Variables and Data Types

A variable is a named container that holds a value. Your program uses variables to remember information — like a person's name, their age, or whether they're a student.

Without variables:

System.out.println(25);  // What does 25 mean? Age? Score? Quantity?

With variables:

int age = 25;
System.out.println(age);  // Clear! This is someone's age.

Variables make your code readable and reusable. Instead of typing 25 everywhere, you type age — and if the age changes, you only update it in one place.

In Java, creating a variable has a strict format. You must tell Java three things:

type  name  =  value;
 ↑     ↑    ↑    ↑
 |     |    |    └── The data you want to store
 |     |    └─────── The equals sign means "assign this value"
 |     └──────────── The name you choose for the variable
 └────────────────── What kind of data this variable holds

Examples:

int age = 25;               // A whole number
double price = 19.99;       // A decimal number
String name = "Alice";      // Text
boolean isStudent = true;   // True or false
char grade = 'A';           // A single character

Why does Java need types?

In JavaScript or Python, you can put anything in any variable. In Java, you must declare the type upfront. This seems annoying at first, but it prevents bugs. If you accidentally try to store text in a number variable, the compiler catches it before your program runs.

int age = "twenty-five";
// Error: incompatible types

let age = "twenty-five";
// Works, but may cause bugs later

Java has many types, but you really only need to know four to get started:

int age = 25;
int score = 100;
int temperature = -5;

double price = 19.99;
double pi = 3.14159;
double weight = 72.5;

String name = "Alice";
String city = "London";
String message = "Hello!";

boolean isStudent = true;
boolean isRaining = false;
boolean hasAccess = true;

Java has exactly 8 primitive types. These store raw values directly in memory. You've already learned the main ones — here's the complete list:

String is the most commonly used reference type. It represents text — any sequence of characters wrapped in double quotes.

String name = "Alice";
String empty = "";              // An empty string (valid!)
String number = "42";           // This is text, NOT a number!

String Concatenation (joining text together):

String first = "John";
String last = "Doe";
String fullName = first + " " + last;  // "John Doe"

// You can also join strings with numbers:
int age = 25;
String message = "I am " + age + " years old";
// Result: "I am 25 years old"

String a = new String("hello");
String b = new String("hello");
a == b        // false (different objects!)
a.equals(b)   // true  (same text content)

Sometimes you have values that should never change — like the value of pi, tax rates, or maximum scores. Use the final keyword:

final double PI = 3.14159;
final int MAX_SCORE = 100;
final String APP_NAME = "MyApp";

// Try to change it?
PI = 3.14;  // ❌ ERROR: cannot assign a value to final variable PI

Sometimes you need to convert data from one type to another. Java has two kinds of casting:

int num = 42;
double d = num;   // 42 → 42.0
// Java does this automatically!

double price = 19.99;
int rounded = (int) price;  // 19
// The .99 is GONE — not rounded!

Converting String to number:

String ageText = "25";
int age = Integer.parseInt(ageText);     // String → int
double price = Double.parseDouble("19.99");  // String → double

Rules (Java will refuse to compile if you break these):

• Names can contain letters, digits, underscores _, and dollar signs $
• Names cannot start with a digit: 1stPlace ❌, firstPlace ✅
• Names cannot be Java keywords: int, class, public are reserved
• Names are case-sensitive: age, Age, and AGE are three different variables

Conventions (not enforced, but everyone follows them):

int x = 25;
String s = "London";
double tp = 19.99;

int age = 25;
String city = "London";
double totalPrice = 19.99;