Variables & Data Types

What Is a Variable?

Think of a variable like a labeled jar on a shelf. The label is the variable's name, and the stuff inside is the value. In Java, you also have to say what kind of stuff goes in the jar — numbers? letters? true/false? That's the data type.

Java is a statically typed language, which means you pick the jar size before you put anything in it. Once you say "this jar holds integers," you can't suddenly shove a sentence in there.

Declaring & Initializing Variables

public class Main {
    public static void main(String[] args) {
        int age = 10;
        double price = 4.99;
        char grade = 'A';
        boolean isHappy = true;
        String name = "Alex";

        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Price: $" + price);
        System.out.println("Grade: " + grade);
        System.out.println("Happy? " + isHappy);
    }
}

Output

Name: Alex
Age: 10
Price: $4.99
Grade: A
Happy? true

The Primitive Types — Java's Building Blocks

Java has 8 primitive types. Think of them as different-sized containers:

byte — tiny box (-128 to 127). Good for saving memory when you know values are small.
short — small box (-32,768 to 32,767).
int — the everyday box (-2 billion to 2 billion). This is your go-to for whole numbers.
long — the giant box (up to about 9 quintillion). Add an L at the end: long big = 9999999999L;
float — decimal number, less precise. Add an f: float pi = 3.14f;
double — decimal number, more precise. This is your default for decimals.
char — a single character in single quotes: 'A'
boolean — only true or false. Like a light switch.

String is NOT a primitive — it's a class (notice the capital S) that internally uses a character array. But you'll use it constantly, so Java gives it special treatment.

Type Casting — Converting Between Types

public class Main {
    public static void main(String[] args) {
        // Widening (automatic) — small jar into big jar, no problem
        int myInt = 42;
        double myDouble = myInt;  // int -> double, automatic
        System.out.println("int to double: " + myDouble);

        // Narrowing (manual) — big jar into small jar, you might lose stuff
        double pi = 3.14159;
        int rounded = (int) pi;  // double -> int, you lose the decimals!
        System.out.println("double to int: " + rounded);

        // String to int
        String numText = "123";
        int parsed = Integer.parseInt(numText);
        System.out.println("Parsed: " + parsed);

        // int to String
        String back = String.valueOf(parsed);
        System.out.println("Back to String: " + back);
    }
}

Output

int to double: 42.0
double to int: 3
Parsed: 123
Back to String: 123

The final Keyword — Constants

public class Main {
    public static void main(String[] args) {
        final double TAX_RATE = 0.08;
        final String SCHOOL = "Riverside Elementary";

        double price = 25.00;
        double total = price + (price * TAX_RATE);

        System.out.println("School: " + SCHOOL);
        System.out.println("Total with tax: $" + total);

        // TAX_RATE = 0.10;  // ERROR! Can't change a final variable
    }
}

Output

School: Riverside Elementary
Total with tax: $27.0

What Is a Variable?

Declaring & Initializing Variables

The Primitive Types — Java's Building Blocks

Type Casting — Converting Between Types

The final Keyword — Constants

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