Variables and Data Types

Java requires every variable to have a type. This catches mistakes at compile time instead of at runtime.

Primitive Types

Java has eight primitives. The most common are int, double, boolean and char.

int age = 16;
double price = 2500.50;
boolean isStudent = true;
char grade = 'A';

Text with String

String is a reference type, not a primitive. Strings are immutable: methods return a new string rather than changing the old one.

String name = "Amina";
String greeting = "Hello, " + name;
System.out.println(greeting.length());
System.out.println(name.toUpperCase());

Casting

Widening conversions happen automatically. Narrowing needs an explicit cast and can lose data.

int whole = 10;
double decimal = whole;       // automatic
int back = (int) 10.75;       // 10, loses the fraction

Constants

Use final for values that must not change. Name constants in uppercase with underscores.

final double VAT_RATE = 0.075;

Practice

1. Declare variables for a student name, age and average score.
2. Convert an int to a double and back.
3. Declare a constant for the number of weeks in a term.

Hands-on exercise

Write a class StudentProfile that:
1. Stores a name, age and average score.
2. Prints each on its own line.
3. Prints the score rounded to one decimal place with String.format.

Recap

Java types are explicit. Strings are immutable and BigDecimal is the right choice for money.