final fields, final variables & immutability

Assigning exactly once — and what immutability really means.

final: assign exactly once

The final keyword on a variable means it can be written exactly once:

final int MAX = 100;      // local: assigned at declaration
final int computed;       // blank final local
computed = calculate();   // OK — assigned exactly once
// computed = 5;          // compile error — already assigned

For a final instance field, it must be assigned either: - at its declaration site, or - in every constructor.

public class Circle {
    private final double radius;  // blank final field

    Circle(double radius) {
        this.radius = radius;  // required — and the only assignment allowed
    }
}

Immutability goes one level deeper: final on a reference prevents reassigning the variable, but the object it points to can still be mutated:

final List<String> list = new ArrayList<>();
list.add("hello");          // OK — mutating the object
// list = new ArrayList<>(); // compile error — reassigning the reference

A class is truly immutable (like String) only when all its fields are final AND those fields hold only primitives or references to other immutable objects.

final locks the arrow, not the box. list.add("hello") changes what the object holds and is legal; list = new ArrayList<>() tries to move the arrow and is not.