Variables and Data Types

Every program stores and manipulates data. Java is statically typed, which means every variable must be declared with a specific type before it can be used. This lesson covers primitive types, strings, type casting, and the var keyword.

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Declaring Variables

int age = 25;             // integer
double price = 19.99;     // floating-point number
char grade = 'A';         // single character
boolean passed = true;    // true or false
String name = "Alice";    // text (reference type)

A variable declaration has three parts:

type  variableName  =  value;

Naming Conventions

Convention	Example	Used For
camelCase	firstName, accountBalance	Variables and methods
PascalCase	BankAccount, StudentRecord	Classes and interfaces
UPPER_SNAKE_CASE	MAX_SIZE, PI	Constants (static final)

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Primitive Data Types

Java has 8 primitive types built into the language:

Type	Size	Range / Precision	Default	Example
byte	1 byte	-128 to 127	0	byte b = 100;
short	2 bytes	-32,768 to 32,767	0	short s = 30000;
int	4 bytes	-2^31 to 2^31 - 1	0	int x = 42;
long	8 bytes	-2^63 to 2^63 - 1	0L	long l = 100_000L;
float	4 bytes	~7 decimal digits	0.0f	float f = 3.14f;
double	8 bytes	~15 decimal digits	0.0d	double d = 3.14159;
char	2 bytes	Unicode character (0 to 65,535)	'\u0000'	char c = 'Z';
boolean	1 bit*	true or false	false	boolean ok = true;

Tip: Use int for whole numbers and double for decimals unless you have a specific reason to choose another type. Use underscores in numeric literals for readability: 1_000_000.

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Strings

String is not a primitive — it is a reference type (a class). However, Java provides special syntax for strings:

String greeting = "Hello";                  // string literal
String fullName = "Alice" + " " + "Smith";  // concatenation
int length = greeting.length();             // 5
char first = greeting.charAt(0);            // 'H'
String upper = greeting.toUpperCase();      // "HELLO"
boolean eq = greeting.equals("Hello");      // true

Useful String Methods