Variables and types

Key points

• Java has eight primitives: int, long, double, float, boolean, char, byte, short.
• A primitive variable holds the value itself. Every other variable holds a reference to an object.
• Default to int for whole numbers and double for decimals. Long literals need an L: 9_000_000_000L.
• double is binary floating point, so 0.1 + 0.2 is not exactly 0.3. Use BigDecimal for money.
• Overflow is silent. Integer.MAX_VALUE + 1 wraps around to the most negative int.
• var (Java 10+) infers the type of a local variable. The type is still fixed at compile time.

Example

public class Main {
    public static void main(String[] args) {
        int count = 42;
        long big = 9_000_000_000L;
        double price = 19.99;
        boolean active = true;
        char grade = 'A';
        String name = "Ada";

        System.out.println(name + " bought " + count + " items at " + price);
        System.out.println("int max      = " + Integer.MAX_VALUE);
        System.out.println("int overflow = " + (Integer.MAX_VALUE + 1));
        System.out.println("long value   = " + big);
        System.out.println("0.1 + 0.2    = " + (0.1 + 0.2));
        System.out.println("grade + 1    = " + (grade + 1) + " as char: " + (char) (grade + 1));
        System.out.println("active       = " + active);

        var scores = new java.util.ArrayList<Integer>();
        scores.add(90);
        System.out.println("var infers ArrayList<Integer>: " + scores);
    }
}

Primitives hold a value; everything else holds a reference to an object.