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.