static members and final

static fields: one copy for the whole class

public class Ticket {
    private static int issued = 0;      // shared by ALL tickets
    private final int number;           // each ticket has its own

    public Ticket() {
        issued++;
        number = issued;
    }
    public int getNumber() { return number; }
    public static int getIssued() { return issued; }   // static method
}
new Ticket(); new Ticket(); Ticket t3 = new Ticket();
System.out.println(t3.getNumber());      // 3
System.out.println(Ticket.getIssued());  // 3 (called on the class)

static methods

A static method does not run on an object, so it has no this and cannot use instance fields directly. It is ideal for utility functions that only need their parameters: Math.sqrt, Integer.parseInt, Arrays.sort. That is also why main is static: the JVM can call it before any object exists.

public final class GstUtil {
    private GstUtil() {}                                // no objects of a utility class
    public static final double STANDARD_RATE = 18.0;   // a constant
    public static double withGst(double amount) {
        return amount * (1 + STANDARD_RATE / 100);
    }
}
double total = GstUtil.withGst(1000);   // 1180.0

Constants: public static final

A constant is a value that never changes and is the same for everyone: public static final int MAX_LOGIN_ATTEMPTS = 3;. Name constants in UPPER_SNAKE_CASE. For a group of related constants (payment modes, order statuses), an enum is usually better.

final in three places

final on a reference variable means the variable cannot point to another object, NOT that the object cannot change: final List<String> list = new ArrayList<>(); list.add("x"); is fine.

static blocks (in brief)

A static { … } block runs once when the class is first loaded, for complex static initialisation such as loading a table of rates. Use it sparingly.