1.2 Variables and Data Types

1. What a data type is

A data type is a set of values and the set of operations allowed on those values. Every data type is in one of 2 categories: primitive or reference 1.2.A.1.

• Primitive types define the values and operations for numbers and Boolean values 1.2.A.2. The variable stores the value directly.
• A reference type is used for objects, that is, for anything that is not a primitive type 1.2.A.3. Examples: String, or a class such as Plant.

The type decides which operations make sense. You can multiply two int values, but you cannot multiply two boolean values.

2. The three primitive types in this course

The three primitive types used in this course are int, double and boolean 1.2.B.1.

Type	Holds	Example values
int	an integer	-4, 0, 120
double	a real number	3.75, -0.5, 87.5
boolean	only true or false	true, false

Outside the course: Java has 5 other primitive types (long, short, byte, float and char). You will not need them on the AP® exam.

3. Declaring variables

A variable is a storage location that holds a value, and the value can change while the program is running. Every variable has a name and a data type. A variable of a primitive type holds a primitive value of that type 1.2.B.2.

int lives = 3;            // whole number
double average = 87.5;    // real number
boolean gameOver = false; // true or false only
String player = "Ana";    // reference type: an object
System.out.println(player + " has " + lives + " lives");
System.out.println("Average: " + average);
System.out.println("Game over? " + gameOver);
lives = 2;                // the value can change
System.out.println(player + " has " + lives + " lives");

Output:

Ana has 3 lives
Average: 87.5
Game over? false
Ana has 2 lives

The name and type stay the same for the life of the variable. Only the value changes.

4. Primitive and reference variables in memory

A primitive variable holds the value itself. A reference variable holds a reference (an address) that leads to an object somewhere else in memory.

Code	What the variable holds
int count = 3;	count holds the value 3.
boolean done = false;	done holds the value false.
Plant p = new Plant("Fern", 30);	p holds a reference to a Plant object, and that object stores name "Fern" and height 30.
Plant q = null;	q holds null: there is no object. Calling a method on q throws a NullPointerException (topic 1.14).

Assigning p to another variable copies the reference, not the object, so both variables then refer to the same object (topics 1.4, 1.12 and 1.14).

Plant p = new Plant("Fern", 30);
Plant other = p;           // copies the reference
other.grow(5);
System.out.println(p.getHeight());

Output:

35

There is only one Plant object. Growing it through other changes what p sees too.

5. Choosing the right type for a specification

Read the specification and ask: what values must this hold? 1.2.A

Specification	Best type	Category
Number of people on a bus	int	primitive
Price of a ticket, average score	double	primitive
Whether a door is locked	boolean	primitive
A player’s name	String	reference

Watch out: do not store a number you will calculate with as text. "12.6" is a String, so you cannot do arithmetic with it.

Background (not tested): naming conventions. Java programmers name variables in camelCase (totalScore, isReady) and classes in PascalCase (Plant, String). Names cannot start with a digit, contain spaces, or be a keyword such as int or new, and Java is case-sensitive (score and Score are different variables). Declaring creates a variable (int x;); initializing gives it its first value (x = 5;).