Data Types and Variables

Most values in a computer program have a type. The type tells Java what the value represents and what operations make sense for it.

Java divides types into primitive and reference types. Common examples include:

• Integer types: int, long, and short (plus byte)
• Floating-point types: double (plus float)
• Character and Boolean types: boolean and char
• Reference types: String, arrays, classes, and interfaces

Variables

A variable is a named storage location. Java is statically typed, so every variable declaration states what kind of data the variable can store.

For example, int num = 10; gives a type, name, and initial value, followed by a semicolon.

int num = 10;
System.out.println(num);      // 10

double salary = 8232.56;
System.out.println(salary);   // 8232.56

Here, num is declared as int, while salary is declared as double. Java checks later assignments against those types.

Naming variables

Good variable names are clear and valid:

• Use letters, digits, and underscores, but do not start with a digit.
• Do not use Java reserved keywords.
• Prefer meaningful English words.

Names like age, studentName, and totalScore are easier to read than a or x1.

Java keywords

Keywords are reserved by Java and cannot be used as ordinary variable names.

Common keywords include:

• true, false, null
• boolean, byte, char, double
• if, else, switch, case
• for, while, break, continue
• void, return, class, interface
• try, catch, finally, throw
• import, package, public, private

Using a keyword as a variable name would make Java unable to tell whether you mean a language feature or an ordinary value.