Variables and Data Types

A variable is a labelled box

• A variable 变量 is a named place in memory that stores a value.
• Give it a type, a name, and (usually) a starting value: int score = 0;
• Read it or change it by name; the box holds the latest value.
• Every variable in Java must be declared with a type before use.

The primitive types

• Java has eight primitive types 基本类型; four matter most for AP:
• int — whole numbers (e.g. 42). double — decimals (e.g. 3.14).
• boolean — true or false. char — a single character ('A').
• Primitives store the actual value directly in the box.

Java's primitive types int, double and boolean, plus char, and String (an object)

Reference types

• Non-primitive types (like String) are reference types 引用类型.
• A reference variable stores the address of an object, not the object itself.
• String name = "Ada"; — name points to a String object.
• Primitive vs. reference is a distinction you'll use all course.

Naming and declaring

• Names start with a letter; use camelCase (highScore, numStudents).
• Declaring reserves the box; initializing gives it a first value.
• A final variable is a constant — its value can't change after it's set.
• Choose clear names — code is read far more than it's written.