1.2 - Variables and Data Types

A data type is basically a set of possible values plus the operations you’re allowed to do on them. Data types fall into two buckets:

Primitive types: numbers and booleans

Numbers (int and double) work with arithmetic operators like + and -, and boolean values work with logical operators like && and || (Unit 2).

Reference types: everything else, used to define objects

Examples: String, Scanner, and any class someone writes, like a Dog or Student class. An object can hold much more than one value (a String holds a whole sequence of characters, for example).

The three primitive types you need for this course are:

Type	Holds
`int`	a whole number
`double`	a real (decimal) number
`boolean`	`true` or `false`

Note: Java has other primitives too (long, short, byte, float, char), but those are outside the scope of this course.

A variable is a named storage location that holds a value, and that value can change while the program runs. Every variable has a name and a data type attached to it.

A variable of a primitive type holds the actual value itself. After int age = 16; runs, the variable age stores the number 16. (Reference type variables work differently, see 1.12.)

You declare a variable by writing its type and then its name. You can give it a value on the same line or later, but once a variable is declared, its type can’t change.


int age = 16;
double gpa = 3.85;
boolean isEnrolled = true;


int students;         // declared, no value yet
students = 28;        // now it holds 28
students = 30;        // the value can change while the program runs
double height = 1.75; // declared and given a value on one line

Variable names can use letters, digits, and the _ symbol, but they can’t start with a digit, can’t contain spaces, and can’t be a Java keyword like int or class. Names are case sensitive, so score and Score are two different variables. By convention, Java programmers use camelCase: start lowercase and capitalize each new word, like totalScore.

Choosing a Data Type

You’ll often need to pick the best type for a value. Ask yourself what kind of value it is:

int: whole numbers you count, like the number of students in a class or the score in a game.

double: values that can have a decimal part, like a GPA, an average, or a temperature.

boolean: anything with only two options, like whether a door is locked or whether a player has won.

Reference type: anything that isn’t a single number or true/false, like a name (a String) or a whole student record (an object of a class).

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