1.2 Variables and Data Types

Primitive Data Types

Java has 8 primitive types, but AP CSA focuses on three: int (whole numbers), double (decimal numbers), and boolean (true/false). Primitives store actual values directly in memory, not references. Each variable must be declared with a type before use, like `int x;` or `double y = 3.5;`.

Declaring and Initializing Variables

A declaration states a variable's type and name: `int score;`. Initialization assigns it a value: `score = 100;`. You can combine both: `int score = 100;`. Variable names must follow Java naming rules — start with a letter, no spaces, and are case-sensitive.

Casting Between Types

Casting converts one data type to another. Widening (int → double) happens automatically. Narrowing (double → int) requires explicit casting, like `(int) 7.9`, which truncates (not rounds) the decimal part, giving 7. Mixing int and double in expressions produces a double result unless cast.

Key terms

Primitive data type

A basic data type (int, double, boolean) that stores its value directly in memory, not as a reference.

Declaration

Specifying a variable's type and name, e.g., int x; — no value assigned yet.

Initialization

Assigning a value to a variable, e.g., x = 5;

Casting

Converting a value from one data type to another; narrowing casts (like double to int) require explicit syntax and truncate decimals.

Widening vs. Narrowing conversion

Widening (int→double) happens automatically; narrowing (double→int) requires explicit casting and loses the decimal part.

Try an AP-style question

Which is a primitive type in Java?

1. String
2. ArrayList
3. Scanner
4. double