. Those rules are the whole character-level test, and every candidate name must pass all of them.

Which candidate names are legal identifiers, and which rule does each one pass or break?

Read the diagram column by column. total_1 passes every rule: it starts with a letter, and the underscore and digit both fall within the allowed characters after the first position. Total also passes, because the rules do not forbid uppercase letters. my var fails because a space is not allowed anywhere in an identifier. 2fast fails for a different reason: its first character is a digit, and an identifier must begin with a letter or underscore. Notice that the illegal names break different rules, so when a name is rejected you should check the start first and then scan for spaces or other forbidden characters.

Reading the Declaration Pattern

A declaration is the statement that introduces a variable to the program. The simple Java declaration pattern is a type name, followed by a variable name, followed by a semicolon. The type name tells the program what kind of variable this is. The variable name is an identifier, so it must follow the rules from the previous section. The semicolon ends the statement.

java

int count;

int count;

Notes on the code

int is the type name, telling the program this is an integer variable
count is the identifier chosen for the variable
the semicolon ends the declaration statement

Java also allows one declaration to introduce several variables, with the names separated by commas. For example, a single statement can declare two names of the same type. Even so, readable style usually favors one variable per statement unless the variables are closely related, because a separate line makes each variable's purpose easier to find and comment on.

In the statement int count = 0;, which token is the type, which is the identifier, and how do they fit together in order?

Read the diagram from left to right, since the tokens must appear in this order. The type comes first, then the identifier. The equals sign and the value after it are an addition to the basic pattern: they give the variable a starting value at the moment it is declared. The source pattern itself shows only the type and the name, so the simplest declaration is int count; and the extra part is optional.

Declaration Before Use

Definition

Declaration is the required introduction of a variable before its use. It introduces the variable's name and tells the program what kind of variable it is.

A Java program cannot use a variable until that variable has been declared. If a line refers to a name and no matching declaration appeared earlier in the code, the program has no established meaning for that name, and the mistake must be corrected before the program can use it. Declarations therefore come first in the flow of the code, and uses come after them.

What do you think happens?

After the statements score, Score, and SCORE are all declared, how many separate variables exist?

Reveal answer

Answer:

Java treats uppercase and lowercase letters as different, so score, Score, and SCORE are three distinct identifiers and therefore three separate variables.

How a Declaration Reaches Memory

A declaration has two connected effects. First, it introduces a name for the program to recognize. Second, when the declaration is executed, the computer sets aside memory for the variable and associates its name with that memory. The name and the storage are two different things that are linked by the declaration. The name is what your code writes; the storage is the space the computer reserves for the value.

After int count; is executed, what does the name count point to, and how is that storage separate from the name?

Read the diagram as a link between two parts. The left box is the name count, which lives in your source code. The right box is the storage that the computer sets aside when the declaration runs. The arrow is the association the declaration creates. Because the name and the storage are separate, the link is what allows later lines that use count to reach the storage that was reserved for it.

Common Naming Mistakes

Common mistake
Using a space inside a name
What it looks like
my var
Why it fails
Identifiers cannot contain spaces.
Do this instead
Use an underscore or a single word, such as my_var or myVar.
Common mistake
Starting a name with a digit
What it looks like
2fast
Why it fails
An identifier must begin with a letter or underscore.
Do this instead
Start with a letter or underscore, such as fast2 or _fast.
Common mistake
Using a reserved word as a name
What it looks like
int
Why it fails
Reserved words such as class, int, and return already have a fixed meaning in Java and are outside the set of legal identifiers.
Do this instead
Pick a descriptive name that is not a reserved word, such as total or itemCount.
Common mistake
Assuming capitalization does not matter
What it looks like
score and Score used as if they were the same
Why it fails
Uppercase and lowercase letters are different, so the two names refer to different identifiers.
Do this instead
Choose one spelling and use it consistently everywhere the variable appears.
Common mistake
Using a variable before declaring it
What it looks like
a line that uses total with no earlier declaration of total
Why it fails
A Java program cannot use a variable until it has been declared.
Do this instead
Place the declaration before the first line that uses the name.

Naming and Commenting Habits

Best practice

Declare one variable per statement unless the variables are closely related. This keeps unrelated variables separate and makes each one easier to read.

Best practice

Add a comment to a declaration when the variable's purpose or other useful information would help a human reader understand the program.

java

int itemCount;   // number of items currently in the cart
int total;       // running sum of item prices

int itemCount;   // number of items currently in the cart
int total;       // running sum of item prices

Notes on the code

each declaration introduces one variable on its own line
each comment states the purpose of the variable for a human reader
both names are legal identifiers and neither is a reserved word

Practice Your Declarations

EASY

Decide whether each candidate is a legal identifier: total_1, Total, my var, 2fast, class. For each illegal name, state which rule it breaks.

Hints

Check the first character before anything else.
Look for spaces anywhere in the name.
Ask whether the word is one the language reserves for its own use.

Your attempt (not saved)

MEDIUM

Write a declaration for a variable that stores a player's score, using a legal identifier and a comment that explains its purpose. Then state which line must come first in the code: the declaration or the first use of the variable.